Weight

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Body weight as a measure of human mass, health status, nutrition, obesity risk, and medical monitoring

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A weighing scale is commonly used to measure body weight in clinical, home, and public health settings.
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In physics, weight is a force caused by gravity; in medicine and everyday health language, body weight usually refers to body mass measured in kilograms or pounds.
Mass and weight are related but not identical. Body mass remains the same, while physical weight changes with local gravity.

Body weight is a commonly used measure of the size and mass of the human body. In everyday language, "weight" usually refers to a person's body mass as measured by a weighing scale and expressed in kilograms, pounds, or stone. In physics, however, weight is technically the force exerted on a body by gravity, while mass is the amount of matter in the body. In medicine, nutrition, fitness, and public health, the term body weight is widely used to describe a person's measured body mass and its relationship to health, nutrition, growth, obesity, malnutrition, metabolism, disease risk, and medical treatment.

Body weight is an important clinical measurement. It is used to assess body mass index, classify underweight, healthy weight, overweight, and obesity, monitor growth in children, calculate some medication doses, guide nutrition therapy, evaluate fluid balance, monitor pregnancy, track heart failure or kidney disease, and follow response to weight loss or weight gain interventions.Adult BMI Categories(link). Centers for Disease Control and Prevention.Obesity(link). World Health Organization.

Overview[edit]

Body weight is influenced by many factors, including genetics, diet, physical activity, sleep, hormones, age, sex, muscle mass, body fat, water balance, medications, pregnancy, medical conditions, and social or environmental factors. A single body weight measurement does not fully describe health, but it is a useful starting point when interpreted with height, body composition, waist circumference, medical history, and metabolic risk factors.

Body weight can be discussed in several overlapping ways:

  • As a physical measurement related to mass, gravity, and force.
  • As a medical measurement used in diagnosis, monitoring, and treatment.
  • As a nutrition marker reflecting energy balance and body composition.
  • As a public health indicator related to undernutrition, overweight, and obesity.
  • As a personal health metric that can change with lifestyle, illness, pregnancy, aging, and treatment.

Terminology[edit]

  • Body weight - Common health term for measured body mass, usually in kilograms or pounds.
  • Body mass - More technically accurate term for the amount of matter in the body.
  • Weight - In physics, the force due to gravity acting on mass.
  • Mass - Quantity of matter in an object or body.
  • Body mass index - Weight in kilograms divided by height in meters squared.
  • Underweight - Body weight below a defined healthy range.
  • Healthy weight - Weight range associated with lower health risk for many people.
  • Overweight - Excess body weight relative to height, often defined by BMI.
  • Obesity - Chronic disease involving excess or abnormal body fat that can impair health.
  • Severe obesity - High level of obesity, often BMI of 40 or greater in adults.
  • Body composition - Relative amounts of fat mass, lean mass, bone, and water.
  • Fat mass - Total body fat.
  • Lean body mass - Body mass excluding fat, including muscle, organs, bone, and water.
  • Skeletal muscle - Muscle attached to bones, important for strength and metabolic health.
  • Adipose tissue - Body fat tissue used for energy storage, endocrine signaling, and insulation.
  • Waist circumference - Measure of abdominal size and central adiposity.
  • Weight management - Strategies to maintain, lose, or gain weight in a healthy way.

Body weight in physics and medicine[edit]

In physics, weight is a force. The gravitational definition of weight is:

W = mg

where W is weight, m is mass, and g is gravitational acceleration. On Earth, standard gravitational acceleration is often approximated as 9.80665 m/s².

In medicine and everyday life, however, a person's "weight" usually means body mass as displayed by a scale. A clinic may record a patient as weighing 80 kg or 176 lb, even though kilogram is technically a unit of mass, not force. This convention is accepted in healthcare because it is practical, familiar, and consistent for routine use on Earth.

Body weight versus mass[edit]

Although the terms are often used interchangeably, body weight and body mass are not identical in physics.

  • Mass is intrinsic and does not change when a person moves from Earth to the Moon.
  • Weight depends on gravity and would be lower on the Moon than on Earth.
  • In clinical practice, measured body "weight" is recorded as kilograms or pounds and treated as body mass.
  • For medical purposes, the distinction rarely affects care because nearly all measurements occur under Earth's gravity.
  • In spaceflight, microgravity changes apparent weight, fluid distribution, muscle mass, bone density, and body composition.

Measurement of body weight[edit]

Body weight is commonly measured using a weighing scale.

Common weighing methods[edit]

Best practices for weighing[edit]

For accurate tracking, body weight should be measured consistently.

  • Use the same scale when possible.
  • Weigh at the same time of day.
  • Wear similar clothing or minimal clothing.
  • Weigh after using the bathroom when tracking home weight.
  • Place the scale on a hard, level surface.
  • Avoid comparing weights from uncalibrated scales.
  • In clinics, record whether shoes, heavy clothing, casts, or assistive devices affect the reading.
  • For infants, use calibrated infant scales and measure without clothing or diaper when clinically appropriate.

Units of body weight[edit]

Body weight is commonly expressed in:

  • Kilogram - Standard SI unit used in medicine and most countries.
  • Gram - Used for newborn and infant weight.
  • Pound - Common in the United States.
  • Ounce - Used with pounds for infant weight in some settings.
  • Stone - Used in some countries, especially the United Kingdom and Ireland.

Conversions:

  • 1 kilogram = approximately 2.20462 pounds.
  • 1 pound = approximately 0.453592 kilograms.
  • 1 stone = 14 pounds.
  • 1 gram = 0.001 kilogram.

Body mass index[edit]

Body mass index (BMI) is a screening measure calculated from body weight and height.

BMI = weight in kilograms / height in meters²

BMI is widely used because it is simple, inexpensive, and useful for population-level assessment. For adults aged 20 years and older, CDC categories include underweight below 18.5, healthy weight from 18.5 to less than 25, overweight from 25 to less than 30, and obesity at 30 or greater.Adult BMI Categories(link). Centers for Disease Control and Prevention.

Adult BMI categories[edit]

BMI Category
Less than 18.5 Underweight
18.5 to less than 25 Healthy weight
25 to less than 30 Overweight
30 to less than 35 Class 1 obesity
35 to less than 40 Class 2 obesity
40 or greater Class 3 obesity or severe obesity

Limitations of BMI[edit]

BMI is useful but imperfect.

  • It does not directly measure body fat.
  • It does not distinguish fat mass from muscle mass.
  • It may overestimate adiposity in very muscular people.
  • It may underestimate adiposity in older adults with low muscle mass.
  • It does not measure fat distribution.
  • It may not reflect health risk equally across all ethnic groups.
  • It should be interpreted with waist circumference, metabolic markers, medical history, and clinical judgment.

Body weight in children and adolescents[edit]

Children and adolescents are still growing, so body weight must be interpreted using age, sex, height, and growth patterns.

CDC child and teen BMI categories are based on BMI-for-age percentiles because body composition changes with growth and puberty.Child and Teen BMI Categories(link). Centers for Disease Control and Prevention.

Body composition[edit]

Body weight alone does not show what the body is made of. Two people can have the same body weight but very different proportions of fat, muscle, bone, and water.

Components of body weight[edit]

Methods of estimating body composition[edit]

Healthy body weight[edit]

A healthy body weight is not a single number. It is a range that supports physical function, metabolic health, quality of life, and disease prevention. For many adults, BMI can help identify a general weight category, but healthy weight assessment should also consider:

  • Waist circumference.
  • Blood pressure.
  • Blood glucose.
  • Lipid profile.
  • Fitness and physical function.
  • Body composition.
  • Age and sex.
  • Ethnicity and family history.
  • Medical conditions.
  • Medication use.
  • Eating pattern.
  • Mental health.
  • Weight history.
  • Personal goals and quality of life.

Underweight[edit]

Underweight refers to body weight below a healthy range. In adults, BMI below 18.5 is commonly classified as underweight.

Causes of underweight[edit]

  • Inadequate calorie intake.
  • Food insecurity.
  • Eating disorders.
  • Malabsorption.
  • Chronic infection.
  • Cancer.
  • Hyperthyroidism.
  • Chronic lung disease.
  • Gastrointestinal disease.
  • Depression.
  • Substance use disorder.
  • Older adult frailty.
  • Medication adverse effects.
  • Excessive exercise without adequate nutrition.

Health risks of underweight[edit]

Overweight and obesity[edit]

Overweight and obesity are defined as abnormal or excessive fat accumulation that may impair health. WHO identifies BMI over 25 as overweight and over 30 as obesity.Obesity(link). World Health Organization.

Obesity is now widely recognized as a chronic, relapsing, multifactorial disease influenced by biology, environment, behavior, medications, sleep, stress, social determinants of health, and access to care.

Health risks associated with excess body weight[edit]

Excess body fat, especially visceral or abdominal fat, can increase risk of:

Obesity as a chronic disease[edit]

Obesity is not simply a lack of willpower. Body weight is regulated by complex interactions involving appetite, satiety, hormones, genetics, brain reward pathways, energy expenditure, food environment, sleep, medications, stress, and social factors.

Energy balance and body weight[edit]

Body weight changes when energy intake and energy expenditure are out of balance over time.

The NIH Body Weight Planner is based on dynamic energy balance, recognizing that weight change is not perfectly predicted by a simple static calorie rule because metabolism and energy expenditure change as body weight changes.About the Body Weight Planner(link). National Institute of Diabetes and Digestive and Kidney Diseases."Energy balance and its components: implications for body weight regulation".American Journal of Clinical Nutrition.2012;95(4)

989-994.PMID:22434603.PMC:3302369.

Factors affecting body weight[edit]

Biological factors[edit]

  • Genetics.
  • Age.
  • Sex.
  • Puberty.
  • Pregnancy.
  • Menopause.
  • Muscle mass.
  • Basal metabolic rate.
  • Appetite hormones.
  • Gut microbiome.
  • Sleep duration.
  • Circadian rhythm.
  • Thyroid function.
  • Insulin resistance.
  • Leptin and ghrelin signaling.

Lifestyle factors[edit]

  • Dietary pattern.
  • Portion size.
  • Physical activity.
  • Sedentary behavior.
  • Sleep quality.
  • Alcohol intake.
  • Stress.
  • Eating speed.
  • Meal timing.
  • Ultra-processed food intake.
  • Sugary beverage intake.
  • Strength training.

Medical factors[edit]

  • Hypothyroidism.
  • Cushing syndrome.
  • Polycystic ovary syndrome.
  • Depression.
  • Anxiety.
  • Eating disorders.
  • Chronic pain.
  • Sleep apnea.
  • Heart failure.
  • Kidney disease.
  • Cancer.
  • Gastrointestinal disease.
  • Inflammatory disease.
  • Medication-related weight gain.

Medication-related weight change[edit]

Some medications may contribute to weight gain or weight loss.

Medications associated with weight gain in some people include:

Medications associated with weight loss in some people include:

Weight gain[edit]

Weight gain can be intentional, unintentional, healthy, unhealthy, temporary, or disease-related.

Common causes of weight gain[edit]

  • Increased calorie intake.
  • Reduced physical activity.
  • Strength training with muscle gain.
  • Pregnancy.
  • Puberty.
  • Menopause-related body composition change.
  • Fluid retention.
  • Medication effects.
  • Sleep deprivation.
  • Stress.
  • Depression.
  • Hypothyroidism.
  • Cushing syndrome.
  • Heart failure.
  • Kidney disease.
  • Liver disease.
  • Binge eating disorder.

Rapid weight gain[edit]

Rapid weight gain over days may reflect fluid retention rather than fat gain.

Possible causes include:

People with heart failure are often advised to monitor daily weight because rapid weight gain can signal fluid overload.

Weight loss[edit]

Weight loss may be intentional or unintentional.

Intentional weight loss[edit]

Intentional weight loss may be recommended for people with overweight or obesity when excess adiposity contributes to health risk. Clinically meaningful weight loss can improve blood pressure, blood glucose, triglycerides, fatty liver disease, sleep apnea, mobility, and quality of life.

Intentional weight loss methods may include:

  • Nutrition therapy.
  • Calorie reduction.
  • Higher protein intake when appropriate.
  • Increased physical activity.
  • Resistance training.
  • Behavioral therapy.
  • Sleep optimization.
  • Stress management.
  • Anti-obesity medication.
  • Bariatric surgery.
  • Treatment of underlying medical conditions.

Unintentional weight loss[edit]

Unintentional weight loss can be a warning sign and should be medically evaluated, especially when significant, persistent, or accompanied by symptoms.

Possible causes include:

  • Cancer.
  • Hyperthyroidism.
  • Diabetes.
  • Chronic infection.
  • Gastrointestinal disease.
  • Malabsorption.
  • Depression.
  • Dementia.
  • Medication side effects.
  • Substance use disorder.
  • Eating disorder.
  • Chronic lung disease.
  • Heart failure.
  • Kidney disease.
  • Food insecurity.

Weight management[edit]

Weight management refers to maintaining a healthy body weight, losing excess fat, gaining needed weight, or preventing weight regain.

Healthy weight maintenance[edit]

Important strategies include:

  • Balanced eating pattern.
  • Adequate protein.
  • High-fiber foods.
  • Regular physical activity.
  • Resistance training.
  • Adequate sleep.
  • Limiting sugary drinks.
  • Reducing ultra-processed foods.
  • Monitoring weight when helpful.
  • Managing stress.
  • Treating sleep apnea, depression, chronic pain, or medications that affect weight.

Medical weight loss[edit]

Medical weight loss is supervised care for people with overweight or obesity and related health risks. It may include:

  • Medical evaluation.
  • Nutrition assessment.
  • Physical activity plan.
  • Behavioral counseling.
  • Laboratory testing.
  • Medication review.
  • Treatment of obesity-related conditions.
  • Anti-obesity medications.
  • Long-term follow-up.
  • Relapse prevention.

Anti-obesity medications[edit]

Modern anti-obesity medications may be used in selected patients when lifestyle treatment alone is insufficient.

Examples include:

Medication choice depends on BMI, comorbidities, contraindications, cost, access, pregnancy plans, adverse effects, and patient preferences.

Bariatric and metabolic surgery[edit]

Bariatric surgery or metabolic surgery may be considered for people with severe obesity or obesity-related complications.

Common procedures include:

Surgery can produce substantial weight loss and improve type 2 diabetes, sleep apnea, hypertension, fatty liver disease, and quality of life in selected patients.

Body weight and pregnancy[edit]

Body weight is important before, during, and after pregnancy.

  • Prepregnancy weight affects pregnancy risk.
  • Gestational weight gain should be individualized.
  • Inadequate weight gain may increase risk of fetal growth restriction.
  • Excessive weight gain may increase risk of gestational diabetes, hypertension, cesarean delivery, and postpartum weight retention.
  • Weight should be monitored during prenatal care.
  • Postpartum weight change is influenced by sleep, breastfeeding, recovery, diet, activity, stress, and support.

Body weight and aging[edit]

Aging changes body composition.

  • Muscle mass often declines with age.
  • Fat distribution may shift toward the abdomen.
  • Bone mass may decline.
  • Appetite may decrease in frail older adults.
  • Unintentional weight loss can signal illness.
  • Obesity in older adults may impair mobility and increase cardiometabolic risk.
  • Sarcopenic obesity combines excess fat with low muscle mass.
  • Strength training and adequate protein can help preserve function.

Body weight and chronic disease[edit]

Diabetes[edit]

Body weight is closely related to insulin resistance and type 2 diabetes. Weight loss can improve glycemic control in many people with overweight or obesity.

Hypertension[edit]

Excess body weight can raise blood pressure. Weight loss may reduce blood pressure in many patients.

Heart disease[edit]

Body weight, waist circumference, blood pressure, cholesterol, glucose, physical activity, and smoking all influence cardiovascular risk.

Sleep apnea[edit]

Obesity is a major risk factor for obstructive sleep apnea. Weight loss can improve sleep apnea severity, though many patients still need CPAP or other treatment.

Fatty liver disease[edit]

Weight loss can reduce liver fat and improve metabolic dysfunction-associated steatotic liver disease in many patients.

Cancer[edit]

Excess adiposity is associated with increased risk of several cancers, including postmenopausal breast cancer, colorectal cancer, endometrial cancer, kidney cancer, and others.

Body weight in clinical care[edit]

Body weight is used in many medical decisions.

  • Medication dosing.
  • Chemotherapy dosing.
  • Pediatric dosing.
  • Anesthesia planning.
  • Fluid dosing.
  • Nutritional assessment.
  • Dialysis planning.
  • Pregnancy monitoring.
  • Heart failure monitoring.
  • Growth monitoring.
  • Critical care nutrition.
  • Obesity treatment eligibility.
  • Bariatric surgery evaluation.

Weight stigma[edit]

Weight stigma refers to negative attitudes, stereotypes, discrimination, or mistreatment based on body weight. Weight stigma can occur in healthcare, education, employment, media, family life, and social settings.

Weight stigma may contribute to:

  • Avoidance of medical care.
  • Psychological distress.
  • Depression and anxiety.
  • Disordered eating.
  • Reduced physical activity.
  • Poor clinician-patient communication.
  • Delayed diagnosis.
  • Lower quality of care.

Respectful care should focus on health, function, evidence-based treatment, patient goals, and compassionate communication rather than blame or shame.

Public health[edit]

Body weight trends are important in public health because both undernutrition and obesity contribute to disease burden. WHO notes that overweight and obesity rates continue to grow in adults and children worldwide, while undernutrition remains a major issue in many settings.Obesity(link). World Health Organization.

Public health strategies include:

  • Food security.
  • Healthy school meals.
  • Physical activity-friendly communities.
  • Safe walking and cycling environments.
  • Nutrition labeling.
  • Reduction of sugar-sweetened beverages.
  • Maternal and child nutrition programs.
  • Obesity prevention.
  • Eating disorder prevention.
  • Access to evidence-based obesity treatment.
  • Addressing social determinants of health.
  • Reducing weight stigma.

Patient education[edit]

Patients should understand that body weight is one health marker, not a complete measure of worth, fitness, or health.

Practical points:

  • Track weight trends rather than overreacting to daily changes.
  • Daily weight can vary with salt intake, menstrual cycle, glycogen, bowel contents, and hydration.
  • Waist circumference and metabolic labs can add useful information.
  • Severe or unexplained weight loss needs medical evaluation.
  • Rapid weight gain may reflect fluid retention.
  • Weight loss should be safe, sustainable, and individualized.
  • Strength training can improve body composition even if scale weight changes slowly.
  • Sleep, stress, medications, and medical conditions can affect body weight.
  • Children should not be placed on restrictive diets without medical guidance.
  • People with eating disorder symptoms should seek professional help.

When to seek medical care[edit]

Seek medical evaluation for:

  • Unintentional weight loss.
  • Rapid unexplained weight gain.
  • Weight loss with fever, night sweats, pain, diarrhea, or blood in stool.
  • Weight gain with swelling, shortness of breath, or chest discomfort.
  • Very low body weight.
  • Suspected eating disorder.
  • Child with poor growth.
  • Severe obesity with difficulty breathing, sleep apnea symptoms, diabetes, or hypertension.
  • Pregnancy with inadequate or excessive weight gain.
  • Weight changes after starting a new medication.
  • Fatigue, hair loss, cold intolerance, or constipation suggesting thyroid disease.
  • Excessive thirst and urination suggesting diabetes.
  • Binge eating, purging, laxative misuse, or compulsive exercise.

Common myths[edit]

  • Myth: BMI is a perfect measure of health. BMI is useful but limited and should be interpreted with other measures.
  • Myth: Body weight is only about willpower. Biology, environment, sleep, stress, medications, hormones, genetics, and social factors all matter.
  • Myth: All weight loss is healthy. Unintentional or excessive weight loss can signal disease.
  • Myth: Muscle weighs more than fat. A pound is a pound, but muscle is denser than fat and takes less space.
  • Myth: Daily weight changes reflect fat gain or loss. Most short-term changes are water, glycogen, food, or bowel contents.
  • Myth: People with obesity cannot be malnourished. Micronutrient deficiencies and poor diet quality can occur at any body weight.

See also[edit]

Further reading[edit]

  • Adult BMI Categories(link). Centers for Disease Control and Prevention.
  • Adult BMI Calculator(link). Centers for Disease Control and Prevention.
  • Child and Teen BMI Categories(link). Centers for Disease Control and Prevention.
  • Obesity(link). World Health Organization.
  • Adult Obesity Facts(link). Centers for Disease Control and Prevention.
  • About the Body Weight Planner(link). National Institute of Diabetes and Digestive and Kidney Diseases.
  • "Energy balance and its components: implications for body weight regulation".American Journal of Clinical Nutrition.2012;95(4)
989-994.PMID:22434603.PMC:3302369.
  • "Role of Physical Activity for Weight Loss and Weight Maintenance".Diabetes Spectrum.2017;PMC:5556592.

External links[edit]


Obesity
Epidemiology


Nutrition information of Weight[edit]


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