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BMI Calculator
Calculate your Body Mass Index (BMI) instantly using metric or US units. See your category.
BMI is a quick screening tool that compares weight to height. It doesn't measure body fat directly — athletes with high muscle mass often score 'overweight' despite being healthy. Always pair BMI with other health markers.
What BMI actually measures
Body Mass Index is a single number derived from height and weight using the formula BMI = weight (kg) / height (m)². For US units, the equivalent formula is BMI = (weight in lb × 703) / (height in in)². It's a screening tool — designed to flag people who may need a more detailed health assessment, not a diagnosis on its own.
BMI was developed by Belgian statistician Adolphe Quetelet in the 1830s as a population-level measure. It's still used today because it's cheap, requires no equipment beyond a scale and tape measure, and correlates reasonably well with body fat percentage across large groups.
BMI categories (adults, WHO)
Underweight: below 18.5 · Normal weight: 18.5 to 24.9 · Overweight: 25.0 to 29.9 · Obesity Class I: 30.0 to 34.9 · Obesity Class II: 35.0 to 39.9 · Obesity Class III (severe): 40.0 and above
When BMI is misleading
Athletes and bodybuilders often have BMIs in the 'overweight' or 'obese' range because muscle weighs more than fat. A 6'0" rugby player at 220 lb (BMI 29.8) isn't unhealthy — they're muscular.
Older adults can lose muscle mass while gaining fat, keeping their BMI stable while body composition worsens. Look at waist circumference too.
Pregnant or breastfeeding women shouldn't use standard BMI categories — your doctor will track weight differently.
Children and teens have separate BMI-for-age percentile charts; adult thresholds don't apply.
Asian populations may face elevated health risks at lower BMIs — the WHO suggests an overweight cutoff of 23 (rather than 25) for many Asian groups.
Quick examples
5'9" and 160 lb → BMI 23.6, normal weight.
5'4" and 145 lb → BMI 24.9, top of normal range.
6'2" and 210 lb → BMI 27.0, overweight.
170 cm and 70 kg → BMI 24.2, normal weight.
Healthy weight range for someone 5'9" tall: roughly 125 to 168 lb (BMI 18.5–24.9).
Better health markers to pair with BMI
Waist circumference: under 35" (women) / 40" (men) is a useful threshold.
Waist-to-hip ratio: below 0.85 (women) / 0.90 (men) is generally healthier.
Body fat percentage from a DEXA scan, BodPod, or smart scale (less reliable).
Resting blood pressure, fasting glucose/A1C, and a basic lipid panel — these tell you far more about cardiovascular risk than BMI alone.
Frequently asked questions
What is a healthy BMI?
A BMI between 18.5 and 24.9 is considered a normal weight for most adults. Under 18.5 is underweight; 25 to 29.9 is overweight; 30 or higher is obese (class I 30–34.9, class II 35–39.9, class III 40+).
Is BMI accurate?
It is a useful screening tool, not a diagnosis. BMI cannot distinguish muscle from fat, so muscular athletes often score as 'overweight' despite low body fat. It also underestimates risk in older adults who have lost muscle mass. Pair it with a waist measurement and body-fat percentage for a fuller picture.
What BMI should I aim for?
Most adults do best in the 18.5–24.9 range, but your ideal weight depends on muscle mass, frame size, age, ethnicity, and overall health. Asian populations use lower healthy cutoffs (overweight starts at 23). Discuss goals with your doctor.
How do I lower my BMI safely?
Aim for 1–2 lb of weight loss per week. Combine a modest calorie deficit (see our calorie calculator) with strength training to preserve muscle. Crash diets drop BMI fast but strip muscle and usually rebound.
Does BMI work for children?
No — children and teens use age- and sex-specific BMI percentiles, not adult cutoffs. Pediatricians compare BMI to growth charts, where the 5th to 85th percentile is considered healthy.
Is BMI different for men and women?
The formula and cutoffs are the same. However, women naturally carry more body fat at a given BMI than men, so the same BMI can reflect different body composition between sexes.
What is Body Mass Index?
Body Mass Index (BMI) is a simple numerical measure that classifies body weight relative to height. It equals weight in kilograms divided by height in meters squared and is used to screen adults as underweight, normal, overweight, or obese. It is a general-information tool, not medical advice.
How is BMI calculated?
Divide weight in kilograms by height in meters squared (kg/m²). In imperial units, divide weight in pounds by height in inches squared and multiply by 703 (lb/in² × 703). Compare the result to the standard BMI categories to assess weight status.
What factors influence BMI?
BMI depends primarily on height and weight. It does not directly account for muscle mass, bone density, or fat distribution, which vary significantly between individuals. Age, sex, and ethnicity also affect how the result should be interpreted. This is general information, not medical advice.
Can BMI be used for all body types?
No. Very muscular individuals may score high on BMI because of extra muscle, not excess fat. Conversely, older adults with low muscle mass can have a healthy-looking BMI while still carrying too much body fat. Consult a healthcare professional for a personalized assessment.
Why does BMI matter?
BMI is a convenient, widely used tool for estimating weight-related health risk. It can help flag potential issues such as heart disease, diabetes, and some cancers linked to overweight or obesity. It is a screening tool, not a diagnostic one — this is general information, not medical advice.
More in Body Composition
Five complementary lenses on body composition: BMI, body fat %, BRI, waist-to-height ratio, and ideal-weight ranges.
- Body Fat Calculator
Estimate body fat percent with the US Navy circumference method, plus a BMI-based check.
- Body Roundness Index (BRI)
Models the body as an ellipse from waist and height — predicts mortality risk better than BMI.
- Waist-to-Height Ratio
Keep your waist under half your height — the simple cardiometabolic risk screen.
- Ideal Weight Calculator
Estimate ideal body weight from height, sex, and frame using Hamwi, Devine, Robinson, and Miller formulas, plus the healthy-BMI range.
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Estimate body fat percent with the US Navy circumference method, plus a BMI-based check.
Ideal Weight Calculator
Estimate ideal body weight from height, sex, and frame using Hamwi, Devine, Robinson, and Miller formulas, plus the healthy-BMI range.
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Daily calorie target for weight loss at your chosen pace, plus weeks to your goal — companion to the TDEE calculator.
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Daily calories to lose, maintain, or gain weight using Mifflin-St Jeor.
Macro Calculator
Daily protein, carbs, and fat targets for cutting, maintenance, or bulking based on your TDEE.
Body Roundness Index (BRI)
Models the body as an ellipse from waist and height — predicts mortality risk better than BMI.
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Best bedtime or wake time based on 90-minute sleep cycles.
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Basal metabolic rate using Mifflin-St Jeor and Harris-Benedict, with activity-multiplier TDEE.