Key takeaways
- Calculates body mass index from height and weight in metric or imperial units, with WHO category labels (underweight, healthy, overweight, obese class I–III).
- BMI is a population screening tool, not a diagnosis. It does not measure body composition, fitness, or individual health.
- For women, we contextualize BMI with notes on muscle mass, life stage, ethnicity-specific cutoffs, and pregnancy adjustments.
- Calculations run in your browser. No tracking, no account, no email.
- For body composition that BMI cannot capture, see our Body Shape Calculator (waist-to-hip and waist-to-height ratios).
How it works
Body mass index (BMI) is a single number derived from height and weight: BMI =
weight (kg) / height² (m²). For imperial inputs, the formula becomes
BMI = (weight (lb) × 703) / height² (in²). The result is the same value either
way; only the units differ.
BMI was developed in the 19th century by Adolphe Quetelet as a population-level proxy for adiposity. It correlates with body fat percentage on average across populations, which makes it useful for screening but unreliable for individuals — particularly muscular athletes (false positive for overweight) and sedentary people with low muscle mass (false negative for healthy weight).
We label BMI according to the WHO classification: under 18.5 is underweight, 18.5–24.9 is healthy, 25.0–29.9 is overweight, 30.0–34.9 is obese class I, 35.0–39.9 is obese class II, and 40+ is obese class III. We optionally apply WHO Asian-specific cutoffs (overweight at 23, obese at 27.5) when selected, reflecting evidence of higher cardiometabolic risk at lower BMI in Asian populations.
The science behind it
What BMI is and isn't
BMI was never designed to diagnose obesity in individuals. The WHO uses it as a population screening tool because it is cheap, fast, and correlates well with average body fat. For an individual, BMI fails in predictable ways: it overestimates adiposity in highly muscular bodies, underestimates it in sarcopenic (low muscle, high fat) bodies, and ignores fat distribution entirely. A waist-to-height ratio above 0.5 is a much better indicator of cardiometabolic risk than a BMI in the overweight range.
Sex differences
Women carry more body fat than men at any given BMI, on average — reproductive-age women typically have 25–32% body fat at a healthy BMI, compared to 13–22% for men. The BMI cutoffs do not reflect this. For women, a body composition measurement (DEXA, BIA, skinfold) and waist-to-hip ratio give a more meaningful read than BMI alone.
Ethnicity-specific cutoffs
The WHO Expert Consultation (2004) recommended lower BMI cutoffs for Asian populations based on evidence that diabetes, hypertension, and cardiovascular disease occur at lower BMI thresholds. The standard recommendation for South Asian, East Asian, and Pacific Islander adults is overweight at BMI 23+ and obese at 27.5+. Some clinical guidelines apply this adjustment routinely; others use it only when clinically indicated.
Conversely, BMI may over-flag adiposity in people of African ancestry because of higher average lean body mass at a given weight. There is no universal correction; clinical judgment and body composition measurement help.
Pregnancy and BMI
BMI calculated during pregnancy is not meaningful — pregnancy weight gain is expected. Clinicians use pre-pregnancy BMI to set expected gestational weight gain ranges (Institute of Medicine, 2009):
- Underweight (under 18.5): gain 12.5–18 kg / 28–40 lb
- Healthy (18.5–24.9): gain 11.5–16 kg / 25–35 lb
- Overweight (25–29.9): gain 7–11.5 kg / 15–25 lb
- Obese (30+): gain 5–9 kg / 11–20 lb
Twin pregnancies have higher targets. Gaining significantly outside these ranges is associated with higher rates of preeclampsia, gestational diabetes, and preterm birth at the high end, and intrauterine growth restriction at the low end.
BMI and fertility
Both very low and very high BMIs reduce fertility. Underweight BMIs suppress ovulation through hypothalamic amenorrhea — the body interprets low energy availability as a signal not to conceive. High BMIs are associated with PCOS, anovulation, and longer time to conception (Wise et al., Hum Reprod 2007). The "fertile sweet spot" is approximately BMI 19 to 25, though there is significant individual variation. If conception is the goal and BMI is outside this range, evaluation by a clinician is reasonable; small weight changes (5–10% of body weight) can restore ovulation in some users.
References
- WHO BMI classification for adults — World Health Organization
- Appropriate body-mass index for Asian populations — WHO Expert Consultation, Lancet 2004
- Weight gain during pregnancy: reexamining the guidelines — Institute of Medicine 2009
- BMI and reproductive function in women — Wise et al., Hum Reprod 2007
How to use this calculator
- Pick your unit system. Metric (cm/kg) or imperial (ft+in/lb). Both give the same BMI value.
- Enter your height. Stand against a wall, look straight ahead, and measure to the top of your head. Mid-day height drops 1–2 cm from morning; either is fine, just be consistent.
- Enter your weight. Best measured in the morning after using the bathroom and before eating. Use the same scale and time of day for trend tracking.
- Read your BMI and category. BMI under 18.5 is underweight, 18.5–24.9 is healthy, 25.0–29.9 is overweight, 30.0+ is obese (with sub-classes).
- Read the contextualization. BMI alone says little. Pair it with body composition, fitness markers, and clinical context — we surface the questions worth asking.
Limitations & medical disclaimer
- BMI does not measure body composition. It does not distinguish muscle from fat.
- BMI does not capture fat distribution. Visceral fat (around organs) is much more strongly associated with cardiometabolic risk than subcutaneous fat.
- BMI cutoffs were derived from primarily white European populations. Apply ethnicity-specific cutoffs where appropriate.
- BMI during pregnancy is not meaningful. Use pre-pregnancy BMI to guide expected gestational weight gain.
- BMI is a screening number, not a diagnosis. Health is determined by many factors — fitness, blood markers, sleep, stress, mental health, and individual circumstances.
- HerCalc is for educational use. For weight-related health concerns, consult a qualified healthcare provider.
HerCalc tools are educational and do not replace professional medical advice. Always consult a qualified clinician for diagnosis or treatment decisions.
Frequently asked questions
How is BMI calculated? +
BMI = weight in kg / (height in meters)². For imperial units: BMI = (weight in pounds × 703) / (height in inches)². The result is the same number; only the calculation steps differ. The WHO categories — under 18.5 underweight, 18.5–24.9 healthy, 25–29.9 overweight, 30+ obese — apply globally for adults aged 20+ regardless of how the BMI was computed.
Is BMI accurate for women? +
BMI uses the same formula and same cutoffs for men and women. This is a known limitation: women generally carry more body fat than men at any given BMI, and the BMI healthy-range cutoffs were derived from mortality data that mixes sexes. For most women, BMI is a useful first screening tool but it should not be the primary measure of health. Body fat percentage, waist-to-hip ratio, and waist-to-height ratio give a more meaningful picture.
What is a healthy BMI for a woman? +
WHO defines 18.5 to 24.9 kg/m² as the healthy weight range for adults regardless of sex. For women trying to conceive, fertility tends to be best in the 19 to 25 range — both very low and very high BMIs are associated with reduced ovulation rates and longer time to conception. For women over 65, mortality data suggest a higher BMI ceiling (up to about 27) may be associated with the lowest mortality.
Should I use a different BMI cutoff if I am Asian, Black, or Hispanic? +
For people of South Asian, East Asian, and Pacific Islander ancestry, the WHO recommends a lower overweight cutoff of 23 (vs. 25) and a lower obesity cutoff of 27.5 (vs. 30). This reflects evidence that cardiometabolic risk rises at lower BMI in these populations. For people of African ancestry, BMI tends to overestimate adiposity at a given weight; the standard cutoffs may flag healthy individuals as overweight. The HerCalc calculator can apply the Asian-specific cutoffs when selected.
What does BMI not measure? +
BMI does not distinguish muscle from fat, does not capture fat distribution (visceral vs. subcutaneous), and does not adjust for age, sex, or ethnicity. A muscular athlete can have a BMI of 28 with very low body fat. A sedentary person with a BMI of 23 can have high visceral fat ("normal-weight obesity"). BMI is a screening number, not a diagnosis. Pair it with waist-to-hip ratio, waist-to-height ratio, and body fat percentage for a fuller picture.
How does pregnancy affect BMI? +
BMI calculated during pregnancy is not meaningful — your weight gain reflects the pregnancy itself, not body composition change. Clinicians use your <em>pre-pregnancy</em> BMI to guide expected gestational weight gain. The Institute of Medicine 2009 ranges are: underweight (BMI under 18.5) gain 12.5–18 kg, healthy (18.5–24.9) gain 11.5–16 kg, overweight (25–29.9) gain 7–11.5 kg, obese (30+) gain 5–9 kg. Twin pregnancies have higher targets.
My BMI is in the obese range but I feel healthy. Should I worry? +
BMI is a starting point, not a verdict. Important questions: are your blood pressure, blood sugar, lipid panel, and waist-to-height ratio in healthy ranges? Do you have stable energy, good sleep, and regular menstrual cycles? Do you exercise regularly and eat a varied diet? If those metrics are fine, "metabolically healthy obesity" is a real category — though long-term cardiometabolic risk is still somewhat elevated and worth tracking. A clinician can run the full panel and interpret it in context.
How does BMI relate to fertility? +
Both very low and very high BMIs are associated with reduced fertility. Underweight (BMI under 18.5) can suppress ovulation through hypothalamic amenorrhea. Higher BMIs (especially 30+) are associated with PCOS, anovulatory cycles, and longer time to conception. The "fertile sweet spot" is roughly 19 to 25, though this varies by individual. If you are trying to conceive and your BMI is outside this range, talk to your clinician about evaluation — small weight changes (5–10% of body weight) sometimes restore ovulation in either direction.