BMI calculator
Body mass index for adults and children, healthy weight, waist measurements, energy needs, pregnancy and tracking. Free, no sign-up: everything is computed in your browser, nothing is sent.
For information only. BMI is a statistical indicator, not a diagnosis: it does not distinguish muscle from fat, nor where fat is stored. Discuss your results with a doctor, especially for a child.
Your measurements
Optional measurements
Waist: halfway between the lowest rib and the top of the hip bone, at the end of a normal breath out. Hips: around the widest part of the buttocks.
Special situations
Your BMI
Child BMI-for-age
In children, BMI changes with age: it is read on a BMI-for-age chart by age and sex, like the charts used by doctors.
Weights for your height
The weight matching each BMI category, for the height entered.
Waist size and fat distribution
Belly (abdominal) fat is more closely linked to heart disease and diabetes risk than BMI alone.
Pregnancy weight gain
Recommended weight gain by pre-pregnancy BMI (Institute of Medicine, 2009).
Other indicators
Complementary indices and classic “ideal weight” formulas. These are estimates, not measurements.
Energy needs
Resting energy expenditure (basal metabolic rate) and estimated daily needs for your activity level.
Tracking over time
Save your measurements to follow your weight and BMI. The data stays in this browser.
How is BMI calculated?
Body mass index (BMI) is weight divided by height squared: BMI = weight (kg) / height (m)².
Example: 70 kg and 1.75 m give 70 / 1.75² = 22.9 kg/m². In US units: BMI = 703 × weight (lb) / height (in)².
Adult BMI categories (WHO)
| BMI (kg/m²) | Category |
|---|---|
| below 16 | Severe thinness |
| 16 to 16.9 | Moderate thinness |
| 17 to 18.4 | Mild thinness |
| 18.5 to 24.9 | Healthy weight |
| 25 to 29.9 | Overweight |
| 30 to 34.9 | Obesity class I |
| 35 to 39.9 | Obesity class II |
| 40 and above | Obesity class III (severe) |
- Asian background: at the same BMI, the risk of diabetes and heart disease is higher. The WHO (2004) suggests action points of 23 (increased risk) and 27.5 (high risk); the UK NICE uses the same thresholds.
- Older adults: the French health authority (HAS) defines undernutrition as a BMI below 22 from age 70 (severe below 20) and below 18.5 before 70 (severe at 17 or less), together with other criteria such as weight loss.
- Athletes: a lot of muscle raises BMI without excess fat; waist size is then more informative.
BMI in children and teens
From birth to 18, BMI changes naturally: it rises in the first year, falls until about age 6, then rises again (the adiposity rebound; an early rebound, before age 6, is an early warning sign of overweight). It is therefore compared with reference charts by age and sex.
- IOTF (International Obesity Task Force, Cole and Lobstein, 2012): the curves passing through a BMI of 16, 17, 18.5, 25, 30 and 35 at age 18. Overweight above IOTF 25, obesity above IOTF 30; Cole's thinness grades below 18.5, 17 and 16. Ages 2 to 18; used in many international studies and in the French child health record, which adds two reference curves from 1 month to 2 years (AFPA – CRESS/Inserm – CompuGroup Medical, 2018) for boys.
- WHO: 2006 growth standards (0–5) and 2007 growth reference (5–19). Under 5: overweight above +2 SD, obesity above +3 SD; 5 to 19: overweight above +1 SD, obesity above +2 SD, thinness below −2 SD. Used in the UK from age 4 with UK90 charts, and in many countries.
- CDC (United States, 2000): ages 2 to 20, underweight below the 5th percentile, overweight from the 85th, obesity from the 95th; severe obesity from 120% of the 95th percentile (class 2) and 140% (class 3).
One measurement is not enough: the shape of the curve matters. A curve rising towards the upper lines, or crossing lines, is worth discussing with a doctor or paediatrician.
Waist measurements
- Waist-to-height ratio: above 0.5, cardiometabolic risk increases (“keep your waist to less than half your height”, NICE 2022); above 0.6 it is high. This rule applies from age 6.
- Waist size (WHO, adults): increased risk from 94 cm (37 in) in men and 80 cm (31.5 in) in women; substantially increased from 102 cm (40 in) and 88 cm (35 in).
- Waist-to-hip ratio (WHO): abdominal obesity from 0.90 in men and 0.85 in women.
Formulas for the other indicators
| Indicator | Formula |
|---|---|
| BMI Prime | BMI / 25 (1.0 = start of overweight) |
| New BMI (Trefethen, 2013) | 1.3 × weight / height^2.5: corrects BMI's overestimation for tall people |
| Rohrer's (ponderal) index | weight / height³ (kg/m³) |
| Estimated body fat (Deurenberg, 1991) | adult: 1.2 × BMI + 0.23 × age − 10.8 × S − 5.4; child: 1.51 × BMI − 0.70 × age − 3.6 × S + 1.4 (S = 1 male, 0 female) |
| Body surface area (Mosteller) | √(height in cm × weight / 3600) m² |
| Devine, Robinson, Miller, Hamwi | base + coefficient × (inches over 5 feet), formulas used in pharmacology |
| Lorentz, Creff | formulas popular in France: H − 100 − (H − 150) / 4 (men) or / 2.5 (women); (H − 100 + age / 10) × 0.9 (H in cm) |
| Basal metabolic rate | Mifflin-St Jeor: 10 W + 6.25 H − 5 A + 5 (men) or − 161 (women) kcal;
Black et al. (1996): 1.083 × W^0.48 × H^0.50 × A^−0.13 (men) or 0.963 × … (women) MJ, H in m |
Daily needs = basal metabolic rate × physical activity level (PAL, 1.4 to 2.2). For lasting weight loss, guidelines suggest losing 5–10% of body weight over 6 months, with support from a health professional.
Pregnancy
| Pre-pregnancy BMI | Total weight gain | Per week (2nd and 3rd trimesters) | Twins |
|---|---|---|---|
| below 18.5 | 12.5–18 kg (28–40 lb) | 0.44–0.58 kg (1–1.3 lb) | — |
| 18.5 to 24.9 | 11.5–16 kg (25–35 lb) | 0.35–0.50 kg (0.8–1 lb) | 17–25 kg (37–54 lb) |
| 25 to 29.9 | 7–11.5 kg (15–25 lb) | 0.23–0.33 kg (0.5–0.7 lb) | 14–23 kg (31–50 lb) |
| 30 and above | 5–9 kg (11–20 lb) | 0.17–0.27 kg (0.4–0.6 lb) | 11–19 kg (25–42 lb) |
Source: Institute of Medicine (2009), also used by ACOG and the CDC. First trimester: 0.5–2 kg (1.1–4.4 lb).
Frequently asked questions
What is a healthy BMI?
In adults, a BMI from 18.5 to 24.9 is classed as a healthy weight, associated with the lowest health risk in large studies. It is not a target to hit to the decimal.
Is BMI accurate?
It is a good indicator for a population but imperfect for an individual: it ignores muscle mass, frame, age and fat distribution. Waist size is a useful complement.
Why do my child's results differ between references?
The references were built from different populations and use different thresholds. Use the one your doctor uses: CDC in the US, WHO/UK90 in the UK, IOTF in France and many international studies.
Is my data sent anywhere?
No. Calculations run in your browser; tracking is stored only on this device (local storage) and can be deleted at any time.
Sources
- WHO: adult BMI classification; Child Growth Standards (2006) and Growth Reference 5–19 years (2007); expert consultation on Asian populations (Lancet, 2004).
- Cole T. J., Lobstein T. Extended international (IOTF) body mass index cut-offs for thinness, overweight and obesity, Pediatric Obesity, 2012 (parameters from the open-source sitar package).
- French child health record BMI charts (2018), AFPA – CRESS/Inserm – CompuGroup Medical (curves under age 2 read from the official chart).
- CDC: 2000 growth charts (LMS parameters from the open-source rcpchgrowth package, RCPCH); severe obesity thresholds (2022).
- French National Authority for Health (HAS): diagnosis of undernutrition in children and adults (2019) and in people aged 70 and over (2021).
- Institute of Medicine: Weight Gain During Pregnancy (2009).
- Ashwell M. et al.: waist-to-height ratio (2012, 2016); NICE guideline on overweight and obesity (2022).