BMI Calculator

What BMI is

Body Mass Index (BMI) is a single number that compares your weight with your height. It is the quickest screening measure doctors use to place an adult into a broad weight category, and it needs nothing more than a tape measure and a scale.

The formula

• Metric: BMI = weight in kilograms ÷ (height in metres)²
• Imperial: BMI = 703 × weight in pounds ÷ (height in inches)²

So someone who is 1.70 m and 65 kg: 65 ÷ (1.70 × 1.70) = 22.5.

BMI categories (WHO)

• Below 18.5 — underweight
• 18.5 to 24.9 — normal weight
• 25.0 to 29.9 — overweight
• 30.0 and above — obese

Many Asian countries use tighter thresholds because health risks rise at a lower BMI. The Chinese standard is:

• Below 18.5 — underweight
• 18.5 to 23.9 — normal
• 24.0 to 27.9 — overweight
• 28.0 and above — obese

Use the standard selector in the calculator to switch between them; the same BMI can fall in a different band.

Healthy weight range

Because BMI is just weight over height squared, you can turn it around: for your height, the healthy range is the weight that keeps your BMI between 18.5 and 24.9. The calculator shows that range in both kilograms and pounds, plus how many kilograms separate you from it.

What BMI does not tell you

BMI is a screening tool, not a diagnosis. It cannot distinguish muscle from fat, so it overstates body fat in athletes and people with a muscular build — someone with a lot of muscle can be flagged "overweight" while being very lean. It also understates body fat in people who have lost muscle mass, which is common in older age.

BMI is not designed for:

• Children and teenagers — they need age- and sex-specific growth percentile charts.
• Pregnant or breastfeeding women.
• Very muscular athletes, bodybuilders and manual labourers.
• Very tall or very short adults, where the squared term drifts.

A single number also says nothing about *where* fat sits. Waist circumference matters: a waist above 94 cm (37 in) for men or 80 cm (31.5 in) for women signals higher risk even at a normal BMI.

Using the number sensibly

• Track the trend over weeks, not the daily fluctuation — weight varies by 1–2 kg with food, water and salt alone.
• Pair BMI with waist measurement and, if you can, a body-composition estimate.
• Set a modest target first: losing 5–10% of body weight already improves blood pressure, blood sugar and cholesterol noticeably.
• If your BMI suggests a health risk, discuss it with a clinician rather than self-diagnosing.

Privacy

Everything is calculated in your browser. Your height and weight are never uploaded, stored or sent anywhere.

Does BMI differ for men and women?

For adults, no. The formula (kg ÷ m²) and every cut-off — underweight below 18.5, overweight from 25, obesity from 30, or 24 and 28 on the China/Asia scale — are identical for men and women. BMI measures weight relative to height; it cannot tell muscle from fat, let alone sex.

Sex does change the surrounding picture in three ways:

• Ideal weight. Clinical ideal-body-weight formulas use different bases for each sex. The Devine formula (1974) used here starts at 50 kg for men and 45.5 kg for women, then adds 2.3 kg for every inch above 5 ft (152.4 cm), subtracting below it. Pick a sex above and the tool shows that figure.
• Body fat at the same BMI. Women carry more essential fat, so at an identical BMI a woman typically has a higher body-fat percentage and may look heavier than a man of the same height and BMI. BMI thresholds were not tuned for this.
• Waist. Abdominal fat is what drives metabolic risk, and the alert lines differ: for Asian populations 80 cm for women and 90 cm for men. The waist-to-height ratio — waist ÷ height, which should stay under 0.5 — is sex-neutral and a better single number than BMI alone.

What BMI is not for

The adult table does not apply to children and teenagers, who need age- and sex-specific percentile charts (a growing child's healthy BMI changes month to month). It is also unreliable during pregnancy, for competitive athletes, and for older adults who have lost muscle mass. Use BMI as a first screen, add a waist measurement, and treat any decision that matters as a conversation with a clinician.

FAQ

Is BMI accurate for athletes or people with a lot of muscle?

No. BMI only knows weight and height, so muscle is counted as if it were fat. Muscular people are frequently labelled overweight while carrying very little fat. Use waist measurement or a body-composition estimate for a fairer picture.

Which BMI standard should I use — WHO or the Chinese one?

The Chinese (and wider Asian) thresholds are tighter because the same BMI carries more metabolic risk in Asian populations. If you are of Asian descent, the lower cut-offs are usually the more meaningful reference. The calculator lets you switch between both.

Is BMI valid for children, pregnant women or older adults?

Not this adult formula. Children and teenagers need age- and sex-specific growth percentiles, pregnancy changes body composition entirely, and older adults lose muscle mass, which makes BMI read lower than the actual fat level.

How do I reach the healthy range shown?

Small, sustainable changes: eat a little less energy than you burn, keep protein and fibre up, and aim for regular movement you can maintain. A loss of 5–10% of body weight already brings measurable health benefits, so treat that as the first milestone rather than a final target.

Does BMI differ for men and women?

For adults the formula and every cut-off are identical for both sexes — BMI screens weight relative to height and cannot separate muscle from fat. Sex matters around it: the ideal-weight formulas differ (Devine: 50 kg base for men, 45.5 kg for women), women carry more body fat at the same BMI, and waist alert lines differ (Asian: 80 cm women, 90 cm men). Children and teenagers are different again — they need age- and sex-specific percentile charts.