BMI Calculator
Body mass index in metric or imperial, with your healthy weight range, waist-to-height ratio, and an honest account of what BMI can and cannot tell you.
Instant answers
Works offline once loaded
Nothing you type is sent anywhere
Metric and imperial
Waist-to-height too
Explains the limits
Measured at the navel, relaxed, not sucked in.
Affects the body-fat estimate only, not BMI.
BMI over-reads muscle as fat; this adds the caveat.
Body mass index
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Under
Healthy
Over
Obese I
II
III
018.525303540
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Healthy weight range for your height—
Distance from that range—
Waist-to-height ratio—
Estimated body fat—
Ponderal index—
BMI Prime—
BMI is a screening tool for populations, not a diagnosis for an individual.
Where the boundaries fall at your height
| Category | BMI | Weight at your height |
|---|
Instructions
How to use this calculator
Step by step
- Choose metric or imperial with the tabs, then enter your height and weight. The two tabs keep separate values, so switching does not scramble what you typed.
- Add your waist measurement if you have a tape to hand. Measure at the navel, standing relaxed, breathing out normally — not pulled in. The waist-to-height ratio it produces predicts health risk better than BMI does.
- Enter age and sex. These do not change BMI at all — BMI is only weight over height squared — but they are needed for the body-fat estimate and for the age-specific caveats.
- Set "build" to notably muscular if that describes you. It does not alter the number; it adds the warning that the number is misreading you, which is the honest thing for a BMI calculator to do.
- Read the needle on the coloured scale for where you sit, and the table underneath for what each boundary means in actual kilograms or pounds at your height.
- Treat the healthy weight range as the useful output. A single index value is much less actionable than "the healthy range for your height is 57–75 kg".
Good to know
- Weigh yourself at the same time of day, ideally in the morning after the toilet and before eating. Day-to-day swings of one to two kilograms are water, not fat.
- If your BMI reads high and your waist is under half your height, the waist measurement is the one to trust. Central fat is what carries the metabolic risk.
- Trends matter more than snapshots. One reading tells you almost nothing; the direction over six months tells you a great deal.
- BMI does not apply to pregnancy, and it is unreliable in people with significant muscle mass, amputees, and anyone with substantial fluid retention.
- A weight loss of 5–10% produces most of the measurable health benefit. You do not need to reach the middle of the healthy range for the change to matter.
- For children and teenagers, ask for the BMI-for-age percentile. The adult categories on this page do not apply and can be genuinely misleading.
The maths behind it
- Body mass index BMI = kg / m² Imperial equivalent: 703 × lb / in².
- Healthy weight range 18.5 × h² to 24.9 × h² h in metres, result in kilograms.
- Waist-to-height ratio WHtR = waist ÷ height Keep it under 0.5 — the same units on both sides, so it works in any system.
- Body fat estimate 1.20 × BMI + 0.23 × age − 10.8 × sex − 5.4 Deurenberg equation; sex = 1 for male, 0 for female. Population-level, ±5 points.
- Ponderal index PI = kg / m³ Scales more sensibly than BMI at very short or very tall heights.
- BMI Prime BMI ÷ 25 Under 1 is within the healthy range; 1.2 means 20% above the upper limit.
Is BMI actually useful?
For populations, yes — it is cheap, standardised, and correlates well with health outcomes across large groups. For individuals it is a crude first screen. It cannot tell muscle from fat, does not know where the fat is, and treats a 150 cm and a 200 cm person as if the same square law applies to both. Use it as a starting question, not an answer.
My BMI says overweight but I lift weights. What now?
This is BMI's best-known failure. Muscle is denser than fat, so a lean, strong person often lands at 26–29 with a perfectly healthy body composition. Look at the waist-to-height ratio instead: if your waist is under half your height, the BMI category is describing your muscle, not your health.
What is waist-to-height ratio and why is it better?
Waist divided by height, in any consistent units. The guideline is simply "keep your waist to less than half your height". It captures central adiposity — fat around the organs — which is the fat most strongly linked to cardiovascular and metabolic disease. Several large studies find it outperforms BMI as a risk predictor, and it needs nothing but a tape measure.
How accurate is the body-fat estimate?
It is a regression fitted to a population, with roughly a ±5 percentage-point standard error for an individual. It is fine for tracking a direction over time and unsuitable for anything else. DEXA is the accurate method; calipers and impedance scales sit somewhere between.
Does BMI work for children?
The calculation is the same but the interpretation is completely different. Children are assessed on BMI-for-age percentile charts because healthy body composition changes substantially through growth. The adult bands shown here should not be applied under 20.
Do BMI thresholds differ between ethnic groups?
Yes. The WHO recommends lower action points for people of South and East Asian descent — overweight from about 23 and obesity from about 27.5 — because metabolic risk rises at a lower BMI in these populations. Some national guidelines adopt these thresholds outright. The bands on this page are the standard international ones.
What is a healthy BMI for an older adult?
Evidence suggests the lowest-mortality range shifts upwards with age, and that over 65 a BMI in the 23–28 region is associated with better outcomes than the standard 18.5–25. Being underweight in later life carries real risk from muscle loss and frailty.