BMI Calculator

Calculator

Why different systems give different answers

The standard WHO cut-offs (18.5, 25, 30) were derived largely from European population data last century. Later research showed that at the same BMI, many Asian populations carry more body fat and face higher cardiometabolic risk, so the WHO Western Pacific office published lower category boundaries (23 for overweight, 25 for obesity), and the WHO's 2004 Expert Consultation kept the international cut-offs while adding action points at 23.0, 27.5, 32.5 and 37.5 where health checks should trigger earlier.

That is why the same person can be 'normal' under one recognised system and 'overweight' under another. Neither answer is wrong; they are calibrated to different populations and purposes. We show them together, labelled, so you can see exactly where you stand under each; most calculators quietly pick one and never tell you.

How BMI is calculated

In metric it is a single division: weight in kilograms over height in metres squared. In imperial units the same ratio needs a conversion factor of 703, because pounds and inches are not metric, which is where the mysterious 703 in American BMI formulas comes from. It is not a fudge factor; it is the unit conversion, and the two forms agree to within a rounding error.

Everything else on this page is arithmetic on that one number. The category is a lookup against a published table. The distance to the nearest boundary is the same formula run backwards to find the weight at a given BMI. Full float precision is carried through every step and rounded only when it is printed.

What BMI cannot tell you

BMI cannot see where your weight is or what it is made of. A muscular 90 kg athlete and a sedentary 90 kg office worker of the same height get the same number with very different health outlooks. That is exactly why guidance since January 2025 asks for BMI plus at least one fat-distribution measure (waist circumference, waist-to-height ratio or waist-to-hip ratio) before concluding anything about excess body fat. Add your waist above and this page runs that check for you; where the tool's competence ends (distinguishing preclinical from clinical obesity needs a doctor's assessment of organ function), it says so plainly.

Height introduces a subtler bias of its own. Dividing by height squared slightly overstates BMI in tall people and understates it in short people, because real bodies do not scale as perfect squares. The ponderal index shown with your result divides by height cubed instead, which behaves better at the extremes, one more reason not to treat a decimal place as a verdict.

Who BMI was never designed for

The formula makes assumptions that some bodies simply do not meet, and in those cases the number is not slightly off; it is answering a different question from the one you asked.

• Anyone under 18: body composition changes continuously through growth, so BMI-for-age percentiles are the only valid reading. Our child BMI calculator uses the CDC growth reference.
• Pregnancy: total mass includes the baby, placenta and a substantially increased blood volume. Pregnancy weight gain has its own guidelines, set by pre-pregnancy BMI category.
• Highly muscular people: the best-documented failure case, and the reason the multi-measure check exists.
• Older adults: the same BMI typically carries more fat and less muscle after 65, and the evidence associates a slightly higher range with better outcomes.
• Amputation and significant oedema: both break the height–weight relationship the formula assumes.
• Very short and very tall adults, where the height-squared term is at its most biased.

How to read your result honestly

Treat the category as the first sentence of a conversation rather than the verdict. If your BMI is raised and your waist is over half your height, the two screens agree and the signal is real. If they disagree (a raised BMI with a modest waist, or a normal BMI with a large one), the disagreement is the most useful thing on the page, and the waist-side measure is usually the truer guide.

Watch the trend rather than the reading. Weight moves a kilogram or two on water and food alone, which is enough to change a BMI by half a point in either direction. A stable BMI of 27 with a shrinking waist is a very different story from the same 27 drifting upward, and no single measurement can tell them apart.

What to do next

If the number surprised you in either direction, the cheapest useful next step is a tape measure: thirty seconds around the middle adds the fat-distribution signal BMI lacks and turns one screen into two. If you want a composition estimate, the tape-based body fat method and the relative fat mass equation both work from measurements you can take at home.

If you are planning to change your weight, the arithmetic starts from maintenance calories rather than from BMI. Our TDEE calculator estimates that, and the calorie deficit calculator turns it into a target with the safety rails already enforced: no target below 80% of maintenance, none below the 1,500 and 1,200 kcal floors, and no goal weight below the healthy range for your height.

How it's calculated

Worked example: 175 cm and 78 kg, with an 88 cm waist

1. Convert height to metres: 175 cm = 1.75 m, and square it: 1.75 × 1.75 = 3.0625.
2. Divide weight by that: 78 ÷ 3.0625 = 25.469, which displays as 25.5.
3. Read the WHO standard table: 25.5 falls in 25–29.9, the 'overweight (pre-obese)' range.
4. Read the WHO Asia-Pacific table: the same 25.5 falls in 25–29.9 there too, but that band is labelled 'obese class I': the same number, two correct answers.
5. Find the nearest boundary: BMI 25 at this height is 25 × 3.0625 = 76.6 kg, so this person is 1.4 kg above the line where 'overweight' begins.
6. Add the waist: 88 ÷ 175 = 0.50 waist-to-height, exactly on the threshold, while the WHO increased-risk waist cut-off for men is 94 cm, so the waist alone sits below it.
7. Run the 2025 check: BMI is raised and one distribution measure sits at its threshold, a borderline result on both screens, which is a reason to re-measure carefully rather than to act on a single reading.

Where this number is used in the real world

• Primary care screening, where BMI is the first number recorded and the trigger for further checks.
• Public-health surveillance, which is what BMI was adopted for: population prevalence of underweight, overweight and obesity is defined by these exact cut-offs.
• Eligibility thresholds for bariatric surgery, some medications and some fertility treatments, which are written in BMI terms.
• Occupational and insurance assessments, where a BMI band is often part of the underwriting question set.
• Clinical research, where BMI is the standard covariate for body size across almost every field of medicine.
• Personal tracking, where the trend across months is more useful than any single reading.

Frequently asked questions

Keep going

A single number rarely tells the whole story. Alongside the BMI result, the waist-to-height calculator, the body fat calculator, the healthy weight range calculator, the asian BMI calculator and the child BMI calculator each add a different angle on the same measurements. For the reasoning behind the numbers, read BMI explained, BMI for athletes and 2025 obesity definition.

Sources

1. WHO Expert Committee. Physical status: the use and interpretation of anthropometry. WHO Technical Report Series 854, 1995. www.who.int/publications/i/item/9241208546
2. WHO Expert Consultation. Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies. Lancet 2004;363:157–63. doi.org/10.1016/S0140-6736(03)15268-3
3. Rubino F, et al. Definition and diagnostic criteria of clinical obesity. Lancet Diabetes Endocrinol 2025;13:221–262. doi.org/10.1016/S2213-8587(24)00316-4
4. Keys A, Fidanza F, Karvonen MJ, Kimura N, Taylor HL. Indices of relative weight and obesity. J Chronic Dis 1972;25:329–43. doi.org/10.1016/0021-9681(72)90027-6
5. NICE. Overweight and obesity management. NICE guideline NG246, 2025. www.nice.org.uk/guidance/ng246