BMI Calculator

How BMI is calculated

BMI is weight in kilograms divided by height in metres squared. Imperial inputs are converted before the division, so a weight in pounds and a height in feet give the same figure as their metric equivalents. It is a ratio, which is why it scales oddly: growing 10 cm taller while keeping the same weight drops your BMI by far more than losing a few kilograms would.

BMI prime is the same figure expressed as a fraction of the top of the healthy band (BMI ÷ 25). A prime of 1.0 means you sit exactly at 25, and 1.12 means you are 12 per cent above the top of the healthy range. It reads more clearly than the raw number once you are outside the band.

What the bands mean

These cut-offs come from population data, not from an individual assessment. Some health bodies use lower thresholds for people of South Asian, Chinese, Japanese and Indigenous Australian descent, where risk rises at a lower BMI: overweight from 23 and obesity from 27.5 is a common alternative framing.

Where BMI misleads

• Muscle weighs more than fat. A trained lifter can read as overweight at 12 per cent body fat.
• BMI says nothing about where fat sits. Fat around the organs carries the risk, and waist circumference catches it better.
• It is not validated for pregnancy, children (who need age-and-sex percentile charts), people under 150 cm, or adults over 65, where slightly higher values are protective.
• Ethnicity changes the thresholds that matter, and a single universal cut-off does not reflect that.

Use BMI as a first pass and then add context: waist measurement, body composition if you have it, blood pressure, blood glucose and how your weight has moved over the past year. Those say more than the ratio alone.

What to do with your result

1. If you are in the healthy band and your waist is in range, the number is unremarkable: treat it as one less thing to worry about.
2. If you are outside it, aim for a change you can hold. Losing 5 to 10 per cent of body weight already improves blood pressure and blood sugar for most people.
3. Prefer changes in habits over changes in the number: a daily walk, protein at breakfast, fewer liquid calories.
4. Recheck no more than monthly. BMI moves slowly, and scale weight moves for reasons that have nothing to do with fat.

BMI is a screening tool, not a diagnosis. If you have a health condition, are pregnant, or are considering a very low calorie diet, speak to a clinician first.

What each input means

The calculator asks for 6 pieces of information. Every field is validated before the maths runs, so an impossible value is refused rather than quietly producing a wrong answer.

A worked example using the default inputs

With units of Metric (kg, cm), weight of 75 kg, height of 175 cm, the calculator returns 24.5 kg/m².

The same arithmetic runs whatever you type into the form above, so you can reproduce any figure by matching the inputs listed here.

How this is calculated

BMI = weight (kg) ÷ height (m)². Imperial inputs are converted before the division.

Working values are kept at full precision and rounded only for display. Impossible dates, zero denominators, negative amounts where they make no sense, and out-of-range values are rejected before any result is shown.

The equations these health tools use

Health calculators are published regression equations, not proprietary formulas. Each was fitted to a population sample, which is why they are reported as estimates and why two tools measuring the "same" thing can differ slightly.

The energy equations assume a healthy adult at rest and do not know whether weight is muscle or fat. Two people with identical height and weight can have maintenance needs twenty per cent apart, which is why the calorie tools present a band rather than a single number and why you should compare your own trend over two to three weeks against the estimate.

BMI is a population screening measure, not a description of an individual. It reads high for muscular athletes and low for older adults who have lost muscle, and it was never designed for children, pregnancy or some ethnic groups.

Three worked examples from this section

BMI for 70 kg at 175 cm

Convert height to metres, square it, then divide weight by that figure: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 ≈ 22.9. That sits inside the 18.5 to 24.9 band the World Health Organization labels as the healthy range.

Maintenance calories for a 30-year-old, 70 kg, 175 cm, lightly active

Mifflin–St Jeor gives 10 × 70 + 6.25 × 175 − 5 × 30 + 5 ≈ 1,649 kcal at complete rest. Multiplying by 1.375 for light activity gives roughly 2,267 kcal a day to hold weight.

Resting energy (BMR)

≈ 1,649 kcal

Lightly active maintenance

≈ 2,267 kcal

A 15% deficit

≈ 1,927 kcal

A 15% surplus

≈ 2,607 kcal

Pace for 5 km in 25:00

Twenty-five minutes over five kilometres is exactly 5:00 per kilometre. In miles that is 5:00 × 1.609 ≈ 8:03 per mile, and the same effort projects a 10 km of about 52 minutes.

Projections assume you hold the same pace over the longer distance. In practice most runners slow by three to eight per cent per doubling of distance, which is why race-day predictions from a short run tend to be optimistic.

What health estimates can and cannot tell you

These tools apply published equations to the numbers you enter. The equations were derived from population samples, so they describe an average body with your measurements, not your body. Individual variation of around 10 per cent is normal and is not a sign that you entered something wrong.

• They do not know your medical history, medication, body composition, pregnancy, disability or family risk.
• A single number is not a diagnosis. Trends over weeks, measured under the same conditions, carry far more information than any one reading.
• If a result surprises you, or you are planning a significant change to training or diet, use it as a prompt for a conversation with a clinician or accredited practitioner rather than as an instruction.

Sources and further reading

• World Health Organization: obesity and overweight
• Centers for Disease Control and Prevention: about adult BMI