BMI Calculator

This BMI calculator (kg and cm, or pounds, feet and inches) works out body mass index from your height and weight and places it on the WHO adult scale. Leave age blank, or enter 20 or older, for the adult result with BMI Prime and the Ponderal Index. Enter an age from 2 to 19 and it switches to the CDC BMI-for-age percentile for a boy or girl of that exact age, from the same CDC growth-chart data the CDC pediatric BMI calculator draws on. Your numbers never leave the browser.

As a body mass index calculator it answers one narrow question, how heavy you are for your height, and says nothing about whether that weight is fat or muscle.

How to use the BMI calculator

Pick Metric or US Units at the top of the form; each tab keeps what you typed, so switching loses nothing. Age is optional. Blank, or 20 and over, gives the adult result: BMI to one decimal, its WHO category, a gauge, a chart, BMI Prime and the Ponderal Index. An age from 2 to 19 gives the CDC percentile instead, and that is the only case where the Male and Female buttons change anything.

The gauge is a BMI visualizer: its needle sweeps to your value across the WHO bands. The chart beneath it works as a height and weight visualizer, plotting BMI for a range of weights at your height with your own point marked, so you can see how far the next band is. Press Calculate after each change; the working, with your numbers substituted, appears under the result.

If you weigh yourself in stones and pounds, the unit the NHS BMI converter also accepts, multiply the stones by 14, add the leftover pounds and use the US Units tab: 11 st 6 lb is 160 lb.

BMI formula in kg and cm, and in pounds and inches

BMI is weight divided by the square of height. In metric units:

With pounds and inches, a factor of 703 puts the answer in the same kg/m² units:

The 703 comes straight from the unit definitions. A pound is 0.45359237 kg and an inch is 0.0254 m, so the exact factor is:

Rounding it to 703 changes a BMI by about 0.01%, far below the one decimal the result shows. This calculator skips even that: on the US Units tab it converts to kilograms and metres first and runs the metric formula, so both tabs always agree.

Worked example: how to calculate BMI by hand

Take an adult who is 175 cm tall and weighs 70 kg.

1. Convert height to metres. Divide height in centimetres by 100: 175 cm is 1.75 m.
2. Square the height. Multiply the height in metres by itself: 1.75 times 1.75 is 3.0625.
3. Divide weight by the squared height. Divide the weight in kilograms by that number: 70 divided by 3.0625 is 22.86, shown as 22.9.
4. Read the category. Find the band: 22.9 lies between 18.5 and 24.9, so it is Normal weight.

The imperial route lands in the same place. 5'10" in centimeters is 177.8, and a 160 lb person at that height gets:

Converted first, 72.57 kg over 1.778 m squared also gives 23.0, which is what the calculator reports.

BMI table for adults

The WHO sorts adults into eight bands. The calculator's headline result uses the four main BMI categories; the table adds the three grades of thinness and the three classes of obesity, with BMI Prime beside each. The CDC and the NHS use the same 18.5, 25 and 30 boundaries.

A healthy BMI range for an adult is 18.5 to 24.9. Class III, a BMI of 40 or more, is the band still widely called morbid obesity. Categories are read from the displayed value, so a result shown as 25.0 is Overweight even when the unrounded figure was 24.96.

Reverse BMI calculator: healthy weight range for height

Solving the formula for weight turns it into a reverse BMI calculator:

The two tables below run that for every height from 4'10" to 6'6" and from 145 to 200 cm. Each boundary is the lightest whole pound or kilogram at which this calculator, after its one-decimal rounding, reports the next category, so the tables and the tool never disagree.

The last column is the practical rule inside the formula: one BMI point equals your height in metres squared, in kilograms. At 175 cm that is 3.06 kg (6.8 lb). Someone at BMI 27 who wants to reach 24.9 has 2.1 points to lose, which at that height is about 6.4 kg, a concrete target instead of “some weight”.

BMI for children and teens: CDC percentiles

A fixed cut-off cannot work for anyone aged 2 to 19. The median 4-year-old has a BMI near 15.5 and the median 17-year-old near 21, and both shift with sex. So for those ages the tool acts as a child BMI calculator and BMI percentile calculator. It looks up the CDC's 2000 growth-chart LMS values for the exact age in months and sex, turns the BMI into a z-score, then into a percentile:

The percentile is the share of the normal distribution below that z-score. The CDC then groups it into four categories, plus a fifth for severe obesity in children that this calculator does not flag separately:

One difference from the CDC's own tool: since 2022 it reports percentiles above the 95th from its extended BMI-for-age charts, so a figure past 95 here can differ slightly from the CDC's. The category does not, because both start from the same 95th-percentile line.

The tables below turn those percentile lines back into BMI values at each birthday, using the same CDC data. A 10-year-old boy with a BMI of 20.0 is above his 85th-percentile line of 19.4 and reads Overweight (88.6th percentile), while 20.0 is mid-range for an adult.

BMI Prime and the Ponderal Index, the two extra results

Both come from the same two measurements as BMI:

BMI Prime divides BMI by 25, the top of the normal range, so 1.00 is the overweight line and 1.20 the obesity line. For the 175 cm, 70 kg example it is calculated as 22.9 divided by 25, which is 0.92, or 8% under the line. The Ponderal Index cubes height instead of squaring it, which treats very tall and very short adults more evenly; the same person scores 13.06 kg/m³. There is no agreed adult category table for it, so the calculator reports it as a reference figure, and neither index is shown for children.

Asian BMI calculator cut-offs and other adjustments

The category label uses the international WHO cut-offs. A WHO expert consultation published in The Lancet in 2004 found that the share of Asian adults at high risk of type 2 diabetes and heart disease is substantial below a BMI of 25, and proposed public health action points at 23 and 27.5. The UK's NICE guideline NG246 (2025) uses the same two numbers for adults of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background. To use this page as an Asian BMI calculator, read your number against the second or third row below rather than the label.

BMI calculator for women, men and older adults

Adults need no separate BMI calculator, female or male: sex and age do not move the cut-offs, so a BMI calculator for men or for women prints the same number and band for the same height and weight. What differs is the meaning. At the same BMI, women carry more body fat than men on average (Gallagher and colleagues, American Journal of Clinical Nutrition, 2000). Age shifts it too. The WHO has no separate senior female weight chart, but a 2014 meta-analysis of adults aged 65 and over in the same journal (Winter and colleagues, 32 studies) found no extra risk of death in the overweight range and a higher risk at the low end of the healthy range, below about 23.

Common mistakes and edge cases

Height errors count double, because height is squared. For small errors the relative change is:

A 1% error in height shifts BMI about 2%. Measured 2 cm short at 175 cm, the example's 22.9 becomes 23.4.

• Measure height barefoot, heels, back and head against a wall, with a flat object laid level on the crown; compare weights taken at the same time of day.
• Pregnancy: BMI during pregnancy is not meaningful. Weight-gain guidance uses the BMI from before the pregnancy.
• Muscle reads as excess. A lifter who is 5'9" and 220 lb has a BMI of 32.5 at any body fat level; our guide to BMI for athletes works through why.
• Turning 20 changes the scale. A 19-year-old boy with a BMI of 26.0 is below his 85th-percentile line of 26.3 and reads Healthy weight; at 20 the same 26.0 reads Overweight.

BMI versus body fat, ideal weight and adjusted body weight

BMI measures weight for height, not fat. The body fat calculator estimates fat percentage from tape measurements, which suits muscular or very lean builds better. The ideal weight calculator returns a single target from formulas such as Devine's, where BMI gives a range. Clinicians who dose some drugs by weight use an adjusted body weight calculation for patients well above their ideal weight:

That is a dosing convention, not a health target. People looking for a military BMI calculator usually need service screening instead: a U.S. soldier over the height-and-weight screening table is taped for body fat under Army Regulation 600-9, which the army body fat calculator follows. NICE also advises measuring waist-to-height ratio, waist ÷ height, alongside BMI; 0.4 to 0.49 is its healthy band, which means a waist under half your height.

Risks of being overweight or underweight

Across large populations, a BMI above 25, and more strongly above 30, is linked with high blood pressure, type 2 diabetes, coronary heart disease and stroke, osteoarthritis, sleep apnea, gallbladder disease and several cancers, including colon and endometrial cancer. The risks of being underweight, below 18.5, get less attention: nutrient deficiencies and anemia, weaker immunity, lower bone density with a higher fracture risk, and irregular periods and fertility problems in women. These are group-level associations, not a forecast for one person; a doctor reads BMI together with blood pressure, blood sugar, cholesterol and waist size.