How it works
Body mass index, or BMI, compares your weight with your height. It’s a quick way to see whether your weight is in the range that’s usually healthy for someone your height, and doctors use it as a first check, not a diagnosis.
You can enter your height in centimeters, meters, or feet and inches, and your weight in kilograms, pounds, or stones and pounds. Whatever you choose, the calculator converts to meters and kilograms using exact conversion factors, then works out your BMI.
You can also choose which BMI ranges to compare with: the standard ranges, or the lower ranges that UK guidance recommends for adults from some ethnic backgrounds.
This calculator is for adults. Children and teens need a different method, explained below.
The formula
BMI = weight ÷ height²
- weight is in kilograms
- height is in meters, so height² means your height multiplied by itself
In pounds and inches, the same formula is usually written as BMI = 703 × weight ÷ height², with weight in pounds and height in inches. The 703 turns pounds per square inch into kilograms per square meter.
For adults, the ranges are:
| Range | Standard BMI | Lower BMI (NICE) |
|---|---|---|
| Underweight | Below 18.5 | Below 18.5 |
| Healthy weight | 18.5 to 24.9 | 18.5 to 22.9 |
| Overweight | 25 to 29.9 | 23 to 27.4 |
| Obesity (class 1) | 30 to 34.9 | 27.5 to 32.4 |
| Obesity (class 2) | 35 to 39.9 | 32.5 to 37.4 |
| Obesity (class 3) | 40 or more | 37.5 or more |
Your BMI is shown to one decimal place, but the range comes from your exact BMI. So a BMI of 24.96 shows as 25.0 and is still in the healthy weight range, because healthy weight runs up to just under 25.
Worked example
Want this with your own numbers? See Show the work above.
Someone is 1.70 m tall (about 5 ft 7 in) and weighs 70 kg (about 11 stone).
- Square the height: 1.70 × 1.70 = 2.89.
- Divide the weight by it: 70 ÷ 2.89 = 24.22, shown as 24.2.
A BMI of 24.2 is in the healthy weight range, 18.5 to 24.9.
For their height, that range runs from 18.5 × 2.89 = 53.5 kg to 24.9 × 2.89 = 72.0 kg.
FAQ
What is a healthy BMI?
For most adults, a BMI of 18.5 to 24.9 is in the healthy weight range. The calculator also shows the weights that give those two numbers for your height, so you can see the range in kilograms or pounds.
How accurate is BMI?
BMI is a useful screening tool, but it doesn’t measure body fat directly. It can overestimate body fat in people with a lot of muscle, such as athletes, and underestimate it in older adults who have lost muscle. It also doesn’t show where fat is stored, which matters for health. It isn’t meant for use during pregnancy. A doctor may look at your waist size and other measures alongside it.
Can children and teens use this calculator?
No. For anyone under 20, BMI is compared with other children of the same age and sex using growth charts, because healthy BMI changes as children grow. Ask a doctor or use a calculator designed for children and teens.
Are the ranges the same for everyone?
Not always. UK guidance from NICE recommends lower thresholds for adults with a South Asian, Chinese, other Asian, Middle Eastern, Black African, or African-Caribbean background, because the risk of heart disease and type 2 diabetes tends to rise at a lower BMI. With these, overweight starts at 23 and obesity at 27.5. Choose “Lower (NICE)” under BMI ranges to use them. Your BMI itself is the same either way; only the range it’s compared with changes.
Why does the pounds-and-inches formula multiply by 703?
BMI is defined in kilograms and meters. Converting pounds and inches to those units works out to multiplying by about 703 (703.07, to be more precise). This calculator converts your height and weight exactly instead, so its result can differ from a 703-based figure, but only in the third decimal place.
Why do I get a slightly different number elsewhere?
Most often it’s rounding. Some calculators round your height or weight before they start, or use 703 instead of the exact conversion. Others round the result instead of showing one decimal. The steps above show every number this calculator uses.
Sources
- The BMI formula in metric and U.S. units: CDC, Calculating BMI
- Adult BMI ranges, and BMI for children and teens: CDC, Adult BMI Categories
- Worked example and one-decimal display: NHS, Calculate your body mass index (BMI) for adults
- Heights and weights for each BMI: NHLBI, BMI Table (NIH Publication No. 22-HL-8190, 2022)
- Standard and lower ranges, including the lower obesity classes: NICE guideline NG246, Overweight and obesity management, recommendations 1.9.10 and 1.9.11
Cite this page
APA (7th edition)
Mathlark. (2026, September 29). BMI calculator. https://mathlark.com/health/bmi-calculator/
MLA (9th edition)
"BMI Calculator." Mathlark, 29 Sept. 2026, mathlark.com/health/bmi-calculator/.
For informational purposes only. This is not medical advice. Talk to a healthcare professional about your situation.