PonyatnoHow it's calculated
BMI = weight (kg) ÷ height (m)². Height in centimetres is divided by 100: at 170 cm and 65 kg, BMI = 65 ÷ 1.7² ≈ 22.5. The result is rounded to one decimal place.
Waist-to-height ratio = waist (cm) ÷ height (cm), rounded to two decimal places. Both must be in the same units.
BMI categories follow the WHO: below 18.5 underweight, 18.5 to under 25 healthy, 25 to under 30 overweight, from 30 obesity class I, from 35 class II, from 40 class III. A value exactly on a boundary goes into the higher category: a BMI of exactly 25 counts as overweight.
Waist-to-height ratio follows NICE (2022): 0.4 to under 0.5 healthy, 0.5 to under 0.6 increased risk, 0.6 or more high risk.
Healthy weight for your height runs from 18.5 × height² to 24.9 × height² (height in metres).
What to know
What the two numbers tell you
BMI quickly shows whether weight fits height, but it can't tell muscle from fat or see where fat is stored. Fat around the belly is more closely linked to type 2 diabetes and heart disease than fat on the hips. That's why NICE has advised adults since 2022 to measure their waist as well and keep it below half their height.
Waist-to-height ratio is especially useful when BMI is below 35: someone with a “healthy” BMI and a large waist can carry more risk than someone who is overweight with a slim waist.
When BMI gets it wrong
In athletes and very muscular people, BMI overstates body fat — the weight is muscle. In older adults it's the reverse: muscle declines with age, so a normal BMI can hide a lot of fat. Also, in later life a little extra weight isn't always harmful, so goals are best set with a doctor.
In people of Asian heritage, the risk of diabetes and heart disease rises at a lower BMI, so the WHO and NICE use lower cut-offs for them. NICE advises similar adjustments for people of Middle Eastern, Black African and African-Caribbean heritage.
Adult cut-offs don't apply under 18: children's and teenagers' BMI is judged against growth percentiles for age and sex. In pregnancy, BMI isn't calculated from current weight — pre-pregnancy weight is used, and weight gain is assessed by the doctor or midwife.
Healthy weight is a guide, not a target
The healthy weight range matches a BMI of 18.5 to 24.9 at your height. It's a guide: blood pressure, blood sugar, cholesterol, activity and how you feel matter too. If your weight is above the range, even losing a few percent already brings benefits.
This calculator is a reference tool, not a diagnosis. Laboratory ranges differ, and the result depends on your health, medicines and other tests. Discuss it with your doctor.
FAQ
What is a healthy BMI?
For adults, the WHO puts it at 18.5 to under 25. Below 18.5 is underweight, 25 and above is overweight, and 30 and above is obesity. The cut-offs are the same for men and women.
How do I measure my waist correctly?
Standing, on bare skin, after breathing out normally. Place the tape level, halfway between your lowest rib and the top of your hip bone — this isn't always at the belly button. Don't suck in your stomach or pull the tape tight.
Why does BMI say I'm overweight when I'm fit?
BMI counts all your weight, muscle included, so it overstates fat in muscular people. Waist-to-height ratio is more telling here: if it's in the healthy range, there's probably nothing to worry about.
Can children or pregnant women use this calculator?
No. Under 18, BMI is read against percentile charts for age and sex — that's for a pediatrician or GP. In pregnancy, pre-pregnancy BMI and weight gain by week are what matter.
Which matters more — BMI or waist?
It's best to look at both. BMI reflects overall weight; the waist reflects belly fat, which is more closely tied to heart and metabolic risk. If both are in range, that's a good sign; if one is out, it's worth discussing with your doctor.
Sources
Formulas and cut-offs follow clinical guidelines — see sources below.
- World Health Organization. Obesity: preventing and managing the global epidemic. WHO Technical Report Series 894. Geneva: WHO; 2000.
- NICE guideline NG246 / CG189 update (2022): waist-to-height ratio to assess central adiposity.