BMI calculator
BMI compares your weight to your height. It is a quick population-level screen, useful as one signal, but it cannot tell muscle from fat and says nothing about where you carry weight. Read the number with that caveat in mind.
Your BMI
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BMI is a rough screen, not a verdict. It cannot tell muscle from fat, so a muscular person can read as "overweight" while carrying little fat. Use it as one signal among several.
These categories are for adults. For children and teenagers, BMI is read against age charts instead. Many Asian populations carry higher health risks at lower BMIs, so international guidance also names action points at 23 and 27.5 for them, alongside the usual 25 and 30.
How to use the BMI calculator
Pick your units first. The calculator takes kilograms and centimetres, pounds with feet and inches, or stone and pounds with feet and inches. Type your weight and height, and your BMI and its category appear straight away. Change either number and the result updates as you type.
The calculator is for adults. It has no age box, so it cannot adjust the result for a child or teenager.
How BMI is worked out
BMI is your weight in kilograms divided by your height in metres, squared. Feet, inches, pounds and stone are converted to metric first. One inch is 2.54 cm, 1 lb is 0.45359237 kg and 1 stone is 14 lb. The calculator then places your number in one of four adult bands.
| BMI | Category shown |
|---|---|
| Under 18.5 | Underweight |
| 18.5 to under 25 | Healthy weight |
| 25 to under 30 | Overweight |
| 30 or more | Obese |
These bands are the international adult cut-offs. Many Asian populations show higher health risks at BMIs below 25. An expert panel kept the usual bands for everyone but named extra public-health action points at 23, 27.5, 32.5 and 37.5. If your family background is Asian, read a BMI of 23 to 25 with that in mind.
A worked example
The calculator opens at 70 kg and 170 cm. Height in metres is 1.70, and 1.70 × 1.70 is 2.89. Then 70 ÷ 2.89 gives a BMI of 24.2, so the category reads “Healthy weight”.
The same sum shows the healthy band for that height. At 170 cm, a BMI of 18.5 is about 53.5 kg (18.5 × 2.89). A BMI of 25 is about 72.3 kg (25 × 2.89). Any weight between those two figures falls in the healthy-weight band at that height.
On the Asian action points, 24.2 sits above the first one at 23. The category shown stays “Healthy weight”, because the calculator uses the international bands.
What your BMI can and cannot tell you
BMI is a screen for whole populations. Your BMI does not measure body fat, and two people with the same BMI can carry very different amounts of fat and muscle. Waist size is another measure used to spot overweight and obesity.
For a closer picture, the body fat calculator estimates your body fat percentage from a tape measure. The FFMI calculator then turns that percentage into lean mass per square metre of height, a figure BMI has no way to give.
BMI and intermittent fasting
Your BMI does not decide which fasting schedule suits you. The fasting calculator works from the time of your last meal and the schedule you pick, such as 16:8 or 18:6.
BMI does matter for safety. We advise against fasting for weight loss if your BMI is under 18.5. Our water-fast estimator refuses to give an estimate below that figure. Anyone under 18 should not fast without a doctor’s advice, so a teenager’s BMI should not be used to plan a fast.
If checking your weight or BMI leaves you anxious or low, talk to a doctor before you start any diet.
Related tools and guides
To turn a BMI result into a plan, read our step-by-step guide for beginners. The TDEE calculator gives a daily calorie target for your goal. If you want to compare schedules first, see 16:8 vs 18:6 vs OMAD.
Frequently asked
What is a healthy BMI for adults?
For adults, a BMI from 18.5 to just under 25 falls in the healthy-weight range. Under 18.5 counts as underweight, 25 or more as overweight and 30 or more as obesity. For many Asian populations, international guidance adds earlier action points at 23 and 27.5.
Is there an intermittent fasting calculator based on BMI?
Your BMI does not change a fasting schedule, so the fasting calculator asks only for your last meal and the schedule you want. BMI matters for safety. We advise against fasting if your BMI is under 18.5, and the water fasting calculator will not give an estimate below that figure.
Can I use this BMI calculator for a child or teenager?
No. The adult categories do not apply to children and teenagers, because their healthy BMI changes with age. For ages 5 to 19, international guidance reads BMI against age charts instead. A doctor can check a child's figure on the right chart.
Why does my BMI say overweight when I am muscular?
BMI uses only weight and height, so 5 kg of muscle counts the same as 5 kg of fat. People who carry a lot of muscle can land in the overweight band with little body fat. The body fat calculator and the FFMI calculator show how your weight splits between fat and lean mass.
Does BMI work the same in kg and in pounds?
Yes. The calculator converts pounds or stone to kilograms and feet and inches to centimetres, then runs the same sum. Take a person of 70 kg and 170 cm, and another of 154 lb and 5 ft 7 in. They get almost the same answer, 24.2 and 24.1. The gap comes from rounding in the imperial figures.