BMI is a screening estimate for adults, not a diagnosis or medical advice. If you are worried about your weight or health, talk to a doctor, nurse or pharmacist.
The BMI calculator takes cm and kg or feet, inches and pounds, and shows your BMI, its category and the healthy weight range for your height.
Work out your BMIThe BMI formula
BMI stands for body mass index. It is your weight divided by the square of your height, and the US CDC gives the formula in two forms:
BMI = weight (kg) ÷ height (m)²
BMI = weight (lb) ÷ height (in)² × 703
The answer is usually rounded to one decimal place. For adults, the WHO and CDC read it the same way: under 18.5 is underweight, 18.5 to 24.9 is a healthy weight, 25 to 29.9 is overweight and 30 or more is obesity. The BMI chart guide has the full tables by height.
How to calculate BMI in kg and metres
- Turn your height into metres: 162 cm is 1.62 m.
- Square it, which means multiplying it by itself: 1.62 × 1.62 = 2.6244.
- Divide your weight in kg by that number: 58 ÷ 2.6244 = 22.1.
If you’d rather keep the height in centimetres, the CDC gives an equivalent form that avoids the decimal point:
BMI = weight (kg) ÷ height (cm) ÷ height (cm) × 10,000
58 ÷ 162 ÷ 162 × 10,000 = 22.1
How to calculate BMI in pounds and inches
- Turn your height into inches: 5 ft 4 in is 5 × 12 + 4 = 64 inches.
- Square it: 64 × 64 = 4,096.
- Divide your weight in pounds by that number: 150 ÷ 4,096 = 0.03662.
- Multiply by 703: 0.03662 × 703 = 25.7.
The 703 converts pounds and inches into the metric units BMI is defined in. A pound is 0.45359237 kg and an inch is 0.0254 m, so 0.45359237 ÷ (0.0254 × 0.0254) = 703.07. Rounding it to 703 changes the answer by less than 0.01 for any adult.
In the UK, weight is often given in stones. One stone is 14 pounds, so 11 st 4 lb is 11 × 14 + 4 = 158 lb.
Converting height and weight before you start
Mixing units is the easiest way to get a wrong BMI. Pick one system and convert everything into it first:
| You have | Convert to | How |
|---|---|---|
| Feet and inches | Inches | feet × 12 + inches: 5 ft 7 in = 67 in |
| Feet and inches | Metres | inches × 0.0254: 67 in = 1.70 m |
| Centimetres | Metres | ÷ 100: 170 cm = 1.70 m |
| Pounds | Kilograms | × 0.4536: 154 lb = 69.9 kg |
| Stones and pounds | Pounds | stones × 14 + pounds: 10 st 8 lb = 148 lb |
Both routes give the same answer. 5 ft 7 in and 154 lb is 154 ÷ 67² × 703 = 24.1, and in metric 69.9 ÷ 1.70² = 24.2; the small gap comes from rounding 67 inches to 1.70 m. Keep more decimals (1.7018 m) and the two agree.
Is the BMI formula different for women?
No. Adult women and men use exactly the same formula and the same categories. The CDC says to use the standard BMI categories for adults 20 and older “regardless of age, sex, or race.” A woman and a man of the same height and weight have the same BMI.
What differs is the body behind the number. Two people with the same BMI can carry very different amounts of muscle and fat, and a muscular person of either sex can score high while carrying little fat. The CDC notes that BMI can’t tell fat from muscle or bone, so it is a starting point rather than a measure of body fat.
Two cases do work differently. Children and teens aged 2 to 19 use the same formula, but the result is compared with growth charts for their age and sex and given as a percentile. And BMI isn’t used to judge weight during pregnancy.
Worked examples
| Height and weight | Working | BMI | Category |
|---|---|---|---|
| 162 cm, 58 kg | 58 ÷ 1.62² | 22.1 | Healthy weight |
| 180 cm, 85 kg | 85 ÷ 1.80² | 26.2 | Overweight |
| 155 cm, 43 kg | 43 ÷ 1.55² | 17.9 | Underweight |
| 5 ft 4 in, 150 lb | 150 ÷ 64² × 703 | 25.7 | Overweight |
| 5 ft 10 in, 165 lb | 165 ÷ 70² × 703 | 23.7 | Healthy weight |
| 5 ft 6 in, 11 st 4 lb | 158 ÷ 66² × 703 | 25.5 | Overweight |
Weight for a target BMI
Run the formula backwards to find the weight that gives a particular BMI at your height:
Weight (kg) = BMI × height (m)²
At 1.62 m, the healthy range of 18.5 to 24.9 is 18.5 × 2.6244 = 48.6 kg up to 24.9 × 2.6244 = 65.3 kg. In US units, divide by 703 instead: at 64 inches, 24.9 × 4,096 ÷ 703 = 145 lb.
The NHS uses lower cut-offs for adults from some ethnic backgrounds, including South Asian, Chinese, Black African and African-Caribbean, because health risks start at a lower BMI. For those groups, 23 counts as overweight and 27.5 as obesity.
Rounding and borderline results
BMI is normally shown to one decimal place, and the categories are written to match: 24.9 is the top of the healthy range and 25.0 the start of overweight. Round only at the end, after dividing, or a result such as 24.96 can land in the wrong band.
A result right at a boundary says very little on its own. Weight can change by a kilogram or so over a day with food and water, and height measured in socks or shoes can be a centimetre or two out. At 1.70 m, one kilogram moves BMI by about 0.35. Treat 24.8 and 25.1 as the same kind of result, and look at the trend over months rather than one reading.
BMI formula in Excel or Google Sheets
With height in cm in A2 and weight in kg in B2:
=ROUND(B2/(A2/100)^2, 1)
With height in inches in A2 and weight in pounds in B2:
=ROUND(703*B2/A2^2, 1)
The brackets matter. Without them, =B2/A2/100^2 divides by 10,000 in the wrong place and returns a number far too small.
Common mistakes
- Using centimetres in the metric formula. 58 ÷ 162² = 0.0022. Divide by 100 first, or use the × 10,000 version.
- Using a remembered height. Adults often lose a little height with age, and a guess can be out by a few centimetres. Measure without shoes against a wall for an accurate figure.
- Forgetting to square the height. 58 ÷ 1.62 = 35.8 looks like a BMI but isn’t one.
- Reading feet and inches as a decimal. 5 ft 4 in is 64 inches, not 5.4 feet.
- Leaving out the 703. In pounds and inches, the result without it is about 0.04.
- Expecting a different answer for men and women. The formula has no sex in it. Calculators that ask for sex use it for other measures, such as body fat or calorie estimates.
Sources
- CDC: Frequently asked questions about BMI. Metric and US formulas, and the same adult categories for all sexes
- CDC: Adult BMI categories. Categories for adults 20 and older
- CDC: About body mass index. Children and teens 2 through 19 use percentiles; BMI doesn’t separate fat, muscle and bone
- World Health Organization: Obesity and overweight fact sheet. Overweight is BMI 25 or more, obesity 30 or more
- NHS: Obesity. Lower thresholds for some ethnic groups
- NIST Special Publication 811: Guide for the Use of the International System of Units. Conversion factors: 1 lb = 0.45359237 kg, 1 in = 0.0254 m