How to Calculate BMI (Metric, Imperial and Stone) and What It Means
By the SnapCalculations team · Updated 2 Nov 2026
The short answer
BMI = weight in kilograms ÷ (height in metres)². In pounds and inches, it's weight × 703 ÷ height². For most adults, 18.5 to 24.9 is the healthy range, 25 to 29.9 is overweight and 30 or more is obese.
What BMI is
Body Mass Index (BMI) is a quick way to compare your weight with your height. It doesn't measure body fat directly, but it's widely used by doctors and health services as a first check, because it needs nothing more than a tape measure and a set of scales.
BMI = weight (kg) ÷ height (m)²
Square the height first: multiply it by itself
How to calculate it in kilograms and metres
- 1Convert your height to metres. 175 cm is 1.75 m.
- 2Square it: 1.75 × 1.75 = 3.0625.
- 3Divide your weight by that number. For 70 kg: 70 ÷ 3.0625 = 22.86.
- 4Round to one decimal place: a BMI of 22.9.
Example: 175 cm and 70 kg
What is the BMI of someone 175 cm tall weighing 70 kg?
- Height in metres = 1.75, and 1.75² = 3.0625.
- BMI = 70 ÷ 3.0625 = 22.86.
Answer: BMI 22.9: in the healthy range
Try it in the calculatorIn pounds and inches (US style)
BMI = weight (lb) × 703 ÷ height (in)²
703 converts the imperial units so the answer matches the metric formula
Example: 5 ft 9 in and 154 lb
What is the BMI of someone 5 ft 9 in tall who weighs 154 lb?
- Height in inches = 5 × 12 + 9 = 69, and 69² = 4,761.
- BMI = 154 × 703 ÷ 4,761 = 108,262 ÷ 4,761 = 22.74.
Answer: BMI 22.7: in the healthy range
Try it in the calculatorIn stone and pounds (UK style)
First turn your weight into pounds: multiply the stones by 14 and add the extra pounds. Then use the pounds-and-inches formula above, or convert to kilograms (1 lb = 0.4536 kg) and use the metric one.
Example: 165 cm and 11 st 4 lb
What is the BMI for a height of 165 cm and a weight of 11 stone 4 pounds?
- Weight = 11 × 14 + 4 = 158 lb = 71.67 kg.
- Height = 1.65 m, and 1.65² = 2.7225.
- BMI = 71.67 ÷ 2.7225 ≈ 26.3.
Answer: BMI 26.3: in the overweight range
Try it in the calculatorWhat your number means
For adults, the World Health Organization uses these ranges:
- Below 18.5: underweight
- 18.5 to 24.9: healthy weight
- 25 to 29.9: overweight
- 30 or more: obese (with higher classes at 35 and 40)
In the UK, NICE guidance uses lower thresholds for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background: overweight from a BMI of 23, and obesity from 27.5. These groups tend to face higher health risks at a lower BMI.
Find your healthy weight range
You can turn the healthy range around to find the weights that fit your height: multiply your height in metres, squared, by 18.5 and by 24.9.
Example: a healthy range for 1.75 m
What is the healthy weight range for someone 1.75 m tall?
- Height squared = 3.0625.
- Lowest healthy weight = 18.5 × 3.0625 = 56.7 kg.
- Highest healthy weight = 24.9 × 3.0625 = 76.3 kg.
Answer: About 56.7 kg to 76.3 kg
Try it in the calculatorWhat BMI can't tell you
- Muscle versus fat. Muscle is heavy, so very muscular people can have a high BMI while carrying little fat.
- Where fat sits. Fat around the waist carries more health risk than fat elsewhere. Waist size, or waist-to-height ratio, adds useful information.
- Children and teenagers. Their BMI has to be compared with growth charts for their age and sex, so adult ranges don't apply.
- Pregnancy, and older age. Body composition changes, so BMI is less reliable.
Treat BMI as a starting point, not a diagnosis. If your result worries you, a GP or doctor can look at the whole picture.
Skip the arithmetic
Our BMI calculator accepts feet and inches or centimetres, and kilograms, pounds or stone, in any mix. It shows your BMI, your category and the healthy weight range for your height, with the working.
Frequently asked questions
Is BMI different for men and women?
The formula and the adult ranges are the same for men and women. Women naturally carry a little more body fat at the same BMI, which is one reason BMI is only a rough guide.
Why do some calculators give slightly different answers?
Usually rounding. The imperial formula uses 703, a rounded conversion factor, so it can differ from the metric result in the second decimal place. The category is almost always the same.
What is a healthy BMI for South Asian adults?
UK NICE guidance treats a BMI of 23 or more as overweight, and 27.5 or more as obese, for people of South Asian background, instead of 25 and 30.
Can I use BMI for my child?
Not with the adult ranges. Children's BMI is compared against age- and sex-specific percentile charts, so use a children's BMI tool or ask a doctor.