BMI Calculator
Body Mass Index compares your weight against your height to give a single number that places you in a broad weight category. It is the standard first-pass screening measure used by the NHS and by health services worldwide, and it takes two measurements to work out.
Formula
BMI = weight (kg) ÷ height (m)²
- Imperial: BMI = 703 × weight (lb) ÷ height (in)²
70 kg at 1.75 m: 70 ÷ 1.75² = 70 ÷ 3.0625 = 22.9
The BMI Formula
BMI is your weight in kilograms divided by the square of your height in metres:
BMI = weight (kg) ÷ height (m)²
In imperial units the same relationship needs a conversion factor:
BMI = (weight (lb) ÷ height (in)²) × 703
Example: someone who is 1.78 m tall and weighs 82 kg has a BMI of 82 ÷ (1.78 × 1.78) = 25.9.
What the Categories Mean
For most adults, the standard ranges are:
- Below 18.5 — underweight
- 18.5 to 24.9 — healthy weight
- 25.0 to 29.9 — overweight
- 30.0 and above — obese
The boundaries are population thresholds, not cliff edges. A BMI of 25.1 is not meaningfully different from 24.9.
Where BMI Falls Down
BMI measures weight, not what that weight is made of, which is why it misreads several groups:
- Muscular people are routinely classed as overweight, because muscle is denser than fat.
- Older adults may lose muscle while gaining fat, and keep a healthy-looking BMI throughout.
- It ignores fat distribution. Waist circumference is a better predictor of cardiovascular risk than BMI alone.
Ethnicity matters too: the NHS advises lower thresholds for people of South Asian, Chinese, Black African and African-Caribbean backgrounds, where health risks begin at a lower BMI.
Quick Reference
| BMI | Category |
|---|---|
| Under 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obese class I |
| 35.0 – 39.9 | Obese class II |
| 40.0 and over | Obese class III |
Frequently Asked Questions
- Is BMI accurate?
- It is accurate as a population screening tool and reasonable for most sedentary-to-moderately-active adults. It is unreliable for athletes, bodybuilders, pregnant women, and anyone with a high muscle mass, because it cannot distinguish muscle from fat.
- What is a healthy BMI?
- For most adults, 18.5 to 24.9. Lower thresholds apply for some ethnic groups, and BMI is not used the same way for children, who are assessed against age and sex percentile charts instead.
- Does BMI apply to children?
- Not directly. Children and teenagers are assessed using BMI centile charts that account for age and sex, because body composition changes rapidly during growth.
- Should I use BMI or body fat percentage?
- Body fat percentage tells you more, because it measures composition rather than total mass. BMI is popular because it needs only a set of scales and a tape measure.