How BMI is calculated
Body Mass Index (BMI) is a simple screening measure that compares your weight to your height. It uses the same formula the World Health Organization and the NHS use:
So a person 1.75 m tall weighing 75 kg has a BMI of 75 ÷ (1.75 × 1.75) = 24.5. If you use pounds and inches, the equivalent is 703 × weight(lb) ÷ height(in)² — this calculator handles the conversion for you.
What your BMI means
The WHO groups BMI for adults into four bands:
- Under 18.5 — Underweight. May indicate you're below a healthy weight; worth discussing with a clinician.
- 18.5 to 24.9 — Healthy weight. Associated with the lowest risk for most people.
- 25 to 29.9 — Overweight. A good moment to review activity and diet habits.
- 30 and above — Obese. Linked to higher risk of conditions like type 2 diabetes and heart disease.
Important limitations
BMI is a useful population-level screen, but it doesn't measure body fat directly. It can over-estimate risk in very muscular people and under-estimate it in others. Age, pregnancy and ethnicity also affect interpretation — some guidance uses a lower overweight threshold (23) for people of South Asian, Chinese and other Asian backgrounds. Pair BMI with your waist-to-height ratio and body fat percentage for a fuller picture.
Tips & next steps
- Track BMI alongside waist measurement — a strong independent risk signal.
- Small, consistent changes beat crash diets. Try the weight-loss planner for a realistic timeline.
- Know your daily energy needs with the calorie / TDEE calculator.
- Re-measure every few weeks, not daily — weight naturally fluctuates.
What is a healthy BMI?
For most adults, a BMI between 18.5 and 24.9 is considered healthy by the WHO and NHS — the range associated with the lowest health risk for the general population.
Is BMI accurate?
BMI is a quick, accurate screen for populations, but it doesn't distinguish muscle from fat. Athletes may score "overweight" despite low body fat. Use it alongside waist measurement and body-fat estimates.
How do I calculate BMI from pounds and feet?
Multiply your weight in pounds by 703, then divide by your height in inches squared. This calculator does it automatically — just switch to the "Imperial" tab.
Is BMI different for men and women?
The formula and adult categories are the same for men and women. Body composition differs between sexes, which is one reason BMI is a screen rather than a diagnosis.
Does BMI work for children?
No — children and teens use age- and sex-specific BMI percentile charts, not the adult categories above. This calculator is intended for adults 18+.
Is this BMI calculator private?
Yes. It runs entirely in your browser. Your height and weight are never uploaded, stored, or shared — the whole idea behind Prescriba.
- World Health Organization — Body Mass Index
- NHS — BMI healthy weight calculator
- CDC — About Adult BMI
Evidence-based · last reviewed July 2026