How to Calculate BMI (Body Mass Index) by Hand
Short answer
BMI = weight (kg) ÷ height (m)². In US units: 703 × weight (lb) ÷ height (in)². Example: 70 kg and 1.75 m → 70 ÷ 3.0625 ≈ 22.9 (healthy range).
Body mass index (BMI) is a quick height-and-weight screen used in clinics and fitness apps. It is not a diagnosis, but knowing how to calculate it by hand helps you understand any chart or calculator result. Use our free BMI calculator to check your arithmetic.
Metric formula
Convert height to metres (175 cm = 1.75 m). Square the height, then divide weight in kilograms by that number.
BMI = kg ÷ m²
Imperial formula (US/UK stones & pounds)
If you use pounds and inches:
BMI = 703 × lb ÷ in²
Example: 154 lb and 69 in → 703 × 154 ÷ 4761 ≈ 22.7.
Worked example
Weight 70 kg, height 175 cm (1.75 m). Height squared = 1.75 × 1.75 = 3.0625. BMI = 70 ÷ 3.0625 ≈ 22.9.
What the ranges usually mean
| BMI | Category (WHO adult) |
|---|---|
| Under 18.5 | Underweight |
| 18.5–24.9 | Healthy weight |
| 25.0–29.9 | Overweight |
| 30.0+ | Obesity |
Cut-offs can differ slightly by country and for children. Always follow your clinician’s guidance.
Limits of BMI
BMI does not separate fat from muscle. Athletes can score “overweight” while carrying little fat. Waist size and body-fat estimates add context — see how to measure body fat at home and the body fat calculator. For calorie targets, the calorie calculator (TDEE) is more useful than BMI alone.
Common mistakes
- Using centimetres in the metric formula without converting to metres.
- Forgetting the 703 factor with pounds and inches.
- Treating BMI as a personal health diagnosis.
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When BMI is useful — and when it is not
BMI is a population screening tool. It helps public-health agencies track trends and gives clinicians a quick first look. It does not measure visceral fat, fitness, bone density or metabolic health. Two people with the same BMI can have very different body compositions.
If your BMI lands near a boundary (for example 24.8 vs 25.1), do not overreact to a single decimal. Weigh yourself under similar conditions, recheck height, and look at waist circumference and how clothes fit. Athletes, older adults and people under 18 need different interpretation.
Worked imperial example in detail
Height 5 ft 10 in = 70 inches. Weight 180 lb. BMI = 703 × 180 ÷ 70² = 126,540 ÷ 4,900 ≈ 25.8. That sits just into the overweight band on adult WHO charts. A strength athlete at the same numbers may still carry a healthy fat percentage — which is why the body fat calculator and professional assessment matter more than a label.
Linking BMI to calories
BMI alone does not tell you how many calories to eat. After you know your size category, estimate maintenance calories with TDEE, then adjust slowly. See how to calculate TDEE and how many calories to lose weight.
Practical checklist
- Measure height without shoes.
- Use a reliable scale on a hard floor.
- Record units before you calculate.
- Repeat weekly averages, not single panic weigh-ins.
- Talk to a clinician before major diet changes.
Frequently asked questions
How do I calculate BMI manually?
Divide weight in kilograms by height in metres squared. Or use 703 × pounds ÷ inches squared. Double-check units and confirm with the linked tool on this page before you rely on the figure for money or school decisions.
What is a healthy BMI?
For most adults, 18.5 to 24.9 is labelled healthy weight on WHO charts. Individual targets can differ. Double-check units and confirm with the linked tool on this page before you rely on the figure for money or school decisions.
Is BMI accurate for athletes?
Often not. Higher muscle mass raises BMI without meaning excess fat. Double-check units and confirm with the linked tool on this page before you rely on the figure for money or school decisions.
Does BMI work for children?
Children use age- and sex-specific percentiles, not the adult cut-offs. Double-check units and confirm with the linked tool on this page before you rely on the figure for money or school decisions.