How to Calculate BMI Manually: Formula, Examples & Common Mistakes
BMI is a single division problem. The trouble is that almost everyone gets the units wrong the first time — so here is the whole calculation laid out both ways, with every step of the arithmetic shown.
The Formula, Both Ways
BMI is defined as body weight in kilograms divided by height in metres squared. Written out, that definition is the metric formula:
If you work in pounds and inches, the same definition needs a conversion constant, because pounds and inches are not metric units:
The 703 is not a fudge factor. It is the number that converts pounds per square inch into kilograms per square metre, so both formulas land on the same value. If you ever wonder whether you are using the right one, remember: the same person in both unit systems must produce the same BMI.
Metric, Step by Step
Take someone 170 cm tall who weighs 65 kg.
- Convert height to metres. 170 ÷ 100 = 1.70 m.
- Square it. 1.70 × 1.70 = 2.89 m².
- Divide weight by that. 65 ÷ 2.89 = 22.49.
- Round once, at the end. BMI = 22.5 — normal range.
Three more so you can check your own arithmetic against a known answer:
| Height | Weight | Height² | BMI | Category |
|---|---|---|---|---|
| 160 cm | 52 kg | 2.56 | 20.3 | Normal |
| 170 cm | 65 kg | 2.89 | 22.5 | Normal |
| 175 cm | 80 kg | 3.06 | 26.1 | Overweight |
| 180 cm | 95 kg | 3.24 | 29.3 | Overweight |
Imperial, Step by Step
The same person in imperial units: 5 ft 9 in and 175 lb.
- Convert height to inches. 5 × 12 = 60, then add the extra 9 = 69 inches.
- Square it. 69 × 69 = 4,761 in².
- Multiply weight by 703. 175 × 703 = 123,025.
- Divide. 123,025 ÷ 4,761 = 25.84.
- Round. BMI = 25.8 — just into the overweight band.
Five Mistakes That Break the Calculation
1. Leaving height in centimetres
65 ÷ 170² = 0.0022. A quick sanity check catches almost every error: a real BMI sits between about 10 and 60. Two digits before the decimal point is normal; anything else means the units are wrong.
2. Squaring the weight instead of the height
The square belongs to height, and only to height. 65² ÷ 1.70 = 2,485, which is not a BMI and never was.
3. Treating feet as a decimal
Height in feet and inches is a base-12 measurement. 5 ft 11 in equals 71 inches; typing 5.11 gives 61.3 inches if you multiply by 12, or 5.11 if you do not. Multiply the feet by 12 first, then add the inches.
4. Mixing unit systems
Kilograms with inches, or pounds with metres, produces a number that looks plausible enough to believe. Convert both measurements into the same system before doing any arithmetic.
5. Rounding in the middle
Round once, at the end. Shortening 2.89 to 2.9 mid-way turns 22.49 into 22.41 — small, but enough to move you across a category boundary in edge cases.
Reading the Number
For adults, the World Health Organization uses these cut-off points:
| Category | BMI |
|---|---|
| Underweight | Below 18.5 |
| Normal weight | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Obese, class I | 30.0 – 34.9 |
| Obese, class II | 35.0 – 39.9 |
| Obese, class III | 40.0 and above |
Some health authorities use lower thresholds for people of South and East Asian descent — 23.0 for overweight and 27.5 for obesity — because the same BMI carries a higher risk of type 2 diabetes at a lower weight. Our BMI calculator lets you switch between the WHO and the lower Asian cut-off standards.
Manual Calculation vs a Calculator
Doing it by hand is worth doing once: the formula stops being a mystery, and you will never again be unsure what the 703 does. For ongoing tracking, though, use a calculator:
- It returns the category and the healthy weight range for your height at the same time, which a bare number does not give you.
- It removes arithmetic slips, and slips are far more common than people expect — especially the feet-to-inches conversion.
- It keeps your history consistent, so week-to-week changes are real movement rather than rounding noise.
What BMI Cannot Tell You
BMI is a screening tool, not a diagnosis. Two people with the same number can have very different health profiles.
- Muscle mass. A consistent lifter can carry a BMI of 27 with 12% body fat. The formula cannot distinguish muscle from fat.
- Where the fat sits. Visceral fat around the organs is the metabolically risky kind, and BMI is blind to it. Waist circumference adds that information — see BMI for women for the 88 cm / 35 in threshold and waist-to-hip ratios.
- Age. Older adults often carry more fat at the same BMI, yet a slightly higher BMI in later life is associated with better outcomes, not worse.
- Pregnancy. BMI is not valid during pregnancy; clinicians use gestational weight-gain ranges instead.
- Children and teenagers. Paediatric BMI is read against age-and-sex percentile charts, never against the adult table above.
From a Number to a Plan
Once you know where you stand, the useful question is what to do about it. If your BMI is above 25 and you want to bring it down, the next step is understanding how much weight one BMI point actually represents at your height — it is not the same number of kilos for everyone. That is exactly what How to Lower Your BMI Safely covers.
Frequently Asked Questions
What is the correct formula for BMI?
In metric units, BMI equals weight in kilograms divided by height in metres squared (kg/m²). In imperial units the equivalent is 703 × weight in pounds ÷ height in inches squared. The 703 is a conversion constant that reconciles the two unit systems, not an arbitrary adjustment.
Is BMI calculated differently for men and women?
No. The formula and the WHO cut-off points are identical for adult men and women. What differs is interpretation: at the same BMI, women typically carry a higher body-fat percentage, and fat distribution differs, so one number can mean different things for metabolic risk.
Why does my manual BMI differ from the calculator's result?
Usually one of three things: height was rounded before squaring, the result was rounded at an intermediate step instead of only at the end, or centimetres were never converted to metres. A gap of 0.1 to 0.2 points is normal rounding; anything larger points to a unit error.