How to Calculate BMI by Hand

By Longevity Lab · Published: August 23, 2026

Based on peer-reviewed research — full source list at the end of this article. This is educational information, not medical advice.

To calculate BMI by hand, divide your weight in kilograms by your height in meters squared: a person of 70 kg and 1.75 m has a BMI of 70 ÷ (1.75 × 1.75) = 22.9. In imperial units, divide your weight in pounds by your height in inches squared and multiply by 703. Both routes produce the same index, because 703 is nothing more than the conversion factor between the two unit systems. The arithmetic takes fifteen seconds; the part worth attention is measuring the two inputs so the answer is stable from one month to the next.

The metric calculation, step by step

Take height in meters — 1.75, not 175. Multiply it by itself: 1.75 × 1.75 = 3.0625. Divide your weight in kilograms by that: 70 ÷ 3.0625 = 22.86, which rounds to 22.9.

The order matters only in that squaring the height first saves you from squaring the whole fraction by accident. If your calculator has an x² key, height goes in first, then the division.

If you know your height in centimeters, divide by 100 before squaring. A common slip is squaring 175 instead of 1.75, which produces a BMI of 0.0023 and a moment of confusion.

The imperial calculation, and where 703 comes from

BMI = weight (lb) ÷ height (in)² × 703. A person of 154 lb and 5 ft 9 in (69 in) works out to 154 ÷ 4,761 = 0.03235, × 703 = 22.7.

The 703 exists because one pound is 0.4536 kg and one inch is 0.0254 m. Divide 0.4536 by 0.0254² and you get 703.07. It carries no medical meaning at all — it converts pounds-per-square-inch into kilograms-per-square-meter and stops there.

The small gap between 22.9 and 22.7 in the two worked examples above is rounding: 5 ft 9 in is 68.9 in, not exactly 69. Rounding height to the nearest whole inch is worth about 0.3 BMI units.

Measuring so the number means something

Weight moves by one to two kilograms across a normal day, mostly water, food in transit and glycogen. Weigh in the morning, after the toilet, before eating or drinking, in light clothing, on the same scale, on a hard floor. A scale on carpet reads differently every time you move it.

Height is the input people get wrong, because they use a number they were told years ago. Stand without shoes, heels and back against a wall, feet together, looking straight ahead so the chin is level. Adults lose roughly one centimeter per decade after 40, and more after 70. A centimeter of phantom height on a 70 kg adult understates BMI by about 0.26 units, and the error compounds in exactly the age group where the index is already least reliable.

Then round nothing until the end. Rounding weight to the nearest 5 kg before dividing is how a 24.6 becomes a 25.4 and a screening result changes category for no reason.

A worked example in both unit systems

Both land in the overweight band, one unit below the obesity threshold — the kind of result where the next measurement matters far more than the last decimal place. Our own BMI calculator runs both unit systems and adds the healthy weight range for your height, and the Body Mass Index (BMI) tool goes further, reporting a Smart BMI score and a mortality-risk ratio alongside the plain index.

Where the arithmetic stops helping

The formula is exact and the conclusion is not. BMI has one input for mass and no input for what the mass is, which is why a well-muscled adult and a sedentary one can share a result while sharing nothing else. The index also says nothing about fat distribution, and abdominal fat is the part that drives cardiometabolic risk.

So finish the calculation and then take one more measurement. Waist circumference divided by height, above 0.5, flags central adiposity at any index value, and it does so earlier than BMI does. Working out a target weight rather than a category is a separate calculation — see Ideal body weight for the competing formulas — while the Waist-to-height ratio (WtHR) tool scores the waist ratio against proper age-adjusted thresholds. If muscle is the reason your result looks wrong, BMI — body mass index as a concept simply cannot settle it, and a body-composition measurement can.

Frequently asked questions

Sources

Disclaimer. This article is for information only and does not replace medical advice. Talk to a qualified clinician before changing anything about your health.