BMI calculation guide
BMI Formula: Metric and Imperial Examples
Body mass index turns your weight and height into a single number by dividing weight by height squared. The exact calculation depends on which units you use: kilograms and metres, or pounds, feet and inches. This guide walks through both versions step by step, with worked examples, so you can see exactly how the number is produced and what it does — and does not — tell you.
What this calculation measures
Body mass index is a screening ratio: it relates how much you weigh to how tall you are, and nothing else. It does not measure body fat directly, and it cannot see muscle, bone density, or where weight is carried on the body. What it gives you is a quick, reproducible number that public-health guidelines use to sort adults into broad reference ranges — a starting point for a conversation, not an endpoint.
The metric calculation
In metric units, weight is entered in kilograms and height in metres. Because height is squared, small differences in height have a larger effect on the result than the same difference in weight.
BMI = weight in kilograms ÷ height in metres²
Worked examples in metric units
Example — height already in metres
An adult weighing 68 kg who is 1.70 m tall: 1.70 × 1.70 = 2.89. Then 68 ÷ 2.89 = 23.5. This result falls in the healthy weight range.
Example — height given in centimetres
Height is often recorded in centimetres, so it needs converting first: divide by 100. A person who is 165 cm tall has a height of 1.65 m. At 60 kg, that gives 1.65 × 1.65 = 2.7225, then 60 ÷ 2.7225 ≈ 22.0 — also in the healthy range.
The imperial calculation
When weight is recorded in pounds and height in feet and inches, the same underlying relationship is expressed differently. Height still needs to be converted to a single unit — total inches — before it can be squared.
BMI = (weight in pounds ÷ height in inches²) × 703
Worked example in imperial units
Example — feet, inches and pounds
An adult who is 5 ft 9 in tall weighs 160 lb. First, convert height: (5 × 12) + 9 = 69 inches. Then 69 × 69 = 4,761. Next, 160 ÷ 4,761 ≈ 0.0336. Finally, 0.0336 × 703 ≈ 23.6 — again in the healthy weight range.
Why the constant 703 appears
The number 703 converts the units so that pounds and inches produce the same scale as kilograms and metres. It is a unit-conversion factor, not a rounding shortcut, so it should always be applied in full.
Common calculation mistakes
- Entering height in centimetres without converting to metres first — this produces a result that is off by a factor of 10,000.
- Forgetting to square the height, or squaring the weight instead.
- Mixing units, such as dividing pounds by height in metres.
- Leaving out the 703 constant when working in feet, inches and pounds.
- Rounding height or weight too early, before the full calculation is complete.
Why rounding can cause small differences
Two calculators can show slightly different results from the same measurements if they round at different points — for example, rounding a converted height to two decimal places before squaring it, rather than carrying the full precision through the calculation. These differences are usually a few tenths of a point and do not change which category a result falls into, but they explain why the same person can see 23.4 on one tool and 23.5 on another.
How the number connects to your category
Once you have a result, it is read against the same adult reference ranges regardless of which units you used to calculate it. A result of 23.5 means the same thing whether it came from kilograms and metres or from pounds and inches. For the full breakdown of each range and what it represents, see the BMI categories guide, and for help interpreting a specific result, see what your BMI result means.
This content is for adults 18 years and older and is intended for informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment.
Limitations of this calculation
- It cannot distinguish muscle from fat, so a muscular adult can calculate a high result while carrying little body fat.
- It does not account for body composition, bone density, or where fat is distributed on the body.
- It is not designed for use during pregnancy, when weight changes reflect factors this calculation does not consider.
- It does not apply to people under 18 — children and teenagers are assessed with age- and sex-specific growth charts instead.
- A single result is a screening reference, not a medical diagnosis, and should be considered alongside other health information.
Common questions about the calculation
Should height be entered in metres or centimetres?
Either works, as long as you convert consistently. If your height is in centimetres, divide by 100 to get metres before squaring it — squaring a centimetre value directly will give a meaningless result.
Why does the imperial formula use 703?
It is a conversion constant that adjusts for using pounds and inches instead of kilograms and metres, so the result lands on the same scale either way.
Why is my result slightly different from another calculator?
Small gaps usually come from rounding at different steps, such as rounding a converted height before squaring it. These differences are typically a few tenths of a point.
Does calculating BMI provide a diagnosis?
No. It produces a screening number that places your result within adult reference ranges. It is not a diagnosis and does not replace an assessment by a qualified healthcare professional.
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How this content is maintained
T9 BMI Check is an educational resource for adults. BMI is a screening tool, not a medical diagnosis, and the information here is general rather than personalized medical advice. We research our content against trusted public-health sources, work to keep it factually accurate, and review it periodically. T9 BMI Check is an independent informational website and is not affiliated with or endorsed by the organizations cited. For guidance about your own health, please consult a qualified healthcare professional.
References
- World Health Organization — Obesity and overweight
- CDC — Adult BMI Calculator
- NHLBI — Calculate Your BMI
- MedlinePlus — Body mass index (BMI)
Prepared and maintained by T9 BMI Check
Last reviewed against the cited sources: July 17, 2026