How To Calculate BMI With Metric And Imperial Units
By The Calcumatix Team Reviewed by Calcumatix Editorial Review 5 min read
Quick Answer
Calculate BMI by dividing weight in kilograms by height in metres squared. A person who weighs 72 kg and stands 1.75 m tall has BMI = 72 / (1.75 x 1.75) = 23.51, rounded to 23.5. For pounds and inches, multiply weight by 703 before dividing by height squared.
BMI compares an adult’s weight with height. The math is short, but units can cause errors. Use kilograms with metres, or pounds with inches, and do not mix the two systems.
Use the BMI Calculator for a quick check. This guide shows each step by hand. You will see both forms of the math, and where the result has limits.
How To Calculate BMI With Metric Units
Metric BMI uses kilograms and metres. Divide weight by height squared. The CDC gives the same form. Convert centimetres to metres before you square height.
- Write down weight in kilograms.
- Write down height in centimetres or metres.
- Divide centimetres by 100 when needed.
- Multiply height in metres by itself.
- Divide weight by that result.
- Round the final BMI to one decimal place.
Metric Formula
- BMI = weight in kilograms / height in metres squared
- Weight = W in kilograms
- Height = H in metres
- BMI = W / H squared
The Metric BMI Math In Full
Take an example adult who weighs 72 kg with a height of 175 cm. First change 175 cm to 1.75 m, then apply the metric formula.
Inputs
- Weight: 72 kg
- Height: 175 cm
- Height in metres: 175 / 100 = 1.75 m
Calculation
- Height squared = 1.75 x 1.75
- Height squared = 3.0625 m squared
- BMI = 72 / 3.0625
- BMI = 23.510204…
- Rounded BMI = 23.5
- Rounding rule: nearest tenth
The result is a weight-to-height ratio, not a health test on its own. The CDC calls BMI a screening measure, and a health review may need more facts.
How To Calculate BMI With Imperial Units
Imperial BMI uses pounds and inches. Multiply weight by 703, then divide by height in inches squared. Change feet and inches into total inches first.
Imperial Formula
- BMI = 703 x weight in pounds / height in inches squared
- Weight = W in pounds
- Height = H in total inches
- BMI = 703 x W / H squared
The Imperial BMI Math In Full
Take an example adult who weighs 180 lb with a height of 5 ft 10 in. Change that height to 70 in, then use the pound-and-inch form.
Inputs
- Weight: 180 lb
- Height: 5 ft 10 in
- Total inches = (5 x 12) + 10
- Total inches = 70 in
Calculation
- Height squared = 70 x 70
- Height squared = 4,900 inches squared
- Weighted value = 703 x 180
- Weighted value = 126,540
- BMI = 126,540 / 4,900
- BMI = 25.824489…
- Rounded BMI = 25.8
- Rounding rule: nearest tenth
Do not type 5.10 as the height. That value is not 5 ft 10 in. Feet and inches are two separate units, so change the full height to inches first.
Does BMI Use Different Rules For Men And Women
Adult BMI uses the same math for men and women. Sex is not part of the adult formula, and age is not part of the math either. Those facts may still matter when a health expert reads the result.
The rule is not the same for children and teens. Their result must be read by age and sex. The CDC uses a child and teen growth chart for that purpose, and the adult chart should not be used in its place.
Pregnancy also needs a different type of review, since body weight changes as part of pregnancy. The standard adult chart does not tell the full story, so ask a health expert for personal care.
The How To Calculate BMI For Athletes guide shows another limit: an adult with significant muscle mass can have a high BMI. The math can be correct while the label needs more context; the formula itself does not change.
Common Errors That Change A BMI Calculation
Most BMI errors come from units. The math is not hard, but a tool can still give a clean result from bad input. Check the unit on each field before you start.
| Error | What Goes Wrong | Fix |
|---|---|---|
| Using centimetres as metres | Height is far too large | Divide centimetres by 100 |
| Not squaring height | The ratio is wrong | Multiply height by itself |
| Typing feet as a decimal | 5.10 does not mean 5 ft 10 in | Use total inches |
| Mixing kg and inches | The two systems clash | Use one full unit set |
| Rounding height too soon | The answer can shift | Round at the end |
| Leaving out 703 | Pound-and-inch BMI is wrong | Add 703 to the formula |
The way you measure can also change the input. Shoes add height, heavy clothes add weight, and a rough guess can move the answer. Measure again when a value looks unusual.
When The Adult BMI Formula Should Not Be Used
Do not use the adult chart for a child or teen, and do not use the chart as a diagnostic test for a health problem. Pregnancy also needs care from a health expert, since one adult ratio cannot cover all of those cases.
BMI does not distinguish fat, muscle, and bone. The CDC says all three add to body weight, and BMI also does not show where fat sits on the body. These limits matter for individual interpretation.
See Is A BMI Calculator Accurate for more on those limits. The Health Calculators hub links BMI, BMR, and TDEE tools. Each tool answers a different type of question, and none can replace personal care.
Sources And Notes For The BMI Formula
This guide provides general health education and does not replace personal care from a doctor, dietitian, or other qualified health professional.
Frequently asked questions
Is BMI calculated differently for men and women?
Adult BMI uses the same weight and height math for men and women. Sex may matter in a wider health review, but sex does not change the adult BMI formula.
How do you calculate BMI from kilograms and metres?
Divide weight in kilograms by height in metres squared. For 80 kg and 1.80 m, BMI = 80 / (1.80 x 1.80) = 24.69, which rounds to 24.7.
How do you calculate BMI from pounds and inches?
Multiply pounds by 703, then divide that value by total inches squared. Keep all digits until the last step, then round the BMI.
Why does my BMI calculator show a different number?
The tools may use a different unit or round at a different step. Check weight, total height, and the full formula, since a small input change can shift the last digit.
Is BMI a direct measure of body fat?
BMI is not a direct body-fat test. The CDC says BMI does not distinguish fat, muscle, and bone, so another test may add more detail.
Can a BMI result diagnose a health problem?
BMI alone cannot diagnose a health problem. The ratio can support a screening process, and a health expert may also use an exam, health history, blood tests, and other facts.