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BMI Calculation Formula for Metric and Imperial Units
BMI calculation formula for metric and imperial units: learn both equations, where the 703 comes from, and how to convert units correctly.
What is the BMI calculation formula in metric and imperial units?
In metric units, the BMI calculation formula is weight in kilograms divided by height in meters squared. In imperial units, it is 703 times weight in pounds divided by height in inches squared. Both versions measure the same thing — your weight relative to your height squared — and give identical results when the conversions are done correctly.
That's the headline version. But the formula is only half the story: unit mix-ups are the number one reason people get wrong BMI results. Let's walk through both versions properly, then nail the conversions.
The metric BMI calculation formula
The metric BMI calculation formula is delightfully simple: BMI equals weight in kilograms divided by height in meters squared. Take 80 kg at 1.80 m: 80 divided by 3.24 equals 24.7. Square the height first, then divide. The units cancel out cleanly, which is why the metric version is the one scientists reach for by default.
If your height is in centimeters, convert first — 180 cm is 1.80 m. Plugging centimeters straight into the formula is the classic mistake, and it produces absurd numbers like 0.25.
The imperial BMI calculation formula
The imperial BMI calculation formula is BMI equals 703 times weight in pounds divided by height in inches squared. Using our example: 176 lb at 70.9 inches — 703 times 176 divided by 70.9 squared — gives 24.7, exactly matching the metric answer. The 703 isn't magic; it's just unit conversion baked in.
Most imperial height readings come as feet and inches, which must become total inches first: 5 feet is 60 inches plus the remainder. A 5'10" person is 70 inches tall.
Where the 703 comes from
One kilogram equals 2.20462 pounds, and one meter equals 39.3701 inches. Converting kg/m² into lb/in² introduces the 703 factor, so you never have to think about it. But knowing it exists stops you from "correcting" the formula or dropping the multiplier — the most common imperial error after the height conversion.
Honestly, you never need to derive 703 yourself. Just remember it exists because metric and imperial disagree, and the formula bridges the gap.
Converting height and weight correctly
For height: inches equals feet times 12 plus inches. For weight: multiply kilograms by 2.20462 to get pounds, or divide pounds by 2.20462 to get kilograms. Never combine pounds with meters or kilograms with inches — the result is meaningless. Double-check which units you're holding before you start.
My rule of thumb: decide on a system before you measure anything, and convert everything to that system first. Mixed units are where calculations go to die.
Worked comparison: both systems side by side
Take 80 kg at 1.80 m. Metric: 80 divided by 3.24 equals 24.7. In imperial, that's 176 lb at 70.9 inches: 703 times 176 divided by 70.9 squared equals 24.7. Same person, same result. Run this comparison on your own numbers once and you'll trust both versions forever.
This is also the best way to check your work: calculate in one system, convert the inputs, and calculate in the other. Matching answers mean you did it right.
Quick-reference cheat sheet
Metric: BMI = kg / m². Imperial: BMI = 703 × lb / in². Height conversion: inches = feet × 12 + inches. Weight conversion: lb = kg × 2.20462. Screening categories (underweight, healthy, overweight, obese) apply identically to both — the formula changes, the interpretation doesn't.
Stick this cheat sheet on your fridge if you're tracking monthly. Future you will be grateful.
Teaching the BMI calculation formula to others
The cleanest explanation starts with the idea: weight relative to height squared. Show the metric version first — it's simpler — then introduce the imperial formula as the same idea with the 703 conversion built in. Walk through the worked comparison together. Most people "get it" the moment they see both systems give 24.7.
From formula to habit: knowing both versions removes every excuse for not tracking. Measure monthly, use one unit system consistently, and watch the trend.
Related BMITool tools and guides
Put the formula into practice with the BMITool BMI calculator, and read more helpful articles on the bmi calculation formula, BMI categories, and healthy weight management.
Key takeaway for BMI Calculation Formula for Metric and Imperial Units
The BMI calculation formula is kg/m² in metric and 703 × lb/in² in imperial — the 703 is just unit conversion. Convert feet-and-inches to total inches first, never mix unit systems, and verify by calculating both ways.
Frequently Asked Questions
What is the BMI calculation formula in metric units?
BMI equals weight in kilograms divided by height in meters squared (kg/m²). Example: 80 kg at 1.80 m gives 80 ÷ 3.24 = 24.7. If your height is in centimeters, divide by 100 first.
What is the BMI calculation formula in imperial units?
BMI equals 703 times weight in pounds divided by height in inches squared (703 × lb/in²). Example: 176 lb at 70.9 inches gives 24.7, matching the metric result. Convert feet-and-inches to total inches before calculating.
Where does the 703 in the BMI formula come from?
It's a unit conversion factor. One kilogram equals 2.20462 pounds and one meter equals 39.3701 inches, so converting kg/m² into lb/in² produces the 703 multiplier. It makes the imperial formula equivalent to the metric one.
Can I mix pounds with meters in the BMI formula?
No — never combine pounds with meters or kilograms with inches; the result is meaningless. Pick one unit system, convert all your measurements to it, then apply the matching formula version.
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