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BMI Formula Explained: How to Calculate Body Mass Index
Learn the BMI formula explained step by step: measure accurately, calculate in metric or imperial, avoid common errors, and interpret your result.
How does the BMI formula work?
The BMI formula is weight divided by height squared: BMI = weight (kg) / height (m)² in metric, or BMI = 703 × weight (lb) / height (in)² in imperial. First measure your height accurately, then weigh yourself, then plug the numbers into the BMI formula. The result maps to standard categories: below 18.5 is underweight, 18.5–24.9 is healthy, 25–29.9 is overweight, and 30+ is obese.
That's the whole journey — measure, weigh, calculate, interpret. Let's do each one well.
Step 1: measure your height accurately
Bad measurements are the biggest source of BMI error, not bad math. Measure height barefoot, standing tall against a wall or door frame, heels together and head level. A few centimeters off can swing the BMI formula's result by a full point — more error than any calculator would.
Garbage in, garbage out. Measure like it matters, because it does.
Step 2: weigh yourself correctly
Weigh yourself in the morning after using the bathroom, before eating, wearing minimal clothing. Place the scale on a hard, flat surface — carpet can quietly shave kilos off the reading. Do it the same way each time; the BMI formula is most useful when the inputs are consistent.
Step 3: apply the BMI formula
With accurate numbers in hand, the BMI formula is one division. Metric: BMI = weight (kg) ÷ [height (m)]². Imperial: BMI = 703 × weight (lb) ÷ [height (in)]². Both versions of the BMI formula are the same equation wearing different units; the 703 is a conversion constant, not magic.
One division. The prep was the hard part.
Worked example: metric
Height 1.68 m, weight 70 kg. Square the height: 1.68 × 1.68 = 2.8224. Divide: 70 / 2.8224 = 24.80. BMI ≈ 24.8 — just inside the healthy range. Notice how the BMI formula rewards doing the squaring before the dividing.
Worked example: imperial
Height 5 ft 6 in (66 in), weight 154 lb. Square the height: 66 × 66 = 4,356. Multiply by 703: 703 × 154 = 108,262, then divide: 108,262 / 4,356 = 24.85. BMI ≈ 24.9 — the same body, the same answer, confirming both forms of the BMI formula agree.
Two systems, one number. Trust the match.
Step 4: interpret your result
For adults, standard WHO categories apply: below 18.5 is underweight, 18.5–24.9 is the healthy range, 25–29.9 is overweight, and 30 and above is obese. These bands were drawn from population data, so treat them as guidance for a conversation with your doctor — not a diagnosis.
The most common calculation errors
Squaring height wrong: height² means height × height, not height × 2 — this mistake roughly doubles the answer. Centimeters into the metric BMI formula: convert to meters first (divide by 100), or your result will be 10,000 times too small. Feet instead of total inches: 5 ft 9 in is 69 inches, not 5.9 of anything — multiply feet by 12 and add the inches. Dropping the 703 in the imperial formula collapses the result to near zero. And rounding too early: keep full precision through squaring and dividing, and round only the final answer to one decimal.
Every classic error is a unit error. Every unit error is a five-second fix.
Three bodies, one height: seeing the formula in action
Fix height at 1.75 m (height² = 3.0625) and watch how weight moves the result. At 65 kg: 65 / 3.0625 = 21.2 — healthy. At 90 kg: 90 / 3.0625 = 29.4 — overweight. At 105 kg: 105 / 3.0625 = 34.3 — obese. Same BMI formula, same height — only the weight changes the story. This is why the BMI formula works best as a tracking tool: re-measure monthly and watch the trend.
Working backward: your healthy weight range
The BMI formula also runs in reverse. Multiply your height squared by 18.5 and 24.9 to find your healthy weight range in kilograms. At 1.75 m, that's 18.5 × 3.0625 = 56.7 kg up to 24.9 × 3.0625 = 76.3 kg. Suddenly the BMI formula isn't a grade — it's a target zone.
How often should you calculate BMI?
For most adults tracking weight, monthly is plenty — body composition changes slowly, and weekly noise will only distract you. Recalculate the BMI formula when your weight has meaningfully changed, or once a month as a steady check-in.
When the BMI formula needs context
The BMI formula was designed for population studies, so know its blind spots. It overstates fatness in muscular people, understates risk in those with normal BMI but high body fat, and needs age adjustment for children and teens. Use it as a first screen, then bring in waist measurements and professional guidance.
Calculator or by hand?
Doing the BMI formula by hand once is worth it — you learn what the number is made of and can sanity-check any tool's output forever after. After that, open the BMI calculator directly, or read more helpful articles on interpreting BMI, body composition, and healthy weight ranges.
Related BMITool tools and guides
Try the BMITool BMI calculator for instant results, and read more helpful articles on the BMI formula, healthy weight ranges, and body composition.
Key takeaway for BMI Formula Explained: How to Calculate Body Mass Index
The BMI formula is weight divided by height squared — measure accurately, convert units carefully, square before dividing, and read the result against the standard bands.
Frequently Asked Questions
How does the BMI formula work?
The BMI formula divides your weight by the square of your height: BMI = kg / m² in metric, or BMI = 703 × lb / in² in imperial. The result is compared against standard categories from underweight to obese.
What are the most common BMI formula mistakes?
Multiplying height by 2 instead of squaring it, feeding centimeters into the meters formula, using feet instead of total inches, dropping the 703 in imperial math, and rounding before the final answer.
How often should I calculate my BMI?
Monthly is plenty for most adults — body composition changes slowly. Recalculate the BMI formula when your weight has meaningfully changed or as a regular monthly check-in.
Why is the BMI formula misleading for some people?
Because the BMI formula can't tell muscle from fat: it overstates fatness in muscular people and needs context for children, teens, the elderly, and pregnant people. It's a first screen, not a diagnosis.