You don’t need an app, a smart scale, or a doctor’s visit to figure out your BMI. The formula is simple arithmetic — all you need is your weight, your height, and about sixty seconds.
The BMI Formula (Metric)
BMI = weight (kg) ÷ height (m) ÷ height (m)
Divide your weight in kilograms by your height in meters squared. That’s the whole thing.
The Imperial Version (Pounds and Inches)
BMI = 703 × weight (lb) ÷ height (in) ÷ height (in)
The 703 is a conversion factor — skip it and your number is meaningless.
Worked Example 1: Metric (70 kg, 1.75 m)
- Square the height: 1.75 × 1.75 = 3.0625
- Divide weight by that: 70 ÷ 3.0625 = 22.857…
- Round: BMI = 22.9 (healthy range)
Worked Example 2: Imperial (180 lb, 70 in)
- Square the height: 70 × 70 = 4,900
- Weight × 703: 703 × 180 = 126,540
- Divide: 126,540 ÷ 4,900 = 25.826… → BMI = 25.8 (just into overweight)
Sanity check: 180 lb ≈ 81.6 kg, 70 in ≈ 1.778 m → 81.6 ÷ (1.778 × 1.778) = 25.8. Same answer.
Get Accurate Inputs First
- Weigh yourself in the morning, after the bathroom, before eating. Evening weights run 1–2 kg higher.
- Measure height barefoot, heels together against a wall. Self-reported heights run 1–2 cm optimistic.
- Use one scale on a hard, flat floor.
What Your Number Means
- Below 18.5 — Underweight
- 18.5 to 24.9 — Healthy weight
- 25 to 29.9 — Overweight
- 30 and above — Obese
These are screening signals, not verdicts — a starting point for a closer look.
Common Mistakes When Calculating by Hand
Forgetting to square the height
Dividing weight by height only once gives a number 3–4× too high. Always square height first.
Mixing units
5’10” = 70 inches, not 5.10. 175 cm = 1.75 m.
Skipping the 703 in imperial
It’s not optional — it converts pounds/inches to the kg/m² scale.
Rounding too early
Keep full decimals until the final step.
When BMI Misleads
- Muscular builds: athletes can score “overweight” while lean — muscle is denser than fat.
- Height extremes: calibrated for average heights.
- Age and body composition: can’t tell muscle from fat or where fat sits.
- Children and pregnancy: standard categories don’t apply.
Pair BMI with a body fat estimate for a fuller picture.
Beyond BMI: What Else to Check
- BMR calculator — calories burned at rest.
- Body fat calculator — distinguishes fat from muscle.
- Health calculators hub — all free health tools.
Skip the arithmetic — get your BMI instantly
Calculate My BMI FreeFrequently Asked Questions
How do I calculate BMI without a calculator?
Round height to something easy to square (1.75 m → use 1.8² = 3.24), then divide weight by it: 70 ÷ 3.24 ≈ 21.6. Rough but useful.
Is the 703 always the same?
Yes — a fixed conversion constant. Never change it.
My BMI says overweight but I’m muscular — worry?
Probably not. Use body fat percentage or waist-to-height ratio alongside BMI.
Does BMI work the same for men and women?
The formula and categories are identical for adults; body composition differs, so look at BMI alongside other measures.
How often should I recalculate?
Monthly is plenty. Weigh yourself first thing in the morning under consistent conditions.