Fitness · 5 min read
How to Calculate BMI
Mass over height squared — and what the number does not mean
Body mass index (BMI) is a screening ratio: body mass divided by the square of height. This tutorial walks through the SI formula, a 70 kg / 1.75 m worked example, and WHO adult category bands you can try with sliders. BMI is not a diagnosis of health, body fat, or disease.
Written by the My Calculator Stack editorial team. About our methods
BMI is a screening ratio, not a medical diagnosis or treatment plan. WHO adult bands cannot replace advice from a qualified clinician.
1.What BMI is
BMI is a screening ratio of mass to height squared. Public-health tables and many clinics use it as a coarse first look at weight relative to stature — not as a medical assessment. The ratio does not measure body fat, muscle, bone density, or where fat sits. Athletes and older adults are common edge cases. Treat the number as a screen, then talk to a qualified clinician if you need a diagnosis or a care plan.
2.The formula
In SI units, BMI = w / h² with w in kilograms and h in meters. US customary inputs convert first: pounds × 0.45359237 → kg, and inches × 0.0254 → m. After that conversion the same formula applies, so Metric and US results stay comparable.
\[ \mathrm{BMI} = \frac{w}{h^{2}} \]
3.Worked example
Take 70 kg and 1.75 m — the same defaults as the BMI calculator. Height squared is 1.75 × 1.75 = 3.0625. Then 70 / 3.0625 ≈ 22.9 kg/m², which sits in the typical WHO adult “normal” band. In US units that pair is about 154 lb and 69 in; convert to kilograms and meters and you get the same BMI.
\[ \mathrm{BMI}=70/(1.75)^{2}\approx 22.9 \]
4.WHO adult categories
For adults, WHO tables commonly group BMI as underweight (below 18.5), normal (18.5 to 24.9), overweight (25.0 to 29.9), and obese (30.0 and above). These bands are a coarse screen. They do not account for muscle mass, bone density, or fat distribution. Child and adolescent BMI uses age- and sex-specific percentiles, not these adult cutoffs. Switch Metric/US, then move weight and height to see BMI update and the marker travel across the bands. This is a teaching widget — no favorites, no full calculator chrome. Open the BMI calculator when you want the standalone tool.
Inline playground
BMI
22.9 kg/m²
Category: Normal
70 / 1.75² = 22.9
Screening aid only — not a diagnosis. Athletes, older adults, and children need different interpretation than these WHO adult bands.
Open the full BMI calculator →5.Open the full calculator
The BMI calculator uses the same equation, the same 70 kg / 1.75 m example, and the same screening disclaimer, with How it works, FAQ, and related fitness tools on one page. Browse other fitness calculators if you need BMR, calories, or body-fat estimates next.
Try it yourself
Open the related calculator and put these formulas to work.