BMI Calculator
Calculate your Body Mass Index.
BMI
24.4
- WHO adult bandNormal weight
Underweight < 18.5 · Normal 18.5–24.9 · Overweight 25–29.9 · Obesity 30+. Same formula for men and women; not a diagnosis.
This BMI calculator turns height and weight into a body mass index and places it on the WHO adult scale. Switch between imperial (pounds, feet and inches) and metric (kilograms, centimetres); the marker on the coloured bar moves as you type.
BMI is a screening number, not a diagnosis. It cannot separate muscle from fat, and it reads differently for athletes, older adults, and people of different ethnic backgrounds. Below are the formula in both unit systems, a worked example, the adult bands, and the point where the number stops being useful.
How to calculate BMI
- Measure height without shoes and weight in light clothing — shoes and a coat can move BMI by half a point.
- Metric: divide weight in kilograms by height in metres squared.
- Imperial: multiply weight in pounds by 703, then divide by height in inches squared.
- Compare the result with the adult bands: under 18.5 underweight, 18.5–24.9 healthy, 25–29.9 overweight, 30 and above obesity.
Example
70 kg at 1.70 m: 1.70² = 2.89, and 70 / 2.89 = 24.2 — the top of the healthy band. The same person in imperial units is 154 lb at 66.9 in: 703 × 154 / 4476 = 24.2. The two formulas agree because 703 is only the unit-conversion constant.
BMI formula
Metric: BMI = kg / m². Imperial: BMI = 703 × lb / in². Healthy weight for a height: kg = 24.9 × m².
How to use this calculator
- Pick imperial or metric. The fields change with the choice and the result is identical either way.
- Enter weight, then height — feet and inches, or centimetres.
- Read the BMI and the band beneath it; the coloured bar shows where the value sits between 10 and 45.
- Change one input at a time to see the sensitivity: at 1.70 m, one BMI point is about 2.9 kg.
What BMI does and does not measure
BMI is weight relative to height, nothing more. It correlates with body fat across a population, which is why insurers and clinicians still use it, but it cannot tell a 95 kg lifter at 12% body fat from a 95 kg sedentary adult at 32%.
It also reads differently across groups: several Asian countries use lower cut-offs (23 for overweight, 27.5 for obesity), and in adults over 65 a BMI of 25–27 is often associated with better outcomes than 22.
Adult BMI ranges
| BMI | WHO band | What it usually means |
|---|---|---|
| Under 18.5 | Underweight | Check for unintended weight loss or illness |
| 18.5 – 24.9 | Healthy weight | Lowest risk band for most adults |
| 25.0 – 29.9 | Overweight | More actionable if waist circumference is also high |
| 30.0 – 34.9 | Obesity class I | Risk rises with waist, blood pressure, and glucose |
| 35.0 – 39.9 | Obesity class II | Sleep apnoea and diabetes screening are common |
| 40 and above | Obesity class III | Specialist assessment usually advised |
WHO adult cut-offs. Not valid for children, pregnancy, or anyone under 18.
BMI for children and teenagers
Under 18, BMI is read against age- and sex-specific percentile charts, not the adult table. A 10-year-old at 19 is not healthy in the adult sense — that value sits near the 85th percentile for that age. Use a paediatric growth chart; every band on this page applies to adults only.
BMI vs. body fat and waist
Waist circumference catches risk that BMI misses. A man at BMI 26 with a 42-inch waist carries more metabolic risk than the BMI alone suggests, and a woman at BMI 27 with a 30-inch waist carries less. Body fat percentage and waist-to-height ratio (keep it under 0.5) are the two cheap upgrades.
What this calculator leaves out
Pregnancy, amputation, significant oedema, and very high muscle mass all break the formula, and age changes what a given value means. This is a screening tool, not a diagnosis, and it says nothing about blood pressure, lipids, or blood glucose — the measurements that actually define metabolic risk.
Common mistakes
- Rounding height down. At 1.70 m, entering 1.69 moves BMI by 0.3 — enough to cross a band boundary.
- Using the adult table for a child. Paediatric BMI needs percentile charts by age and sex.
- Treating overweight as a diagnosis. It is one input; waist, blood pressure, and blood work carry more weight.
FAQ
Is BMI calculated differently for men and women?
No, the formula is identical. Body composition differs — women typically carry more fat at the same BMI — so the same number can mean slightly different things, but the bands do not change.
What is a healthy BMI?
18.5 to 24.9 for adults under 65. Several Asian guidelines prefer 18.5–22.9, because type 2 diabetes risk rises at lower values in those populations.
Does BMI depend on age?
The adult bands stay fixed, but their meaning shifts: at 70, a BMI of 26 is associated with lower mortality than 22. For anyone under 18, use age- and sex-specific percentiles instead.
What is my BMI if I am a man weighing 75 kg?
It depends on height. At 1.75 m: 75 / 3.06 = 24.5, just inside the healthy band. At 1.70 m the same weight gives 26.0, which is overweight.
How much weight is one BMI point?
One BMI point equals height in metres squared, in kilograms: 2.89 kg (6.4 lb) at 1.70 m and 3.06 kg (6.7 lb) at 1.75 m.
Is BMI accurate for athletes?
No. Muscle is denser than fat, so a lean lifter can score 27–30 while carrying 12% body fat. Use body fat percentage or waist-to-height ratio instead.
What is the BMI formula in pounds and inches?
703 × weight in pounds / height in inches squared. The 703 converts kg/m² into lb/in² so the result matches the metric formula.
What is a good BMI for a woman?
The same 18.5–24.9 band applies. Waist circumference adds what BMI misses: above 35 inches (88 cm) raises risk regardless of the BMI value.