BMI Calculator — Free Body Mass Index Calculator
Check your Body Mass Index in seconds. Enter your height and weight — in metric or imperial units — and instantly see your BMI, your weight category, and your healthy weight range.
Enter your height and weight
A BMI calculator estimates your Body Mass Index — a number calculated from your height and weight that screens for weight categories which may signal health risks. Doctors and health organizations worldwide, including the WHO and CDC, use BMI as a quick first check before looking deeper at body composition, waist size, and lifestyle.
How to Calculate BMI
The BMI formula divides your weight by the square of your height. This calculator runs the math instantly, but here is exactly what happens behind the scenes:
Imperial: BMI = 703 × weight (lb) ÷ height (in)2
Worked example: a person who weighs 70 kg and is 175 cm (1.75 m) tall has a BMI of 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9 — comfortably inside the normal range.
BMI Categories (WHO)
Once you have your number, compare it against the standard adult BMI categories:
| BMI | Category | What it suggests |
|---|---|---|
| Below 18.5 | Underweight | May indicate undernourishment; worth discussing with a clinician |
| 18.5 – 24.9 | Normal weight | Associated with the lowest risk of weight-related disease |
| 25.0 – 29.9 | Overweight | Elevated risk of heart disease, type 2 diabetes, and high blood pressure |
| 30.0 – 34.9 | Obese – Class I | Moderately increased risk; lifestyle changes plus clinical follow-up advised |
| 35.0 – 39.9 | Obese – Class II | Severely increased risk; medical treatment is usually recommended |
| 40.0 and above | Obese – Class III (severe) | Very severely increased risk; specialist care typically needed |
Do BMI Cutoffs Differ by Ethnicity?
For many Asian populations, health risks rise at a lower BMI than the standard cutoffs above. A WHO expert consultation proposed these public-health action points for Asian adults:
| BMI | Interpretation (Asian cutoffs) | Standard (general) cutoff |
|---|---|---|
| 18.5 – 22.9 | Acceptable range | Normal (18.5 – 24.9) |
| 23.0 – 27.4 | Increased risk (overweight) | Overweight (25.0 – 29.9) |
| 27.5 and above | High risk (obese) | Obese (≥ 30.0) |
These are guidance thresholds, not diagnoses — if you are of South or East Asian descent and your BMI is above 23, it is worth discussing with a clinician even if the standard chart calls you “normal.”
What Is a Healthy Weight for My Height?
Multiply the square of your height in meters by 18.5 and 24.9 to get your healthy range. For example, at 175 cm (1.75 m): 18.5 × 3.0625 = 56.7 kg and 24.9 × 3.0625 = 76.3 kg. The calculator above shows this range automatically for any height you enter.
Waist Size Matters Too: Waist-to-Height Ratio
BMI cannot tell where fat is stored — and belly fat carries more risk than fat elsewhere. Two quick checks complement your BMI:
- Waist-to-height ratio: measure your waist at navel level, divide by your height (same units). Aim to keep it below 0.5 — your waist should be less than half your height.
- Waist circumference: general guidance flags increased risk above about 102 cm (40 in) for men and 88 cm (35 in) for women.
A person with a “normal” BMI but a waist-to-height ratio above 0.5 can still carry meaningful metabolic risk — a pattern sometimes called normal-weight obesity.
BMI Limitations You Should Know
BMI is a screening tool, not a diagnosis. It does not distinguish muscle from fat, so athletes and very muscular people can score as “overweight” while being perfectly healthy. It also does not account for age, sex, bone density, or where fat is stored — belly fat carries more risk than the number alone suggests. Pregnant people and children need different charts (BMI-for-age percentiles). Use your result as a starting point for a conversation with a healthcare professional, not a final verdict.
Who Should Not Rely on BMI?
BMI works as a population screening tool, but it is misleading for some groups:
- Children and teens (under 18): growing bodies need BMI-for-age percentiles, not adult cutoffs — this calculator’s categories don’t apply to them.
- Pregnant or breastfeeding people: weight gain is expected and healthy; use your clinician’s guidance, not a BMI chart.
- Athletes and very muscular people: muscle is dense, so a fit lifter can land in “overweight” with low body fat. Our body fat calculator is a better fit here.
- People with eating disorders: tracking a number can be harmful — focus on professional care rather than self-screening tools.
- Older adults: some research finds slightly higher BMIs less risky in old age, partly because BMI misses muscle loss (sarcopenia).
How to Reach a Healthy BMI
Small, steady changes beat crash diets. Aim for a calorie deficit of roughly 300–500 kcal per day through a mix of food and movement, which typically produces 0.5–1 kg of loss per week. Strength training preserves muscle while you lose fat, and 150 minutes of moderate activity weekly is the widely recommended baseline. Recheck your BMI monthly — not daily — because weight fluctuates with water, food, and hormones.
BMI Prime and the Ponderal Index
Two lesser-known refinements, for the curious:
- BMI Prime = your BMI ÷ 25 (the upper limit of normal). A BMI Prime of 1.0 means you’re at the top of the healthy range; 1.2 means 20% above it. It’s just a rescaled BMI — same information, easier ratio to read.
- Ponderal Index = weight (kg) ÷ height (m)³. Because it uses height cubed instead of squared, it scales better for very tall or very short people — though it never caught on clinically the way BMI did.
Sources & Medical Disclaimer
BMI categories follow the World Health Organization (WHO) classification; adult guidance reflects the CDC’s Adult BMI guidance; Asian action points come from the WHO Expert Consultation on appropriate BMI for Asian populations. BMI is a screening measure, not a diagnosis — only a qualified clinician can assess your health. If you have a medical condition, are pregnant, or are under 18, talk to a professional rather than acting on a calculator result alone.
Last updated: October 2026.
BMI FAQs
What is a good BMI number?
For most adults, a BMI between 18.5 and 24.9 is considered the healthy or “normal” range and is associated with the lowest risk of weight-related disease.
How is BMI calculated?
Divide your weight in kilograms by your height in meters squared. In imperial units, multiply your weight in pounds by 703, then divide by your height in inches squared.
Is BMI accurate?
BMI is a useful screening tool but an imperfect one. It cannot tell muscle from fat, so muscular athletes may score high while being healthy. Doctors pair it with waist measurements and other checks.
What BMI is considered obese?
A BMI of 30 or higher is classified as obese, with 30–34.9 class I, 35–39.9 class II, and 40+ class III (severe) obesity.
Does BMI differ for men and women?
The formula is identical for men and women. However, women naturally carry more body fat at the same BMI, so clinicians interpret borderline results alongside other measures.
What is my ideal weight for my height?
Multiply your height in meters squared by 18.5 and 24.9. That gives your healthy weight range — the calculator above does this automatically.
Can children use this BMI calculator?
Children and teens should use BMI-for-age percentile charts rather than adult cutoffs, because healthy BMI changes as they grow. Ask a pediatrician for the right chart.
What are the BMI cutoffs for Asian populations?
A WHO expert consultation proposed lower action points for Asian adults: 23.0–27.4 signals increased risk (overweight) and 27.5+ signals high risk (obese), versus the standard 25 and 30 cutoffs.
What is BMI Prime?
BMI Prime is your BMI divided by 25. A value of 1.0 means you’re at the top of the healthy range; above 1.0 means above it. It’s the same information as BMI, expressed as a simple ratio.
What is a healthy waist-to-height ratio?
Keep it below 0.5 — your waist measured at navel level should be less than half your height. It’s a useful companion to BMI because it captures belly fat, which BMI misses.
Can my BMI be normal but still unhealthy?
Yes — sometimes called normal-weight obesity. If your BMI is normal but your waist-to-height ratio is above 0.5 or you carry little muscle, metabolic risk can still be elevated. Body composition matters, not just the number.
Is BMI accurate for athletes and muscular people?
Often not — muscle is dense, so fit, muscular people can score “overweight” with low body fat. A body fat measurement (e.g. the Navy tape method) is more meaningful for them.
At what BMI should I see a doctor?
Consider it at 30+ (or 27.5+ with Asian cutoffs), or below 18.5 — and sooner if you have symptoms like high blood pressure, blood sugar issues, or rapid weight change. BMI is a screening prompt, not a diagnosis.
What is the Ponderal Index, and is it better than BMI?
The Ponderal Index divides weight by height cubed (kg/m³), which scales better for very tall or short people. It never replaced BMI clinically, but it’s a fairer comparison across extreme heights.