NumberTally BMI Calculator — Body Mass Index & Healthy Range
Calculate your BMI instantly in imperial or metric units. See your WHO weight category, your BMI prime and ponderal index, the exact healthy weight range for your height, your waist-to-height ratio — and how much to gain or lose to reach it.
Advanced options
We’ll show the weight that hits this BMI at your height.
Ponderal index (kg/m³) scales weight by height cubed — fairer for very tall or short people.
What BMI actually measures
Body mass index is simply your weight relative to your height squared. It was devised in the 1830s by Adolphe Quetelet as a population statistic, not a personal health grade — but because it needs only a scale and a tape measure, it became the world’s most-used screening tool for weight categories.
The WHO cutoffs (18.5, 25, 30) are screening thresholds. They flag who might benefit from a closer look, the same way a smoke alarm flags possible fire. They don’t diagnose anything on their own.
When BMI misleads — and what to check instead
BMI can’t tell muscle from fat, so lifters and athletes often land in “overweight” while carrying little body fat. It also misses where fat sits: belly fat predicts risk far better than the number alone. If your BMI surprises you, check two things next — your waist circumference and your body fat percentage (our body fat calculator estimates it from tape measurements).
Estimates only — not medical advice. Talk to a healthcare professional about your health.
Frequently Asked Questions
What is a healthy BMI range?
The World Health Organization defines a healthy BMI for adults as 18.5 to 24.9. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is obesity. These are screening thresholds for adults, not diagnoses.
Why does the imperial BMI formula multiply by 703?
703 is a unit-conversion factor. BMI is defined as kg/m²; converting pounds to kilograms and inches to meters gives 39.3701² ÷ 2.20462 ≈ 703. The step-by-step box above shows both formulas giving the same answer.
Can BMI be wrong for muscular people?
Yes — BMI sees only total weight, so a muscular athlete can score as overweight or obese while lean. Pair it with a body-fat estimate or waist measurement for the full picture.
What BMI counts as obese?
A BMI of 30 or higher is classified as obesity, further divided into Class I (30–34.9), Class II (35–39.9), and Class III (40+).
What is a healthy waist-to-height ratio?
Keep your waist under half your height — a ratio below 0.5. Between 0.5 and 0.6 signals increased health risk and 0.6 or above high risk, whatever your BMI says. Enter your waist above to see yours.
Is BMI enough to judge my health on its own?
No. It’s one screening data point alongside waist size, blood pressure, blood sugar, fitness, sleep, and family history. Use it as a starting point, not a verdict.