What BMI actually is
Body Mass Index is your weight divided by your height squared — a number invented in the 1830s to describe populations, not people. It says nothing about muscle, bone or where you carry fat. It is a cheap, useful screening signal, and a poor verdict. Read it next to your waist and the direction it moves over time.
WHO categories
| Category | BMI (kg/m²) |
|---|---|
| Underweight | below 18.5 |
| Healthy weight | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Obese · class I | 30.0 – 34.9 |
| Obese · class II | 35.0 – 39.9 |
| Obese · class III | 40.0 and above |
World Health Organization cut-points (Technical Report Series 894). The same thresholds apply to adult men and women. Some Asian populations reach equivalent risk at lower BMI (≈23 and 27.5).
Why we show a range
Most calculators hand you one decimal and an air of certainty. We run two formulas: the classic BMI and Oxford mathematician Nick Trefethen’s “new BMI” (1.3 · weight ÷ height^2.5), which corrects a known flaw — classic BMI makes tall people look heavier and short people lighter than they are. Near average height the two agree; at the extremes they part ways, and that gap is the honest signal that no single BMI number is exact.
For muscular builds
Muscle weighs more than fat, so athletes often score “overweight” at a healthy body fat. If your BMI is high but your waist is fine, that’s the likely story.
For older adults
Muscle is lost with age, so a “healthy” BMI can hide low muscle and higher fat. Waist and strength matter more than the number here.
Not for everyone
BMI isn’t valid during pregnancy or for children (who use age-specific percentile charts). For under-18s, see a pediatrician.
What BMI misses — and what to add
BMI can’t see fat distribution, the single biggest thing it gets wrong. Pair it with:
- Waist-to-height ratio — a measured check for central fat; often a better early signal than BMI.
- Body fat percentage — separates fat from muscle, which BMI cannot.
If you’re above the healthy band, the evidence is encouraging: a sustained loss of just 3–5% of body weight already brings meaningful health benefits, and 5–10% more — per the 2013 AHA/ACC/TOS obesity guideline. Small and steady beats a number by a deadline.
FAQ
Is BMI accurate?
As a population screening tool, yes — it correlates with health risk across large groups. For any one person it can be misleading, especially for muscular, very tall, very short, or older bodies. Use it as one signal among several.
What is a healthy BMI?
The WHO healthy band is 18.5 to 24.9. But a number inside that band with a large waist, or above it with lots of muscle, both tell you to look past BMI alone.
Why does BMI overestimate fat in athletes?
BMI only knows weight and height — it counts a kilogram of muscle exactly like a kilogram of fat. Muscular people are dense, so they score high without excess fat.
What’s the difference between BMI and body fat percentage?
BMI is weight-for-height; body fat percentage is how much of your weight is fat. Two people with the same BMI can have very different body fat. BMI is quicker; body fat is more informative.
Should I use BMI or waist-to-height ratio?
Use both. Waist-to-height captures fat around the middle — the kind most tied to health risk — which BMI ignores entirely. When they disagree, the waist number is usually the one to trust.
Does this calculator store my data?
The calculator runs in your browser — your measurements aren't sent anywhere to be calculated. If you save your history, it's kept in this browser. When accounts and the diary launch, saving to your account will be your choice, and you'll be able to export or delete everything at any time.