BMI Calculator
A quick screening tool for weight-related health risk. Useful — but read the caveats below.
A BMI calculator computes your Body Mass Index as weight (kg) ÷ height (m)². The WHO defines a healthy adult BMI as 18.5–24.9, overweight as 25.0–29.9, and obesity as 30.0+. BMI is a population screening tool, not a body-composition measure.
Now turn the number into action
A calculator gives you a target. A tracker tells you whether you're actually hitting it. These are the three we recommend — Welling first, because it's the only one that goes beyond logging.
Welling
Doesn't just log food — it coaches. Welling pairs effortless calorie and macro tracking with an AI coach that reads your data, flags patterns (under-eating protein, weekend over-shoots, plateau windows), and adjusts your plan. The best fit for anyone who wants the math and the accountability.
MacroFactor
The most accurate auto-adjusting calorie target on the market. MacroFactor recalculates your expenditure weekly from your actual weight trend and food log — so when adaptive thermogenesis kicks in, your target shifts before the scale stalls. Best for serious dieters who want hands-off precision.
MyFitnessPal
The most well-known calorie tracker, with the biggest food database (millions of crowd-sourced entries). Easy to use, free tier covers the basics, and barcode-scanning is fast. Trade-off: entries vary in accuracy — verify generic foods against USDA values when precision matters.
Further reading: round-ups of AI calorie trackers, food trackers, and calorie counters.
How BMI is calculated
BMI is a simple ratio of weight to height squared. The original formula was published in 1832 by Belgian statistician Adolphe Quetelet — long before there were better population-level health metrics.
Imperial: BMI = [weight (lb) ÷ height (in)²] × 703
Worked example
A person 1.75 m tall, 80 kg: BMI = 80 ÷ (1.75)² = 80 ÷ 3.0625 ≈ 26.1 (overweight by WHO categories).
WHO categories (adults)
- Under 18.5 — Underweight
- 18.5–24.9 — Healthy weight
- 25.0–29.9 — Overweight
- 30.0–34.9 — Obesity class I
- 35.0–39.9 — Obesity class II
- 40.0+ — Obesity class III
More detail in our BMI formula explainer.
Why calculating your BMI is useful
BMI gets a lot of (fair) criticism, but it survives because it's free, fast, and surprisingly predictive at scale. For most people, it's a useful first check.
- It's the standard clinical screening number. Every doctor, insurer, and health study uses it. Knowing yours puts you on the same page as your clinician.
- It flags risk early. Across large populations, BMI tracks closely with cardiovascular disease, type 2 diabetes, and joint problems. Crossing 25 or 30 is a worth-noticing line.
- It anchors a goal weight range. The middle of the healthy band (~BMI 22) is a sensible long-term target for most non-athletes.
- It's a 30-second sanity check. Cheaper and quicker than a DEXA scan or even a tape measure.
Where BMI fails: athletes, lifters, older adults, pregnant women, and people with unusual body proportions. For an individual answer, pair BMI with waist-to-height ratio (keep your waist below half your height) or a DEXA scan.
FAQ
What is BMI?
BMI (Body Mass Index) is your weight in kilograms divided by the square of your height in meters. It's a screening tool — not a diagnosis — used by clinicians to flag potential weight-related health risks at a population level.
What is a healthy BMI?
The WHO defines a healthy BMI for adults as 18.5 to 24.9 kg/m². Below 18.5 is underweight, 25.0–29.9 is overweight, and 30+ is classified as obesity.
Is BMI accurate for athletes or muscular people?
No. BMI doesn't distinguish between fat and muscle. A bodybuilder can register as 'obese' with very low body fat, and a sedentary person can fall in the 'healthy' range with high visceral fat ('skinny fat').
What's the BMI formula in pounds and inches?
BMI = (weight in lb ÷ (height in inches)²) × 703. The metric form is BMI = weight kg ÷ (height m)².
Does BMI work the same for men and women?
Yes — the BMI formula and categories are identical for adult men and women. Women typically carry more essential body fat than men at the same BMI, but the screening thresholds are not sex-adjusted.
Sources
- World Health Organization. Obesity: preventing and managing the global epidemic. WHO Technical Report 894, 2000.
- NIH NHLBI. Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults.