BMI calculator

What BMI is (and what it isn’t)

BMI is weight (kg) divided by height (m) squared. Adolphe Quetelet, a Belgian statistician, built it in the 1830s to describe populations — not individuals. At population scale, BMI tracks body fat reasonably well. At individual scale, two people at the same BMI can have very different body compositions, and the same BMI can mean different things at 25 and at 75.

BMI categories and the healthy BMI range

The WHO categories for adults:

• Under 18.5 — underweight. Often reflects insufficient intake or underlying conditions worth investigating.
• 18.5–24.9 — healthy weight range. Lowest population-level mortality risk in adults aged 18–65.
• 25.0–29.9 — overweight range. Modestly raised risk of type 2 diabetes and cardiovascular disease.
• 30.0–34.9 — obese I range.
• 35.0–39.9 — obese II range.
• 40.0+ — obese III range. Medical support for structured weight loss is typically the right path here.

The thresholds are conveniences, not cliff edges — BMI 24.9 and 25.1 are indistinguishable in actual risk.

Where BMI fails — and what to use instead

Four groups get consistently mis-classified:

• Muscular people. A resistance-trained body at BMI 26 can have less body fat than an untrained body at BMI 22. Gallagher 2000’s body-fat reference data shows this clearly³.
• Older adults. Winter 2014’s meta-analysis of 197,940 adults aged 65+ found the lowest all-cause mortality at BMI 23.0–29.9, not the standard 18.5–24.9².
• Non-European ancestry populations. The WHO’s 2004 expert consultation concluded the standard 25 cutoff under-classifies risk in Asian populations¹. The NHS uses BMI 23 as the overweight threshold for South Asian adults. Deurenberg 1998 showed body-fat-to-BMI relationships vary systematically across ethnic groups⁴.
• People with central adiposity. Weight carried around the waist is more strongly linked to cardiometabolic risk than overall BMI. Waist-to-height ratio — keep your waist below half your height — is often a better single signal.

Alternatives depending on what you care about:

• A personalised target weight → ideal weight calculator
• Body composition → body-fat percentage via our metabolic age tool
• A daily plan → calorie + protein calculators

Why this calculator doesn’t apply to children

The calculator above is for adults. Under 18, BMI is assessed against percentile-based growth charts that vary by age-in-months and sex — not against fixed category cutoffs. A 12-year-old at BMI 19 sits near the 50th percentile (neither healthy nor unhealthy, just middle of the distribution); a 14-year-old at the same BMI might be near the 10th. Childhood BMI is about where a child sits within the typical range for their exact age, not about crossing a single line.

And the references for those percentiles differ by country: NHS / UK-WHO for British children, CDC for American children, WHO Child Growth Standards as an international reference. None is universally “right” — childhood obesity is real and worth addressing, but the number that flags it is population-specific. If you’re checking a child’s BMI, use the tool your healthcare system points you at.

Using BMI as one signal among many

BMI works best as the cheapest of several signals, not the only one. Paired with waist measurement, blood pressure or body composition, BMI becomes useful. Alone — especially near category boundaries — it barely tells you anything you couldn’t eyeball. The healthy-range output is the most useful thing this calculator gives you. Our healthy balanced dinners and meal-plan walkthrough cover what a week of eating around that goal looks like.

When to see your GP

Book a GP appointment if:

• Your BMI is 35+ and you’re considering a structured weight-loss plan
• BMI 30+ and a pre-existing condition (type 2 diabetes, high blood pressure, sleep apnoea)
• BMI below 18.5 without intentional weight loss
• Weight changed 10%+ over a short period without an obvious cause

Otherwise BMI is a number to note and move on from. The calculators and habits downstream of it are where the useful work happens.

Take the free 2-minute quiz and we’ll build a meal plan around the healthy weight range this calculator gave you.