Type “BMI” into a search bar and you’ll get a number back in seconds. What you won’t get, most of the time, is any sense of what that number is actually good for, and where it quietly stops being useful.
What BMI actually measures
Body Mass Index is just weight divided by height squared. That’s it. It was developed in the 1830s by a Belgian mathematician, Adolphe Quetelet, as a population-level statistic, not a personal health diagnosis, and the World Health Organisation later adopted it as a simple screening tool because it’s cheap and easy to calculate for large groups of people.
For most adults, in most circumstances, it gives a reasonable first signal. The WHO’s healthy range (18.5 to 24.9) correlates well enough with health outcomes across large populations that it remains the standard starting point doctors use.
Where it breaks down
BMI cannot tell the difference between muscle and fat. A muscular athlete and a sedentary person of the same height and weight get the same BMI, despite having very different body compositions and very different health profiles. This is the single most common criticism of BMI, and it’s a fair one.
It also says nothing about where fat is stored. Two people with an identical BMI can have very different amounts of visceral fat (the kind around organs, linked to higher cardiometabolic risk) versus fat stored elsewhere. A waist-to-height ratio, or a body fat percentage estimate, captures this in a way BMI structurally cannot.
It wasn’t built with age or sex-specific body composition changes in mind either. Older adults naturally carry more body fat at the same BMI as younger adults due to age-related muscle loss, and women typically carry a higher body fat percentage than men at an identical BMI, since men tend to carry more muscle mass.
Why a BMI Calculator Can Seem “Wrong” About You
When people say their BMI calculator is wrong, the calculator is almost never miscalculating. BMI is a simple formula, so every correct calculator gives the same number. What’s usually happening is one of these:
- You carry a lot of muscle. Muscle is denser than fat, so strength athletes and very active people often land in “overweight” with low body fat.
- You’re older. With age, muscle tends to be replaced by fat at the same body weight, so a “healthy” BMI can hide more body fat than it would in a younger adult.
- Your ethnic background shifts the risk. The WHO notes that many Asian populations face higher health risks from a BMI of about 23, below the standard overweight line of 25, so a “normal” result can be falsely reassuring.
- You’re very tall or very short. Because BMI squares height, it tends to slightly flatter short people and slightly penalise tall ones.
- The numbers went in wrong. Centimetres instead of metres, or 5.9 feet instead of 5 ft 9 in, will give a nonsense result.
Is BMI Realistic for You? A Quick Check
BMI is most reliable for adults who aren’t especially muscular, aren’t pregnant, and aren’t at the extremes of height. It’s least reliable for:
- bodybuilders, athletes and people who do heavy strength training
- adults over about 65
- pregnant or breastfeeding women
- children and teenagers, who need age- and sex-specific BMI-for-age charts instead
If you’re in one of these groups, a waist measurement will usually tell you more than your BMI does.
What Doctors’ Organisations Now Say
Medical bodies have become more explicit about BMI’s limits. In 2023 the American Medical Association adopted a policy calling BMI alone an imperfect clinical measure and recommending it be used alongside other measures such as waist circumference, body composition and relative fat mass. In 2025 a Lancet Diabetes & Endocrinology commission, endorsed by more than 75 medical organisations, went further: it said excess body fat should be confirmed with a waist-based measure or a direct body fat measurement, not diagnosed from BMI alone.
What to check alongside it
A more complete picture usually comes from combining BMI with at least one other measurement:
- Waist-to-height ratio — a waist circumference under half your height is associated with lower cardiometabolic risk, independent of BMI.
- Body fat percentage — even an estimated figure (from skinfold calipers, bioelectrical impedance, or a formula-based estimate) tells you what BMI can’t: how much of your weight is fat versus lean mass.
- Trend over time — a single BMI reading is a snapshot. Whether it’s moving up, down, or holding steady over months tells you more than the number itself.
For waist-to-height ratio, all you need is a tape measure: wrap it around your waist midway between your lowest rib and the top of your hip bone, then divide by your height. Under 0.5 is the healthy target recommended by the UK’s NICE.
Our BMI Calculator goes beyond the single number too: alongside your BMI it shows a body fat estimate, your healthy weight range and your daily calorie needs, because the number alone doesn’t tell the whole story. To keep track of your trend, log your measurements week by week in our free Fitness Planner.
Frequently Asked Questions
Is the BMI calculator accurate?
The calculation itself is exact: every correct BMI calculator gives the same number. What’s less precise is what BMI tells you about body fat and health, especially if you’re muscular, older, of Asian descent, or a child.
Why does my BMI say I’m overweight when I’m fit?
BMI can’t tell muscle from fat, so muscular people often score as overweight. Check your waist: if it’s under half your height, your “overweight” BMI is very likely down to muscle.
Is BMI still used by doctors?
Yes, as a quick first screen. But current guidance from the AMA and the 2025 Lancet commission is to pair it with other measures, such as waist circumference, before drawing conclusions.
What is more accurate than BMI?
For body fat, a DEXA scan is the most accurate widely available method. For everyday use, waist-to-height ratio is simple, free and better than BMI at flagging risky belly fat.
This article is for general information only and is not medical advice.
Sources: American Medical Association (2023); Lancet Diabetes & Endocrinology Commission (2025); NICE; WHO Expert Consultation (2004).