BMI Calculator Online — Free, Instant Results

What your BMI number actually tells you

BMI (Body Mass Index) is a single number that compares your weight to your height — a quick screening figure for whether you sit in a healthy weight range for your frame. Enter your details in metric (kilograms and centimetres), imperial (feet, inches and pounds), or stone, and this BMI calculator returns your BMI value, your WHO weight category, your healthy weight range, and an ideal weight target, plus a built-in waist-to-hip ratio (WHR) tab for a second angle on the same question.

Every calculation runs locally in your browser — your height, weight and measurements are never uploaded, stored, or sent to a server. Treat the result as a screening tool, not a diagnosis: it flags where to look, it isn’t medical advice, and it can’t see the muscle, bone and fat behind the number.

The BMI formula, worked in metric and imperial

Body Mass Index is the same quantity in every unit system — only the arithmetic that gets you there changes. The metric formula divides your weight by your height squared; the imperial formula does the same with pounds and inches and multiplies by a fixed constant so the scale lines up:

Metric: BMI = weight (kg) ÷ height (m)²  ·  Imperial: BMI = weight (lb) ÷ height (in)² × 703

Take a person of 175 cm and 70 kg — roughly 5 ft 9 in and 154 lb — and run both formulas:

Metric   (70 kg, 175 cm = 1.75 m):
                  1.75 x 1.75      = 3.0625 m2
                  70 / 3.0625      = 22.9      -> Normal weight

                Imperial (154 lb, 5 ft 9 in = 69 in):
                  69 x 69          = 4,761 in2
                  154 / 4,761      = 0.0323
                  0.0323 x 703     = 22.7      -> Normal weight

Both land squarely in the healthy 18.5–24.9 band. The tiny 22.7-vs-22.9 gap isn’t a real difference — it’s just rounding in the pounds-and-inches conversion. Under the hood this calculator converts whichever units you type into kilograms and metres before it divides, so identical input always returns an identical BMI whether you enter it as metric, imperial, or stone.

WHO BMI categories for adults

Your BMI is sorted into one of four World Health Organization bands, shown on the colour gauge under your result. These adult cut-offs are the same in metric and imperial:

Why BMI can mislead — and who it misreads

BMI only knows two things about you: your height and your weight. It can’t tell muscle from fat, or see where that weight sits. For most sedentary adults it tracks body fat well enough to be useful, but for several groups it’s a poor stand-in — which is exactly why the result should be a starting point, not a verdict.

Athletes and muscular builds

Muscle is denser than fat, so BMI counts it as excess weight. A lean, muscular person can read Overweight while carrying very little fat. Tick the “Muscular / athletic build” box for a result note that reflects this, and check your waist-to-hip ratio instead.

Older adults

We tend to lose muscle and gain fat with age, so a “normal” BMI can hide low muscle mass (sarcopenia). Some guidance uses a slightly higher healthy range for older adults.

Pregnancy

Weight gain is expected and healthy during pregnancy, so the standard adult BMI categories don’t apply. Follow the guidance from your own maternity care instead.

Children and teens

Under-18s are assessed with age- and sex-specific BMI percentiles, not the fixed adult cut-offs this calculator uses. Read a child’s BMI only against a paediatric growth chart.

Ethnicity

Health risk can begin at a lower BMI in some populations, and several health bodies apply lower overweight and obesity thresholds for people of South and East Asian descent. The same number can carry a different meaning across groups.

Beyond the number: healthy range, ideal weight, and fat distribution

A bare BMI is hard to act on, so the result turns it into targets. The healthy weight range is the span of weights that keeps you between BMI 18.5 and 25 at your height, shown in kg, pounds, or stone. The ideal weight is the weight at BMI 22 — near the middle of the healthy band — and the calculator tells you roughly how much to lose or gain to reach the healthy range. To plan how to get there, the daily calorie and TDEE calculator turns that goal into a daily calorie target, and the kg-to-lbs and cm-to-inches converter handles any unit you’re missing.

Because BMI ignores where you carry weight, the second tab calculates your waist-to-hip ratio — waist measurement divided by hip measurement. Abdominal (apple-shaped) fat is more strongly linked to heart-health risk than fat carried on the hips (pear-shaped), so WHR often catches risk that a “normal” BMI misses, and it’s especially useful for the muscular builds where BMI overstates fat. The ratio is unit-independent, so centimetres and inches give the same answer:

Common mistakes when reading your BMI

Rounding your height carelessly. BMI is far more sensitive to height than weight because height is squared — a 3 cm error shifts the result more than a 1 kg change in weight. Measure your height once, properly, before you trust the number.

Reading the category as a diagnosis. Overweight or Underweight is a prompt to look closer — at your waist, body composition, and bloodwork — not a medical verdict on its own.

Mixing units by hand. Keep feet and inches together, or stone and pounds together, and let the toggles convert for you rather than pairing a metric height with an imperial weight yourself. The imperial × 703 factor only works when both height and weight are imperial.

Comparing BMI across very different bodies. Two people at BMI 26 — one sedentary, one a rugby player — carry very different health risk. Pair the number with waist-to-hip ratio before drawing any conclusion.

Judging a single reading. Weight swings day to day with hydration and food. A trend over several weeks tells you far more than any one BMI on any one morning.

Frequently asked questions