BMI Calculator — Free Online Body Mass Index Calculator
Calculate your Body Mass Index (BMI) and see which category you fall into. Supports metric and imperial units. All calculations happen locally — nothing leaves your browser.
What is BMI? Body Mass Index is your weight in kilograms divided by your height in metres squared (BMI = kg/m²), a single number used to screen adults for underweight, healthy weight, overweight and obesity. The World Health Organization treats 18.5–24.9 as the healthy adult range, 25.0–29.9 as overweight and 30.0 or above as obese. BMI estimates body size, not body fat, and is a screening figure rather than a diagnosis.
How to Use the BMI Calculator
- Pick a unit system — Switch between metric (kg/cm) and imperial (lbs/ft+in). Both paths produce the same BMI — only the conversion constant differs.
- Enter your height — Measure without shoes, standing straight against a wall. A 2 cm error shifts an adult BMI by roughly half a point, so measure rather than estimate.
- Enter your weight — Weigh yourself at the same time of day, ideally in the morning before eating. Swings of 1–2 kg across a day are fluid, not fat.
- Read the BMI and category — The large figure is your BMI to one decimal place, colour-coded by category, with BMI Prime (your BMI divided by 25) shown alongside it.
- Check your healthy weight range — The Ideal Weight card converts the 18.5–24.9 band back into kilograms or pounds for your exact height, so you get a target range rather than a single number.
- Copy the result — Copy Result puts the BMI, category and range on your clipboard for a training log or a note to bring to a checkup.
How the BMI Formula Works
BMI was devised in the 1830s by the Belgian statistician Adolphe Quetelet, who was describing populations rather than individuals — which is why it is still sometimes called the Quetelet Index. The metric definition is one division:
BMI = weight (kg) / height (m)²At 70 kg and 1.75 m that is 70 / (1.75 × 1.75) = 70 / 3.0625 = 22.9. The imperial form multiplies by 703 to convert pounds and inches into the same scale:
BMI = weight (lb) × 703 / height (in)²At 154 lb and 69 in that is 154 × 703 / 4,761 = 22.7 — the small gap from the metric example is rounding in the height, not a different method. Because height is squared, height errors matter about twice as much as weight errors: getting your height wrong by an inch changes the result more than being a pound off on the scale.
BMI Prime and the ideal weight range
BMI Prime is simply your BMI divided by 25, the top of the healthy band. A BMI Prime of 0.92 means you are at 92% of the overweight threshold; anything above 1.0 means you are past it. It is a quick ratio for people who do not want to memorise category boundaries, and it makes progress easy to read — moving from 1.16 to 1.04 is visibly the same distance as moving from 29 to 26.
The Ideal Weight figure runs the formula backwards. Multiplying 18.5 and 24.9 by your height in metres squared gives the weight band that lands inside the healthy category: at 1.75 m that is 56.7 kg to 76.3 kg. A range that wide is the point — there is no single correct weight for a given height, and two people at the same BMI can carry very different amounts of muscle.
BMI Categories
| Category | BMI Range | BMI Prime |
|---|---|---|
| Underweight | Below 18.5 | Below 0.74 |
| Normal | 18.5 – 24.9 | 0.74 – 1.0 |
| Overweight | 25.0 – 29.9 | 1.0 – 1.2 |
| Obese (Class I) | 30.0 – 34.9 | 1.2 – 1.4 |
| Obese (Class II) | 35.0 – 39.9 | 1.4 – 1.6 |
| Obese (Class III) | 40.0 and above | 1.6 and above |
What BMI Cannot Tell You
The formula has only two inputs, so everything it cannot see follows from that. BMI does not know whether a kilogram is muscle, fat, bone or water, which is why a heavily trained rugby player and a sedentary person of the same height and weight get an identical number. It does not know where fat sits, and abdominal fat behaves differently from fat on the hips and thighs. It does not know your age, sex or frame size, all of which change what a given weight means.
The categories are also least reliable at the edges of the population they were drawn from. Older adults lose muscle and bone with age, so a stable BMI can hide a real change in body composition. Pregnancy makes the number meaningless. Children and teenagers are still growing and are assessed against age-and-sex percentile charts instead of fixed adult cut-offs. Risk at a given BMI also varies between ethnic groups: several health authorities apply a lower overweight threshold — around 23 — for people of South Asian, Chinese and other Asian backgrounds.
This calculator is an informational screening tool, not a medical assessment. A BMI outside the healthy range is a prompt for a conversation, not a diagnosis — talk to a doctor or a registered dietitian before acting on it, especially if you are pregnant, under 18, an athlete, or managing a health condition.
Measurements Worth Taking Alongside BMI
BMI is cheap and repeatable, which is exactly why it survives — but it is most useful as one line in a short list. Waist circumference, measured at the midpoint between the lowest rib and the top of the hip bone, captures abdominal fat that BMI ignores. Waist-to-height ratio is easier still: a common rule of thumb is keeping your waist under half your height, which needs no chart and works across most adult heights.
Body fat percentage, from skinfold calipers, bioimpedance scales or a DEXA scan, answers the question BMI dodges — how much of your mass is fat. Bioimpedance readings drift with hydration, so compare them under the same conditions each time rather than across devices. Alongside those, resting blood pressure and standard blood work say more about cardiometabolic risk than any single body measurement.
The practical approach is to track trend rather than absolute value. A BMI that has moved from 27.4 to 26.1 over a training block while your waist dropped 4 cm tells a coherent story. A single reading on a single day tells you very little, which is the main reason BMI gets over-interpreted.
Frequently Asked Questions
The World Health Organization defines the healthy adult range as 18.5 to 24.9. Below 18.5 is classed as underweight, 25.0 to 29.9 as overweight, and 30.0 or above as obese, split into three classes at 35 and 40. These cut-offs apply to adults aged 18 and over and are the same for men and women. Children are assessed against age-and-sex percentile charts instead.
No. BMI uses only height and weight, so it cannot separate muscle from fat. A muscular athlete can land in the overweight band while carrying very little fat, and an older adult who has lost muscle can sit in the healthy band while carrying a lot. It is also unsuitable during pregnancy and for anyone under 18. Treat it as a screening number that flags who might benefit from a closer look.
No. The formula runs in JavaScript in your browser, and nothing you type is transmitted, logged or stored. There is no account, no cookie holding your measurements, and closing the tab clears everything. That also means the page works offline once loaded, and you can read the source to confirm what it does.
Multiply your weight in pounds by 703, then divide by your height in inches squared. The 703 constant converts lb/in² into kg/m², so the imperial and metric formulas produce the same result for the same body. For 154 lb at 5 ft 9 in (69 inches): 154 × 703 / 4,761 = 22.7. Switching the unit pills on this page does the conversion for you.
BMI Prime is your BMI divided by 25 — the upper edge of the healthy range. A value of 0.85 means you are at 85% of the overweight threshold, and anything above 1.0 means you have crossed it. It expresses the same information as BMI as a proportion, which makes comparisons between people of different heights and progress over time easier to read at a glance.
They answer different questions. BMI measures mass relative to height and takes five seconds; body fat percentage measures composition but needs calipers, a bioimpedance scale or a DEXA scan, each with its own error margin. If you are lifting weights or losing weight, BMI alone can mislead — pair it with a waist measurement, which is nearly as easy and reflects abdominal fat that BMI cannot see.
Not with these categories. Body composition changes throughout growth, so paediatric BMI is plotted on age-and-sex growth charts and reported as a percentile — the same BMI can be normal at 8 and high at 16. Use the charts your national health service or paediatrician provides rather than the adult cut-offs shown here.
Usually inputs, not maths. Clinic scales record clothing and shoes, height is measured against a stadiometer rather than estimated, and results are often rounded to a whole number. A 2 cm difference in height or 1 kg of clothing is enough to move BMI by half a point and can flip a borderline reading between categories. Re-measure barefoot and use the same scale each time for comparable numbers.
Use Cases
Tracking a Training Block
Recalculate at the start and end of a 12-week programme to see whether weight moved in the direction you intended, and read it next to a waist measurement so muscle gain is not mistaken for a setback.
Preparing for a Checkup
Arrive at an annual physical with your own figure already in hand, so the appointment starts from a shared number and you can spend the time on what to do about it rather than on the arithmetic.
Setting a Realistic Target Weight
Use the ideal-weight band for your height to pick a goal inside the healthy range instead of an arbitrary round number, then work backwards to a rate of change you can sustain for months.
Reading Forms in Another Unit System
Fill in a gym induction form, a visa medical or an insurance questionnaire that asks for kg/cm when you think in lbs and feet — switch the unit pills and read both the measurements and the BMI without converting by hand.
Teaching the Formula
Show a maths or PE class how a squared denominator behaves: change the height by one centimetre and watch the result move further than a one-kilogram weight change does, which explains why measurement care matters.