BMI Calculator
BMI is the quickest screening number in medicine: your weight scaled to your height. Enter yours below, in kilograms and centimetres, and you get your body mass index, your category, and the healthy weight range for your height. You also get something most calculators skip: an honest read on what this number cannot tell you, written by a physiotherapist who weighs and measures people every clinic day.
Your result
The standard World Health Organization formula, shown in full below. Nothing you type leaves this page.
How to use this BMI calculator
Three steps, under a minute. The calculator updates as you type, so you can also test target weights to see where your BMI would land before you commit to a goal.
Measure properly
Weigh yourself in the morning, after the bathroom, in light clothing. Measure height barefoot against a wall. Small errors in height move BMI more than small errors in weight, because height is squared in the formula.
Enter your numbers
Kilograms and centimetres by default, pounds and feet with the unit switch. Add your age and sex: they do not change the maths, they change how the result should be read.
Read the whole result
Your category, your healthy range and the notes below matter more than the single number. Then check your waist: the section further down explains why it completes the picture.
BMI categories for adults
These are the standard World Health Organization bands for adults aged 20 and over. They were built from population data linking BMI ranges to disease risk, which is why they work well for groups and only roughly for individuals.
| Category | BMI | What it usually means |
|---|---|---|
| Underweight | Below 18.5 | Too little mass for your height; muscle and bone are often part of what is missing |
| Normal weight | 18.5 to 24.9 | The range linked with the lowest average health risk in adult populations |
| Overweight | 25.0 to 29.9 | Above the healthy range; joint load and metabolic risk start climbing |
| Obese, class I | 30.0 to 34.9 | Obesity; knee, back and blood sugar risk rise steeply |
| Obese, class II | 35.0 to 39.9 | Higher risk again; supervised plans work better than solo dieting |
| Obese, class III | 40.0 and above | The highest band; structured medical support is the sensible first step |
One caution before you judge yourself by this table: the bands were drawn mostly from European population data. For South Asian adults, diabetes and heart risk climb at lower BMIs, which is why many clinicians here start paying close attention from a BMI of 23, and treat 27.5 the way this table treats 30. If you are South Asian, read your result as slightly more serious than the label suggests.
What BMI misses
BMI was designed in the 1830s as a tool for population statistics, and it still does that job well. What it has never been able to do is look inside the body. It counts kilograms. It cannot tell muscle from fat, bone from water, or a healthy frame from a risky one. Four situations where the number misleads:
Muscle mass and athletes
Muscle is denser than fat, so a strength athlete or a physical labourer can land in the overweight band with low body fat and excellent health. I see this in clinic with kabaddi players and gym regulars whose blood work is perfect, and whose knees are loaded by sport, not by fat. If you train hard and your waist is lean, treat a high BMI as a prompt to check body composition, not as a verdict.
Pregnancy
BMI is meaningless as a target during pregnancy, because the weight includes the baby, the placenta and extra fluid. Clinicians use your pre-pregnancy BMI to set healthy weight gain ranges instead. This calculator is not for use while pregnant.
Older adults losing muscle
After 60, the trap runs the other way. Weight stays stable while muscle quietly shrinks and fat takes its place, a pattern called sarcopenia. The BMI still reads normal while strength, balance and bone protection drain away. For older adults, grip strength, sit to stand speed and waist size say more than BMI does. A slightly higher BMI with strong legs is often safer than a textbook BMI with weak ones.
Children and teens
Growing bodies change proportions month by month, so adult bands do not apply under 20. Children and teenagers are plotted on percentile charts that compare them with others of the same age and sex. If you are checking a young person, this is the wrong tool; a paediatrician or a percentile calculator is the right one.
Beyond BMI: check your waist
If BMI is the headline, waist size is the article. Fat carried around the abdomen, called visceral fat, is the fat most closely tied to diabetes, heart disease and fatty liver, and a tape measure finds it better than a scale does.
Measure at the midpoint between your lowest rib and the top of your hip bone, tape snug, stomach relaxed, at the end of a normal breath out. Two minutes, no maths beyond halving your height. A high BMI with a waist inside the line is a different risk picture from the same BMI with a waist outside it, and the waist is the number you can actually watch move, week to week.
Turn the number into a plan
A BMI result tells you where you stand. What you do next depends on the direction you need to travel, and both of these tools are free:
TDEE Calculator
Your maintenance calories are the base for any weight change. This works yours out from your age, size and activity level.
Macro Calculator
Once calories are set, protein comes first. This turns your target into daily protein, carb and fat grams, with the formula shown.
Book an assessment
Losing or gaining weight with knee or back pain needs a plan your joints can survive. We will build one around your body, not a template.
Start with the TDEE calculator for your calorie target, then split it with the macro calculator. Both keep the maths on the page, the same way this one does.
BMI calculator: frequently asked questions
What is a healthy BMI for adults?
For adults aged 20 and over, a BMI between 18.5 and 24.9 is classed as the healthy range. It is a screening band drawn from population risk data, not a personal diagnosis. Plenty of healthy people sit just outside it, and plenty of unwell people sit inside it, which is why this page pairs your BMI with your waist measurement and a healthy weight range rather than stopping at the label.
How is BMI calculated?
Divide your weight in kilograms by your height in metres squared. Take a person who weighs 70 kg and stands 1.75 m tall: 70 divided by (1.75 times 1.75) equals 22.9, inside the healthy range. This calculator does the same maths in imperial units after converting pounds and inches to metric first.
Is BMI accurate for athletes and muscular people?
Often not. The formula cannot separate muscle from fat, and muscle is heavy for its size, so athletes, soldiers and manual workers are regularly classed overweight, or even obese, at low body fat. If that sounds like you, judge by waist size, fitness and blood markers instead, or get a body composition test. The Beyond BMI section on this page explains the waist check.
Can I use this BMI calculator for a child or teenager?
No. Under 20, BMI has to be read as a percentile against growth charts for the same age and sex, because children’s proportions change as they grow, and adult cutoffs can mislabel healthy teenagers. Use a child BMI percentile calculator, or ask your paediatrician at the next visit.
What BMI should I aim for during pregnancy?
Do not aim for a BMI during pregnancy at all. Clinicians use your pre-pregnancy BMI to set a recommended total weight gain range for the pregnancy. Eat for the pregnancy you are in, follow your midwife or obstetrician on the numbers, and come back to this calculator a few months after delivery if you want a reference point.
My BMI is high but I feel fine. Should I be worried?
Treat it as a flag to check further, not as a sentence. Run the waist check on this page, and look at blood pressure, fasting sugar and cholesterol if you have recent results. If your waist is under half your height and those markers are healthy, your risk is lower than the BMI alone suggests, but it is not zero, and it tends to climb with age. If your waist is over the line too, that combination is worth acting on now, while change is still easy.
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