BMI Calculator: Body Mass Index in Seconds

About this BMI Calculator

This calculator turns two numbers, your Weight (kg) and your Height (cm), into a body mass index and names the standard WHO adult category for it. You do not need to convert centimetres into metres. The calculator does that and squares the height for you. The result updates as you type, so there is no button to press.

It is for adults who want a quick screening figure, and for students working on health or biology exercises. Parents sometimes try it too, but children and teenagers need age-specific charts, which this tool does not provide. It is also useful when someone wants to understand what a doctor or a fitness form means when it quotes a BMI.

People usually look up BMI before a health check, when filling a form, when starting or ending a fitness plan, or after a doctor mentions it. It is a convenient first number, cheap and quick to find. It is also easy to over-read, which is why this page spends as much time on what BMI cannot tell you as on how to calculate it.

How to use the BMI Calculator, step by step

1. Weigh yourself properly. Use the same scale each time, ideally in the morning, in light clothing and before eating. Round to the nearest tenth of a kilogram if the scale allows. Weight changes during the day, so a consistent routine gives you more comparable numbers.
2. Measure your height in centimetres. Stand straight against a wall without shoes, heels together, and mark the top of your head. Measure from the floor in centimetres. If you know your height in feet and inches, multiply total inches by 2.54.
3. Enter Weight (kg). Type your weight in the Weight (kg) box. If your scale shows pounds, divide by 2.2046 first. The default value of 70 is just a placeholder, so replace it with your own number.
4. Enter Height (cm). Type your height in the Height (cm) box, such as 168. Do not type metres, because 1.68 would be read as 1.68 cm and give an absurd result. The calculator converts centimetres to metres on its own.
5. Read the BMI and Category. BMI shows your index to one decimal place. Category names the WHO band: Underweight, Normal weight, Overweight or Obesity. Note that the calculator uses the standard WHO cut-offs for adults, not the lower cut-offs suggested for Asian populations.
6. Use the result as a starting point. Treat the number as a prompt, not a verdict. Look at the weight range table for your height, think about your waist, fitness and family history, and bring any concerns to a doctor who can look at the full picture.

The formula behind the BMI Calculator

BMI compares your weight with the square of your height. Using metric units, the formula needs no extra constants.

BMI = Weight (kg) ÷ Height (m)²
Height (m) = Height (cm) ÷ 100
Weight for a target BMI = BMI × Height (m)²

What each symbol means

BMI

Body mass index, in kg per square metre.

Weight (kg)

Body weight in kilograms.

Height (m)

Height in metres, found by dividing centimetres by 100.

Height (m)²

Height multiplied by itself.

How the formula works

The formula squares height because body size grows in more than one direction. If two people are built in the same proportions and one is taller, the taller person is wider and deeper as well as longer, so weight rises faster than height. Dividing by height squared is a simple way to adjust for this. It is a rough fix, which is one reason BMI is a screening number and not a measure of health.

The categories are cut at fixed points. For adults, the WHO bands are below 18.5 for underweight, 18.5 to 24.9 for normal weight, 25 to 29.9 for overweight, and 30 or above for obesity. They are population-level markers linked to health risk, not sharp lines. Someone at 24.8 is not meaningfully different from someone at 25.1. The calculator applies these cut-offs exactly as written.

BMI has real limits. It cannot tell muscle from fat, so a muscular athlete can score high without carrying much extra fat. It does not show where fat sits on the body, and it ignores age, sex and ethnicity. Older adults can lose muscle while their weight stays the same. Children and teenagers must use growth charts based on age and sex, and pregnancy changes weight for good reasons, so adult BMI does not apply in the usual way.

Guidance for Asian populations notes that health risks can begin at lower BMI levels. WHO Asia-Pacific and Indian guidance uses lower cut-offs, with overweight from about 23 and obesity from about 25. This calculator shows the standard WHO categories only, so if you are of South Asian origin, a reading in the upper part of the normal band may still deserve a conversation with your doctor. Waist size and other tests give a fuller picture than BMI alone.

Worked examples

Slightly below the normal range

Divya is 165 cm tall and weighs 48 kg. She wants to know her category and how far she is from the normal range.

1. Height in metres: 165 ÷ 100 = 1.65.
2. Height squared: 1.65 × 1.65 = 2.7225.
3. BMI = 48 ÷ 2.7225 = 17.63, shown as 17.6.
4. The normal range starts at 18.5. The weight for BMI 18.5 is 18.5 × 2.7225 = 50.37 kg.
5. Gap: 50.37 − 48 = 2.37 kg.

Result: BMI 17.6, which falls in the Underweight band; about 2.4 kg below the lower edge of the normal range for her height.

Above the normal range

Arjun is 172 cm tall and weighs 82 kg. He wants to see how much would bring him to the top of the normal range.

1. Height in metres: 172 ÷ 100 = 1.72. Height squared: 1.72 × 1.72 = 2.9584.
2. BMI = 82 ÷ 2.9584 = 27.72, shown as 27.7.
3. The normal range ends at 24.9. The weight for BMI 24.9 is 24.9 × 2.9584 = 73.66 kg.
4. Difference: 82 − 73.66 = 8.34 kg.

Result: BMI 27.7, which is in the Overweight band; the top of the normal range for 172 cm is about 73.7 kg, roughly 8.3 kg below his current weight.

Finding a healthy weight range for a height

Sana is 168 cm tall and wants the weights that correspond to the WHO normal range, from 18.5 to 24.9.

1. Height in metres: 168 ÷ 100 = 1.68. Height squared: 1.68 × 1.68 = 2.8224.
2. Lower weight: 18.5 × 2.8224 = 52.21 kg.
3. Upper weight: 24.9 × 2.8224 = 70.28 kg.

Result: For 168 cm, weights from about 52.2 kg to 70.3 kg fall in the normal BMI range.

Reference tables

Weight ranges by height for adults

Weights are computed from BMI × height squared. The first two columns give the WHO normal range of 18.5 to 24.9. The last column shows the weight at a BMI of 23, the point where Asian-population guidance starts to treat weight as overweight.

WHO adult BMI categories used by the calculator

These are the standard bands applied to adults aged 18 and above. The calculator treats anything below 25 and at or above 18.5 as normal weight.

Common mistakes and tips

Typing height in metres or feet

The Height (cm) field needs centimetres. Typing 1.7 or 5.7 produces a nonsense result. Convert feet and inches first: 5 feet 7 inches is 67 inches, and 67 × 2.54 = 170.2 cm.

Treating BMI as a diagnosis

BMI is a screening figure. A high or low number says a conversation might be useful. It does not say you are unwell or healthy. Doctors combine it with waist measurement, blood tests, history and symptoms.

Using adult BMI for children

Children and teenagers are growing, so their BMI is read against age- and sex-specific growth charts. The adult categories here do not apply. If you are worried about a child, speak to a paediatrician or the school health service.

Ignoring muscle mass

Strength athletes and very active people can have a high BMI with a low body-fat percentage. If that is you, other measures such as waist size or body composition checks are more informative than this number.

Chasing a single number

Weight moves with water, food and time of day. A change of one or two kilograms from one day to the next is ordinary. Look at the trend over weeks, not a single weigh-in, and avoid crash methods.

Forgetting the Asian cut-offs

If your family background is South Asian, the standard categories may understate risk at a given BMI. Ask your doctor how the lower guidance cut-offs, around 23 for overweight, apply to you.

Where people use this calculator

Preparing for a health check

Knowing your BMI before a visit lets you ask better questions. Combine it with a waist measurement and your family history, and let the doctor interpret the full picture.

Setting a sensible weight range

Use the table or the formula to find the weights that match the normal range for your height. A range is more realistic than one exact target and avoids fixating on a number.

Following progress over time

If you are changing your routine, calculate BMI every few weeks with a consistent scale. Notice trends, and also track other signs such as energy, sleep and fitness.

Studying health and biology

Students can use BMI to practise unit conversion, squaring and ratios, and to discuss why simple indices are useful but limited. The worked examples above can be used as practice problems.

Understanding forms and reports

Insurance, gym and school forms often ask for BMI, and fitness reports quote it. This calculator lets you check the figure and see which category it falls into.

Frequently asked questions

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Results are estimates for general information. See our disclaimer and about page for how we build and check these tools.

Last reviewed: October 2026. Spotted a mistake? Tell us on the contact page.