BMI Calculator

Using the calculator

The calculator takes two numbers, both in metric units. There is no pounds or feet option, so convert first if you use imperial units (see the second example below).

1. Weight (kg) — your weight in kilograms, for example 70 or 68.5.
2. Height (cm) — your height in centimetres, for example 175. Not metres: type 175, not 1.75.
3. Press "Calculate BMI". If either box is empty, you will see a message asking for both values.

You get three results:

• Your BMI, rounded to one decimal place.
• Category — Underweight, Normal, Overweight or Obese.
• Healthy Range — the weight in kg that would give a BMI from 18.5 to 24.9 at your height. This is often the most useful number, because it turns the BMI into a weight you can compare with the scale.

A marker on the colour bar shows where you sit. The bar runs from a BMI of 10 on the left to 45 on the right. Values outside that span stay pinned at the ends.

Worked example in kilograms and centimetres

BMI is weight in kilograms divided by height in metres squared. Take a person who weighs 70 kg and is 175 cm tall.

So the tool shows 22.9, Normal, and a healthy range of 56.7 - 76.3 kg.

Worked example in pounds and inches

Say you weigh 180 lb and are 5 ft 10 in tall. Convert each number before you type it in.

You can check this with the imperial formula: 703 × pounds ÷ inches². Here that is 703 × 180 ÷ (70 × 70) = 126,540 ÷ 4,900 = 25.8. Same answer.

The ranges this calculator uses

These are the standard adult categories from the World Health Organization (WHO). The right-hand column shows what they mean in kilograms for someone 175 cm tall, so you can see how wide each band is.

One small detail: the category is decided on the exact BMI, but the number is rounded for display. At 175 cm, 76.5 kg gives a BMI of 24.98. The tool shows "25.0" but the category stays Normal, because 24.98 is still below 25. The Healthy Range uses 24.9 as its top value, so it ends a little lower, at 76.3 kg.

Common input mistakes

• Height in metres instead of centimetres. Typing 1.75 instead of 175 gives a BMI in the hundreds of thousands. If the result looks impossible, check the height box first.
• Feet written as a decimal. 5 ft 9 in is not 5.9 feet. 5.9 feet is 179.8 cm, but 5 ft 9 in is 175.3 cm. At 70 kg that mistake moves the BMI from 22.8 to 21.6.
• Pounds typed into the kg box. 154 lb entered as 154 kg at 175 cm gives 50.3 instead of the correct 22.9. Divide pounds by 2.205 first.
• Weighing at different times. Body weight can change by a kilogram or more during a day. If you track BMI over time, weigh yourself at the same time of day, on the same scale.

When BMI can mislead you

BMI only uses height and weight. It cannot tell muscle from fat, or where fat is stored. Keep these limits in mind:

• Muscular people. A 90 kg, 180 cm athlete has a BMI of 27.8 and is labelled Overweight, even with low body fat.
• Older adults. People often lose muscle with age. A "Normal" BMI can hide a higher share of body fat. Some research suggests the lowest-risk BMI may be a little higher in older age, so talk to a doctor before aiming to lose weight.
• Children and teens. The adult categories above do not apply under age 20. For ages 2 to 19, BMI is compared with growth charts for the same age and sex, and the result is a percentile, not a category from this table.
• Pregnancy. Weight gain in pregnancy is expected, so current BMI is not meaningful. Doctors usually use BMI from before the pregnancy to guide weight-gain advice.
• Where fat sits. Fat around the waist is linked to higher health risk than fat on the hips. A tape measure can add useful information. One simple guide used in UK health advice is to keep your waist below half your height.

Different cut-offs for some populations

Research shows that many people of Asian background have higher risk of type 2 diabetes and heart disease at a lower BMI than people of European background. A WHO expert group in 2004 suggested extra action points at a BMI of 23 and 27.5 for Asian populations. The UK's NICE guidance uses lower thresholds for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background: overweight from 23 and obesity from 27.5. Some countries set their own national cut-offs too. This calculator uses the standard WHO ranges only, so keep these lower points in mind if they apply to you. At 175 cm, a BMI of 23 is about 70.4 kg.

This page is for general information only. It is not medical advice. Speak to a doctor or other qualified health professional about your own weight and health.