BMI Calculator

What BMI actually measures

Body mass index compares your weight to your height. That is all it does.

BMI = weight (kg) ÷ height (m)²

Someone 1.75 m tall weighing 70 kg: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9.

In imperial units the sum is 703 × pounds ÷ inches². The 703 is not a mystery constant — it is the metric conversion folded in, so the two forms agree.

BMI was devised in the 1830s by a Belgian statistician studying populations, not patients. That origin explains both its usefulness and its limits: it is a fast way to describe a group, and a blunt way to describe a person.

The standard scale

BMI	Category
Below 18.5	Underweight
18.5 – 24.9	Healthy weight
25.0 – 29.9	Overweight
30.0 – 34.9	Obese, class I
35.0 – 39.9	Obese, class II
40.0 and above	Obese, class III

If you are South Asian, the numbers above are the wrong ones

This is the part most calculators leave out, and it matters enormously for readers in Pakistan, India, Bangladesh and across East Asia.

A WHO expert consultation, published in The Lancet in 2004, examined why type 2 diabetes and heart disease appear at lower body weights in Asian populations. It found that people of Asian descent tend to carry a higher proportion of body fat — and more of it around the abdomen — at any given BMI. The same risk arrives earlier on the scale.

WHO therefore recommends lower action points:

	Standard	Asian populations
Overweight from	25.0	23.0
Obese from	30.0	27.5

The practical consequence: a BMI of 24 is “healthy” on the standard scale and “overweight” on the Asian one. A 1.75 m man weighing 74 kg is comfortably inside the healthy range by one measure and above it by the other.

Use the scale switch above the calculator. If your family background is South Asian or East Asian, the second scale is the one the evidence supports.

Healthy weight ranges by height

Standard scale, from BMI 18.5 up to the 25 threshold:

Height	Healthy weight
1.55 m (5’1”)	44.4 – 60.1 kg
1.60 m (5’3”)	47.4 – 64.0 kg
1.65 m (5’5”)	50.4 – 68.1 kg
1.70 m (5’7”)	53.5 – 72.3 kg
1.75 m (5’9”)	56.7 – 76.6 kg
1.80 m (5’11”)	59.9 – 81.0 kg
1.85 m (6’1”)	63.3 – 85.6 kg

On the Asian scale the upper figure drops by roughly 8%: at 1.75 m the top of the range is 70.4 kg rather than 76.6 kg.

What BMI cannot tell you

Four honest limitations, because a screening number presented as a verdict does real harm:

• It cannot tell muscle from fat. Muscle is denser. A trained athlete can register as obese while carrying very little fat. If you lift seriously, BMI is close to meaningless for you.
• It ignores where the weight is. Fat around the abdomen carries markedly more metabolic risk than fat on the hips and thighs. Two people with identical BMIs can have very different risk profiles.
• It is unreliable at the edges of the population. Children, pregnant women, older adults losing muscle, and people who are very tall or very short are all poorly served by a formula that squares height.
• It says nothing about health behaviour. Fitness, diet quality, sleep, blood pressure and blood sugar all predict outcomes better than BMI alone — and all of them can improve without the number moving.

Measurements worth having alongside it

• Waist circumference. Measured at the navel. Risk rises above roughly 94 cm for men and 80 cm for women, with lower thresholds again for South Asian populations — about 90 cm and 80 cm.
• Waist-to-height ratio. Your waist should be less than half your height. It is simple, works across ethnicities, and often outperforms BMI.
• What your blood says. Blood pressure, fasting glucose and cholesterol describe your actual cardiovascular risk. BMI only estimates it.

If your BMI concerns you

Weight changes slowly and safely at around 0.5 kg a week, which is a deficit of roughly 500 calories a day. Faster than that and you lose muscle along with fat, which lowers the rate at which your body burns energy and makes the loss harder to keep.

Speak to a doctor before starting, especially if you have a health condition or take regular medication. A number from a web page is a prompt to ask a question, not an answer to one.