How is BMI calculated?

BMI is your weight divided by your height squared. In metric it's kilograms ÷ metres²; in imperial it's 703 × pounds ÷ inches². A 70 kg, 1.75 m adult has a BMI of 70 ÷ 1.75² = 22.9.

BMI is a screening tool, not a body-fat measurement. It works well across large groups but can mislabel an individual — a muscular athlete and a person carrying excess fat can share a BMI. Treat it as a first flag, not a verdict.

What BMI is considered obese?

A BMI of 30 or above is classed as obesity: class I is 30–34.9, class II is 35–39.9, and class III (severe) is 40 and above.

Is BMI different for men and women?

The formula and the adult category thresholds are the same for men and women. Body composition differs between sexes, which is one reason BMI is a rough screen rather than a precise measure of health.

Does BMI work for children?

Not with these fixed bands. Children and teens are scored against age- and sex-specific percentiles (BMI-for-age), because healthy ranges change as they grow.

How can I lower my BMI?

BMI moves only with weight (at a fixed height), so it changes through sustained changes in diet and activity. The calculator shows the healthy weight range for your height — the target that lands you in the normal band.

Is a higher BMI ever fine?

In older adults a slightly higher BMI can be protective, and very muscular people sit high without excess fat. BMI is a starting point; waist measurement and a clinician's assessment add the context it misses.

Health

BMI Calculator

Enter your weight and height to get your Body Mass Index, your weight category, and the healthy weight range for your height — in metric or imperial, updating as you type.

Your measurements

Mix units freely — kg with feet is fine.

Weight

Height

23.6BMI

Normal weight

1518.5253040

Healthy weight for this height: 125.2–168.7 lb

No sign-up · Free

The basics

What BMI measures

Body Mass Index compares your weight to your height. It's a quick way to screen whether your weight sits in a healthy range for how tall you are — one number, the same formula for every adult. It doesn't measure body fat directly, which is why it's a starting point rather than a full picture.

BMI = kg ÷ m²

The metric formula

Weight in kilograms divided by height in metres, squared.

70 ÷ 1.75² = 22.9

BMI = 703 × lb ÷ in²

The imperial formula

The same idea in pounds and inches — the 703 factor converts the units.

703 × 160 ÷ 69² = 23.6

Method

Work out your BMI by hand

Three steps once your weight and height are in matching units.

1
Get the units consistent
Convert height to metres (or total inches). 175 cm is 1.75 m; 5 ft 9 in is 69 inches.
175 cm → 1.75 m
2
Square the height
Multiply the height by itself — this is the area term the formula divides by.
1.75² = 3.0625
3
Divide weight by it
Divide weight by the squared height. In imperial, multiply by 703 first.
70 ÷ 3.0625 = 22.9

The scale

The BMI categories

The adult thresholds set by the WHO. They're the same bands the calculator and the chart below use to colour every result.

UnderweightBelow 18.5

Normal weight18.5 – 24.9

Overweight25.0 – 29.9

Obesity, class I30.0 – 34.9

Obesity, class II35.0 – 39.9

Obesity, class III40.0 and above

Quick chart

BMI chart by height and weight

The classic at-a-glance grid. Find a weight and a height and read the BMI straight off the cell — colour-coded by category.

Find your weight (down) and height (across) — the cell is your BMI.

lb \ ht	5′0″	5′3″	5′6″	5′9″	6′0″	6′3″
120	23.4	21.3	19.4	17.7	16.3	15
140	27.3	24.8	22.6	20.7	19	17.5
160	31.2	28.3	25.8	23.6	21.7	20
180	35.2	31.9	29	26.6	24.4	22.5
200	39.1	35.4	32.3	29.5	27.1	25
220	43	39	35.5	32.5	29.8	27.5
240	46.9	42.5	38.7	35.4	32.5	30
260	50.8	46.1	42	38.4	35.3	32.5

UnderweightNormal weightOverweightObesity

Worth knowing

Where BMI falls short

BMI is fast and useful at scale, but it misses things a single number can't hold.

muscle weighs more than fat

Muscle reads as mass

BMI counts every kilogram the same, so a muscular body can land in the overweight band without carrying excess fat.

athlete → high BMI, low fat

screen ≠ diagnosis

It flags, it doesn't diagnose

BMI can suggest risk, but it doesn't measure body fat or where that fat sits — the part that actually drives health risk.

high BMI → look closer

thresholds vary

Age and ancestry shift it

Older adults can be healthy at a higher BMI, and some populations face weight-related risk at lower numbers than the standard bands assume.

risk bands differ by group

kids → percentiles

Children are scored differently

For children and teens, BMI is read against age- and sex-specific percentiles, because a healthy range moves as they grow.

BMI-for-age, not fixed bands

Where it shows up

Why BMI is everywhere

One cheap number, computed from two measurements, that scales to whole populations.

first-pass flag

Clinical screening

In a checkup, BMI is the quick screen that decides whether weight is worth a closer look.

cheap at scale

Population health

Because it needs only height and weight, BMI underpins national obesity statistics and public-health research.

threshold gates

Eligibility & cover

BMI cut-offs help decide access to some treatments and programs, and feed into insurance underwriting.

FAQ

Common questions

For adults, a BMI from 18.5 to 24.9 is the normal, healthy range. Below 18.5 is underweight, 25–29.9 is overweight, and 30 or above is in the obesity range.

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