What this calculator works out
Body mass index compares your weight against your height. This calculator divides weight in kilograms by height in metres squared, and shows which of the standard adult categories the result falls into.
BMI is a population screening measure. It was designed to describe groups, not to assess individuals, and it cannot tell the difference between muscle and fat. What it is good for, and what it is not, is set out below.
Enter your details
Select Calculate to see where a result falls on this scale.
- Below the healthy range 14 to 18.5
- Healthy range 18.5 to 25
- Overweight range 25 to 30
- Obese range 30 to 45
Before you read the result
- Use height in centimetres and weight in kilograms. The calculator converts height to metres itself.
- Weigh yourself at a consistent time — normally first thing, before eating. Weight varies by a kilogram or more across a day.
- The standard categories apply to adults. BMI is interpreted differently for children and is not used during pregnancy.
- Lower thresholds are recommended for people of South Asian, Chinese, Black African and African-Caribbean family background, because health risks appear at a lower BMI.
Body mass rises roughly with volume, which scales with the cube of height, while BMI divides by height squared. The mismatch is deliberate — squaring produced the best statistical fit across a population — but it means BMI slightly overstates readings for tall people and understates them for short people.
How this calculator works
One division, then a comparison against fixed thresholds:
BMI = weight in kg ÷ (height in metres)²
Under 18.5 — below the healthy range
18.5 to 24.9 — healthy range
25 to 29.9 — overweight range
30 and above — obese rangeThese thresholds come from World Health Organization definitions and are used across the NHS. They are population cut-offs rather than individual diagnoses, and they are the same regardless of age or sex for adults.
The calculator does not adjust for ethnicity, muscle mass, pregnancy or age, all of which change how a given BMI should be read.
Worked example: 75 kg at 175 cm
Using the default figures — 75 kg and 175 cm:
- Height in metres: 1.75, squared = 3.0625
- BMI: 75 ÷ 3.0625 = 24.5
- Category: healthy range, close to the upper boundary
Now consider two people with that identical result. A regular weight trainer at 75 kg may carry very little body fat; someone with little muscle at the same height and weight may carry a good deal more. BMI reports them identically. That is not a flaw in the arithmetic — it is what happens when a single ratio is asked to describe body composition it cannot see.
Common mistakes
- Treating BMI as a measure of health. It is a ratio of two measurements and nothing more.
- Applying adult categories to children. Children are assessed against age and sex percentile charts instead.
- Using it during pregnancy. It is not meaningful and is not used clinically for that purpose.
- Ignoring the ethnicity adjustment. Standard thresholds understate risk for several population groups.
- Tracking it daily. Normal weight fluctuation makes short-term changes meaningless.
Frequently asked questions
Is BMI actually useful?
At population level, yes — it is cheap, consistent and correlates well with health outcomes across large groups, which is why public health bodies use it. For an individual it is a rough screening figure. It works best as one input alongside waist measurement, blood pressure, blood tests and how you actually feel, which is how a GP would use it.
Why does it not work for muscular people?
Muscle is denser than fat, so a very muscular person weighs more for the same body size. BMI reads weight only, so athletes and regular strength trainers frequently land in the overweight or obese categories while carrying little fat. The waist-to-height ratio handles this better, because it measures where the tissue is rather than how much you weigh.
What should someone of South Asian background use?
NHS guidance recommends lower thresholds for people of South Asian, Chinese, Black African and African-Caribbean family background, because the risk of type 2 diabetes and cardiovascular disease rises at a lower BMI. The Healthy Weight Range Calculator has editable BMI limits so you can use thresholds appropriate to your background.
My BMI has not changed but I look different. Why?
Because body composition changed while weight did not. Gaining muscle and losing fat at a similar rate leaves BMI static while the body underneath is measurably different. This is one of the more common reasons BMI frustrates people who are actually making progress.
Is my weight information stored?
No. Height and weight are processed entirely in your browser and are never transmitted or retained. If thinking about weight, food or body shape is causing you distress, these tools are unlikely to help and may make things harder. Beat, the UK eating disorder charity, runs a free helpline and online support, and a GP can refer you to specialist services.
Related tools
References
- NHS — BMI thresholds, the ethnicity adjustment and healthy weight guidance
- World Health Organization — the international BMI classification these categories come from
Sources are checked at publication and can change — how I choose and check references.