BMI Calculator (UK / NHS)

How to use the BMI calculator

Choose metric or imperial units, then enter your height and weight. The calculator instantly works out your Body Mass Index (BMI), tells you which NHS weight category you fall into, and shows the healthy weight range for someone of your height. It takes a few seconds and doesn't store or send your details anywhere.

BMI is one of the most widely used screening tools in UK healthcare — it's the measure GPs, practice nurses and the NHS itself use as a quick, first-pass indicator of whether someone's weight might be putting their health at risk, before considering other factors in more detail.

The BMI formula

BMI is calculated as your weight in kilograms divided by the square of your height in metres:

BMI = weight (kg) ÷ height (m)²

If you enter your details in imperial units — feet, inches, stone and pounds — the calculator converts them to metric behind the scenes before applying the same formula, so you get an identical result either way.

NHS BMI categories

The NHS uses the following standard categories for adults:

• Below 18.5 — Underweight
• 18.5 to 24.9 — Healthy weight
• 25 to 29.9 — Overweight
• 30 to 34.9 — Obese (Class I)
• 35 to 39.9 — Obese (Class II)
• 40 and above — Obese (Class III)

Worked example

Take someone who is 1.70m tall and weighs 70kg. Their BMI is 70 ÷ (1.70 × 1.70) = 70 ÷ 2.89, which comes to approximately 24.2 — within the healthy weight range. For their height, a healthy weight would fall roughly between 53.5kg and 72.0kg (18.5 to 24.9 multiplied by 2.89).

In imperial terms, that's the same as someone around 5'7" weighing 11 stone — the calculator converts feet, inches, stone and pounds into the metric figures needed for the formula automatically.

Why BMI has limitations

BMI is a useful population-level screening tool precisely because it's simple — but that simplicity is also its biggest limitation. It can't tell the difference between weight from muscle and weight from fat, so very athletic or muscular people can register a “high” BMI despite having a low body fat percentage. It also doesn't account for where fat is carried on the body, which matters for health risk — this is why the NHS often recommends pairing a BMI check with a waist measurement, particularly for people in the overweight category.

The NHS also highlights that some ethnic groups — including people of South Asian, Chinese, Black African and African-Caribbean heritage — can face a higher risk of conditions like type 2 diabetes and heart disease at a lower BMI than the standard thresholds suggest, so healthcare professionals sometimes apply adjusted, lower cut-off points for these groups.

What to do with your result

A BMI result on its own isn't a diagnosis — it's a starting point. If your BMI falls outside the healthy range, or you have any concerns about your weight, the best next step is a conversation with your GP or practice nurse, who can look at the fuller picture alongside factors like waist circumference, activity levels, family history and any existing health conditions.

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