Is BMI accurate for women?

What BMI gets right

It's easy to be dismissive of BMI, so it's worth starting with why it's still used everywhere from GP surgeries to the NHS Health Check.

• It needs nothing but a tape measure and scales, and it takes seconds.
• Across whole populations, a higher BMI really does track with higher rates of type 2 diabetes, heart disease and several cancers.
• For most people who aren't unusually muscular, it's a fair first guess at whether weight might be affecting their health.

The trouble starts when a number designed for populations gets treated as a precise verdict on one person.

Where it goes wrong for individual women

It can't tell muscle from fat

Muscle is denser than fat, so a woman who lifts weights or plays a lot of sport can land in the overweight band of the BMI chart while carrying very little fat. The reverse happens too. Someone with little muscle can sit comfortably in the healthy band with more body fat than is good for her.

It can't see where fat sits

Fat around the tummy is more closely linked to health problems than fat on the hips and thighs. BMI treats both the same. This matters more as you get older, because around the menopause fat tends to move towards the middle even if your weight doesn't change.

The same number means different things for different people

The risk of type 2 diabetes rises at a lower BMI for people of South Asian, Chinese, other Asian, Middle Eastern, Black African and African-Caribbean backgrounds. That's why NICE uses lower thresholds for these groups: overweight from 23 and obesity from 27.5.

It doesn't work for everyone

BMI isn't designed for pregnancy, and it needs age-specific centile charts for girls under 18. It's also why our calculator is meant for women aged 18 to 65.

Body fat percentage for women

Body fat percentage is the share of your weight that's fat. Women need more of it than men for hormones and reproduction, so the healthy ranges sit higher. The categories most widely used come from the American Council on Exercise:

"Essential fat" is the minimum your body needs to function. Dropping near it isn't a fitness goal. For women it's often linked with periods becoming irregular or stopping.

Ways to measure body fat

None of these is perfect, and they often disagree with each other by several percentage points. What matters more is using the same method each time and watching the trend.

• DEXA scan. A low-dose X-ray that maps fat, muscle and bone. It's the most accurate option you're likely to come across, but it's mainly used in research and private clinics rather than routine NHS care.
• Smart scales. These pass a tiny current through your body and estimate fat from how easily it travels. They're convenient but sensitive to how hydrated you are, so the reading can shift a lot from morning to evening.
• Skinfold callipers. A trained person pinches and measures fat at several points. Fairly good in experienced hands, much less so when you do it yourself.
• Tape-measure formulas. Methods such as the US Navy formula estimate body fat from your height, neck, waist and hip measurements. Rough, but free and repeatable.

So which number should you trust?

For most women, the most useful pairing is BMI plus waist-to-height ratio. You can do both at home in five minutes, and both have clear, evidence-based cut-offs. Body fat percentage is interesting extra detail if you have a reliable way to measure it, but you don't need it to get a good read on your health.

When BMI and your waist disagree, the waist measurement usually tells you more. Our waist-to-height ratio guide explains how to measure it properly and what to do with the result.