BMI and Waist-to-Height Ratio: What They Can and Cannot Tell You

BMI is a quick screening number, but it misses a lot. See BMI categories, their limits and why keeping your waist under half your height is a useful check.

Body mass index (BMI) is probably the best-known health number after blood pressure. It is quick and free to calculate, but it is also widely misunderstood. Pairing it with your waist-to-height ratio gives a more useful picture of health risk, especially the risk linked to fat stored around your middle.

You can calculate both in seconds with our free BMI and waist-to-height calculator. Here is how to read the results.

What BMI measures

BMI is your weight in kilograms divided by your height in metres squared. For adults, the World Health Organization uses these categories:

• Below 18.5: underweight
• 18.5 to 24.9: healthy weight range
• 25.0 to 29.9: overweight
• 30.0 and above: obesity

At a population level, higher BMI is linked with higher rates of type 2 diabetes, high blood pressure, heart disease and some cancers. That is why it is used as a first screening step.

What BMI cannot tell you

BMI does not measure body fat directly, and it says nothing about where fat is stored or how healthy you are overall. Its main limits include:

• Muscle: muscle is dense, so athletic people can have a high BMI with little body fat.
• Age: older adults tend to lose muscle and bone, so they may carry more fat at the same BMI.
• Sex: women generally carry more body fat than men at the same BMI.
• Ethnicity: health risks begin at lower BMIs in some populations (see below).
• Pregnancy: BMI is not meaningful during pregnancy. Pre-pregnancy BMI is used instead to guide weight gain.

BMI also cannot capture fitness, blood pressure, blood sugar, cholesterol, sleep, mental health or how you feel day to day. It is a starting point for a conversation, not a verdict.

Lower cut-offs for some populations

People of South Asian, Chinese and other Asian backgrounds tend to develop type 2 diabetes and heart disease at lower BMIs. For this reason, many guidelines, including the UK's NICE, suggest lower thresholds for these groups, with overweight starting at a BMI of 23 and obesity at 27.5. NICE applies the same lower thresholds to people of Middle Eastern, Black African and African-Caribbean family background.

If this applies to you, a "normal" BMI result may still be worth discussing with your provider, particularly if you also have a family history of diabetes or a larger waist.

Waist-to-height ratio: a simple extra check

Fat stored deep in the belly, around the organs, is more strongly linked with type 2 diabetes, high blood pressure and heart disease than fat stored elsewhere. Waist-to-height ratio helps capture this.

In 2022, NICE recommended a simple message for adults: keep your waist measurement to less than half your height. The categories are:

• 0.4 to 0.49: healthy, no increased health risk
• 0.5 to 0.59: increased health risk
• 0.6 or more: high, further increased health risk

For example, a woman who is 165 cm tall would aim for a waist under 82.5 cm. The ratio works across sexes and ethnic backgrounds and is useful for people with more muscle, where BMI can mislead. NICE recommends it mainly alongside BMI for adults with a BMI under 35. As with BMI, it is not meant for use during pregnancy.

How to measure your waist

• Use a flexible tape measure and stand up straight, with your feet together.
• Find the point halfway between the bottom of your lowest rib and the top of your hip bone. For many people this is near the belly button.
• Wrap the tape around your bare skin, keeping it level and snug but not tight.
• Breathe out normally and measure at the end of the breath. Do not suck in.
• Use the same units for waist and height, then divide waist by height.

Using these numbers kindly

Numbers can feel loaded. Neither BMI nor waist-to-height ratio defines your worth or your health on its own. If your results suggest increased risk, small steps such as regular movement, more fibre-rich foods, better sleep and less alcohol can improve blood pressure, blood sugar and how you feel, often before the scale changes much.

When to get medical advice

Talk with your doctor or nurse practitioner if:

• your waist-to-height ratio is 0.5 or higher, or your BMI is outside the healthy range for your background
• you have lost weight without trying, especially if you also feel unwell
• you have gained weight quickly without a clear reason
• you have signs of high blood sugar, such as increased thirst or urination
• food, eating or body image feels distressing or out of control

Your provider can check blood pressure, blood sugar and cholesterol, and refer you to a registered dietitian if helpful.