Track both, for different jobs. BMI (weight in kg divided by height in metres squared) is a quick screening number, but it cannot tell muscle from fat. Waist size shows fat stored around the abdomen, which is linked to higher health risk. The NHS suggests keeping your waist under half your height.
Key takeaways
- BMI is weight in kg divided by height in metres squared; 25 or more is overweight and 30 or more is obesity in adults.
- BMI cannot tell muscle from fat and does not show where fat is stored.
- The NHS advises keeping your waist to less than half your height.
- The NHLBI links a waist over about 89 cm (women) or 102 cm (men) to higher risk.
- Track weight as a weekly average and waist every two to four weeks.
What does BMI actually measure?
Body mass index (BMI) is your weight in kilograms divided by your height in metres squared. Someone who weighs 80 kg (176 lb) and is 1.70 m tall has a BMI of 80 ÷ (1.70 × 1.70) = 80 ÷ 2.89 ≈ 27.7. It takes ten seconds with a calculator, needs nothing but a scale, and is used worldwide, which is exactly why doctors and health agencies rely on it for screening.
For adults, the WHO and the CDC use the same cut-offs:
| BMI | Category (adults) |
|---|---|
| Under 18.5 | Underweight |
| 18.5 to under 25 | Healthy weight |
| 25 to under 30 | Overweight |
| 30 or more | Obesity |
The NHS notes that people from South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean backgrounds are at risk of health problems at a slightly lower BMI. For them it uses 23 to 27.4 as overweight and 27.5 or more as obesity. The CDC calls BMI "a screening measure" that should be considered alongside other factors, not a diagnosis on its own.
What does BMI miss?
BMI only knows your weight and height. It cannot see what that weight is made of or where it sits on your body. The NHS says it plainly: the BMI calculator cannot tell the difference between muscle and fat, so a muscular person can land in the overweight or even obese range without carrying much body fat.
It also misses the opposite case. Someone can have a BMI in the healthy range while storing a fair amount of fat around the abdomen. That matters because where fat is stored affects risk. The US National Heart, Lung, and Blood Institute (NHLBI) explains that if most of your fat is around your waist rather than your hips, your risk of heart disease and type 2 diabetes is higher.
Why does waist size matter?
A tape measure around your middle gives a rough picture of the fat stored in your abdomen, which BMI cannot show. There are two common ways to read it:
- Waist-to-height ratio. The NHS advice is to keep your waist to less than half your height. At 170 cm, that means a waist under 85 cm; at 180 cm, under 90 cm. It works for any height and sex, which makes it easy to remember.
- Fixed waist cut-offs. The NHLBI says a waist of more than 35 inches (about 89 cm) for women or more than 40 inches (about 102 cm) for men increases health risk.
The WHO also lists waist circumference as one of the measures that can be used to identify overweight and obesity alongside BMI.
How do the two compare?
| BMI | Waist size | |
|---|---|---|
| What you need | Scale and height | Tape measure and height |
| What it reflects | Total weight for your height | Fat stored around the abdomen |
| Muscle | Can read muscle as excess weight | Barely affected by muscle elsewhere |
| Day-to-day noise | Moves with water, food and salt | Moves with bloating and how you measure |
| Best use | Quick screening, comparing populations | Spotting risk BMI misses, tracking fat loss |
Three people, same story?
Here is how the numbers can disagree, all for people 170 cm tall:
- Person A: 80 kg, BMI 27.7, waist 80 cm. Waist-to-height ratio 0.47. BMI says overweight; the waist suggests much of that weight may be muscle.
- Person B: 80 kg, BMI 27.7, waist 96 cm. Ratio 0.56. Both measures point in the same direction.
- Person C: 65 kg, BMI 22.5, waist 90 cm. Ratio 0.53. BMI looks fine, but the waist is above half of height.
Neither number is a verdict on your health or your worth. They are signals, and two signals are more informative than one. If they disagree, that is a good reason to talk it through with a doctor rather than to worry.
So which one should you track?
For most adults who want to lose fat, the most useful answer is both, at different speeds:
- Weight, as a weekly average. Weigh yourself under the same conditions, for example in the morning after using the toilet, and look at the average over the week. Daily jumps are mostly water; our guide to water weight versus fat loss explains why.
- Waist, once every two to four weeks. Measure just above your hip bones, right after you breathe out, with the tape level and snug but not pressing into the skin. Use the same spot each time.
- BMI, occasionally. Check it every few months as a broad screening number, not as a weekly score.
A few habits make waist numbers more reliable. Measure at the same time of day, ideally in the morning before eating, because a large or salty meal the evening before can leave you more bloated. Write each number down with the date and compare month to month rather than week to week; very small changes are usually just measurement noise. If you can, ask someone to check that the tape is level at the back.
If you are strength training, the waist can shrink while the scale barely moves. That is often a sign of progress, not a stall. For setting a target that fits your body, see how to set a realistic goal weight.
What brings waist size down?
You cannot choose where fat comes off; as our article on losing belly fat explains, the waist follows overall fat loss. That means a moderate calorie deficit, regular activity, strength training and enough protein. A typical safe pace is about 0.5–1 kg (1–2 lb) a week, and women should not go below about 1,200 kcal a day or men below about 1,500 kcal a day without medical supervision.
Protein-rich meals help you stay full in a deficit. A few options from our meal catalog, as Sporty's per-serving estimates (real values depend on recipe and portion): grilled salmon with arugula at 460 kcal and 40 g of protein, chicken fajitas at 460 kcal and 38 g, chili con carne at 480 kcal and 34 g, and ceviche at 220 kcal and 26 g. Logging meals for a couple of weeks, for example by snapping a photo in Sporty, shows how your days actually add up.
When should you talk to a doctor?
This article is general information, not medical advice. The NHS suggests seeing a GP if your BMI is in the overweight or obese range and your waist is more than half your height, or if you have conditions such as pre-diabetes, diabetes or high blood pressure. Adult BMI cut-offs do not apply to children and teenagers, and neither measure fits well during pregnancy. If tracking numbers makes you anxious, or you have or think you may have an eating disorder, speak to a doctor or registered dietitian before you start measuring.
Frequently asked questions
Is waist size better than BMI?
Neither is better in every case. BMI is a simple screening tool for weight relative to height, while waist size shows fat around the abdomen that BMI can miss. Used together, they give a clearer picture.
What is a healthy waist size?
The NHS suggests keeping your waist to less than half your height, for example under 85 cm at 170 cm tall. The NHLBI says a waist over 35 inches (about 89 cm) for women or 40 inches (about 102 cm) for men increases risk.
How do I measure my waist correctly?
Stand up, breathe out normally and wrap a tape measure around your middle just above your hip bones. Keep the tape level and snug without pressing into the skin, and measure in the same spot each time.
Can I be overweight by BMI but healthy?
BMI cannot tell muscle from fat, so muscular people can fall into the overweight range. A waist under half your height is reassuring, but a doctor can look at the full picture, including blood pressure and blood tests.
Sources
This article is general information, not medical advice. If you have a health condition, are pregnant or under 18, talk to a doctor or registered dietitian before changing your diet.