Both of these turn your body into a single number, and both take about ten seconds. The difference is what they pay attention to. BMI looks at your weight against your height. Waist-to-height ratio looks at where the weight sits, and for predicting health risk that turns out to matter more.
The two calculations
BMI is weight in kilograms divided by height in metres squared. Someone who weighs 70 kg and stands 1.75 m tall gets 70 divided by 3.06, which is 22.9. In imperial units, multiply pounds by 703 and divide by inches squared.
Waist-to-height ratio is even simpler. Measure your waist, divide by your height, use the same units for both. A 34 inch waist on a 70 inch frame gives 0.49.
The threshold everyone quotes is 0.5. Keep your waist under half your height and you are on the right side of it.
Why waist size carries more information
Fat around the organs behaves differently from fat under the skin. It sits close to the liver and pancreas and is more strongly associated with insulin resistance, type 2 diabetes and heart disease. Fat on the hips and thighs is far less troublesome.
Your waist measurement is a crude but effective proxy for the first kind. BMI cannot distinguish between them at all, because it only sees total weight.
That is the whole argument, and it explains the two cases where BMI fails hardest. The muscular person flagged as overweight who is nothing of the sort. And the person in the normal BMI range carrying very little muscle and a thick middle, whose risk profile is worse than their score suggests. Waist-to-height ratio catches the second one, which is the one that gets missed.
Measuring your waist properly
This is where the method falls apart in practice, because people measure inconsistently and then compare readings that were never comparable.
Use a soft tape. Measure against skin, not over a jumper. Find the point midway between the bottom of your ribs and the top of your hip bones, which is usually around the navel but not always. Stand up straight, breathe out normally, and do not pull the tape tight enough to dent yourself.
Do it at the same time of day, ideally in the morning before eating. Take the same measurement each time or the trend is meaningless.
Where BMI is still fine
It is quick, it needs no tape measure, and it uses two numbers most people already know. For screening large groups it works well, and the research base behind it is enormous.
If your BMI is 35, that number is telling you something real. Context does not make it go away. Where BMI struggles is the middle of the range, from about 25 to 30, where muscle mass and fat distribution decide whether the score means anything at all. Our piece on BMI accuracy goes through the failure cases in detail.
Use both, they take a minute
There is no need to pick a side here. Calculate both, and pay attention when they disagree.
Normal BMI with a ratio above 0.5 is the combination worth acting on, because it says you are carrying more around the middle than your weight suggests. High BMI with a ratio below 0.5 usually means muscle, and it is far less concerning.
Both metrics describe where you are. Neither tells you what to do next, and that is the limit of what any ratio can offer. If you want a number that changes your week, work out your maintenance calories and start from there. If you want to know what your weight is actually made of, our guide to measuring body fat covers the practical methods.
What to do with the answer
Track the trend rather than the reading. One waist measurement means very little. Six months of them means a great deal.
And remember that a tape measure is not a diagnosis. Blood pressure, blood glucose, how much you move and how well you sleep all matter, and none of them show up in either of these numbers.