Why the ratio matters more than the weight
Where you store fat predicts metabolic risk better than how much you weigh. Visceral fat, packed around the organs inside the abdomen, is metabolically active: it releases free fatty acids and inflammatory signals straight into the portal circulation, which drives insulin resistance, raises triglycerides and pushes blood pressure up. Fat stored subcutaneously on the hips and thighs does far less of this. Waist-to-hip ratio is the cheapest way to tell the two patterns apart, and in the INTERHEART study of about 27,000 people across 52 countries it predicted heart attack risk better than BMI did.
The calculation is simply waist circumference divided by hip circumference, with both measured in the same units, so the result is the same whether you work in inches or centimeters. The World Health Organization treats 0.90 or above in men and 0.85 or above in women as substantially increased risk. Ratios below 0.90 for men and 0.80 for women are considered low risk, with a moderate band in between.
A worked example
A man with a 34-inch waist and 39-inch hips has a ratio of 34 / 39 = 0.87, which is below the 0.90 cutoff and in the low-risk band. If his waist grew to 38 inches with no change at the hips, the ratio would rise to 0.97, and at 40 inches it would be 1.03, squarely in the high-risk range. A woman with a 31-inch waist and 40-inch hips has a ratio of 0.78, comfortably under the 0.85 line.
Add height and you get a third, arguably better, number. Waist-to-height ratio in the example is 34 / 69 = 0.49. The rule of thumb is to keep your waist under half your height: below 0.50 is the healthy band, 0.50 to 0.59 signals increased central fat and 0.60 or more substantially increased risk. Because it scales automatically, the same threshold works across heights, sexes and most ethnic groups, and the UK's NICE guidance now recommends it as a first-line screen.
Measuring properly, and the limits
Use a flexible tape, stand relaxed with feet together and arms at your sides, and read the measurement at the end of a normal breath out. The waist is measured either at the navel or midway between the lowest rib and the top of the hip bone; the hips at the widest part of the buttocks. Keep the tape horizontal and snug without denting the skin, do not suck in, and use the same landmarks every time so the numbers are comparable over months. Measuring twice and averaging removes most of the error.
Absolute waist circumference carries its own thresholds: the NIH flags more than 40 inches (102 cm) in men and more than 35 inches (88 cm) in women as increased risk regardless of BMI, and for people of South Asian, Chinese, Japanese and some other ancestries the International Diabetes Federation uses lower cutoffs, around 35 inches for men and 31 for women. The ratio has blind spots too. Very muscular hips and glutes lower it without lowering risk, weight loss that shrinks both measurements can leave it unchanged, and pregnancy makes it meaningless. Read it alongside BMI, blood pressure, fasting glucose and lipids rather than on its own.