How to measure your waist: the number the scales miss.

Sunday morning, in the bathroom. You're holding a tape measure and wondering how to measure your waist. Above the navel, below it, where your belt sits?

Many people I work with measure at random. One day in the evening, the next over a shirt, the next holding their breath. Then they can't tell whether their belly has changed or just the way they measured.

Scales or tape measure?

The scales weigh everything at once: fat, muscle, water, last night's dinner. The tape looks at one spot. Between them you get four cases.

• Weight steady, waist going down. Often you're losing fat and keeping muscle. A kilo of fat takes up about three times the space of a kilo of muscle.
• Weight going down, waist the same. Sometimes water or muscle goes and the fat stays put.
• Weight jumping overnight. It can swing by two or three kilos in a day from water and food. It's rarely fat.
• Weight the same, waist going up. This is the one not to put off. At the same weight, a bigger waist tends to mean more risk.

Why the scales aren't enough

Two bodies, one weight. Two people of the same height and weight can have opposite bodies. One has muscle and a narrow waist, the other thin arms and a belly. The scales can't tell them apart.

The fat that matters sits in one place. The deep fat in your abdomen, wrapped round your organs, is the kind most closely tied to blood sugar, inflammation and your heart. I cover it in my article on belly fat after 40.

Scales that promise your body-fat percentage. They send a mild current from one foot to the other, so they read your legs and work out the rest with a formula built for the average person. Drink a little more or less, or step on with sweaty feet, and the result shifts. Use them for your weight.

How do you measure your waist?

Standing, tape level with your navel, on bare skin, without pulling your stomach in. First thing in the morning, after the loo and before breakfast, once a month.

The spot. Your navel, because it's always in the same place. Your belt moves with your trousers, and a little higher gives you a different number.

The stance. Feet together, stomach relaxed, breathing normally. The tape rests on your skin without digging in, at the same height front and back. Check side-on in the mirror.

The timing. Morning is the one time you can repeat. By evening, the day's meals and drinks are in the measurement too. Same day each month. If you have periods, keep to the same week of your cycle, as your body holds more water in the days before. Weigh yourself once a week.

The figures. For waist size in men, 94 cm is where to start paying attention. For waist size in women, it's 80 cm.

And 'hips'? I get asked this a lot. That's a separate measurement, taken at the widest point, and most useful for women, who store more fat there. On someone in good shape, the waist is narrower than the hips.

One measurement on its own tells you little. It's your starting point. Take it tomorrow morning, write it down and leave it alone for a month.