Is your watch's calorie number right?

Not very. Your watch measures steps and heart rate well, then estimates calories from them with a model of a body like yours. In a Stanford test of seven devices, every one missed calories by more than 20%, and the worst by 93%. Use the number to compare your own days, never to decide how much to eat.

Why does the number look so exact?

The watch says you burned 2,743 calories today. Not about 2,700, but 2,743. A number that precise feels measured, so people eat to it, and many food apps add the watch’s exercise calories straight onto the day’s budget.

Precise and right aren’t the same. A kitchen clock that runs ten minutes fast still tells you how long the roast has been in the oven. It’s no use for catching a train. A watch’s calorie number is a bit like that clock, with one more catch: how far off it runs changes with what you’re doing. So compare like with like, a walking day with a walking day.

What’s in the number?

A watch can’t measure calories. Labs measure them from the oxygen you breathe in and the carbon dioxide you breathe out. Your watch measures your movement and your heart rate, and turns them into calories with a model. Apple says its watch uses your height, weight, gender and age to work out how many calories you burn, and that calibrating it improves the distance, pace and calorie numbers.

The number comes in two parts. In Apple’s health records, active energy is “the energy that the user has burned due to physical activity and exercise”, and it leaves out resting energy. Resting energy is what your body burns to keep its normal resting state: breathing, circulating blood, keeping cells going. Your total for the day is the two added together.

That split is where people double count. A maintenance number from a calculator, or from your scale, already includes your movement. Adding the watch’s active calories on top counts your walking twice.

How far off is it?

Strong Many lab studies, two large reviews, the same answer: heart rate and steps are good, calories are not.

We found no watch or ring with published tests showing its calories within 10% across most activities. On a 2,500 calorie day, a 20% miss is 500 calories, the size of a whole deficit.

Should you eat back what it says?

No. Your body also makes up for part of every workout. In a 2021 study of 1,754 adults, only about 72% of the extra calories burned in activity showed up as extra calories burned that day. So even a perfect watch would overstate what a workout leaves you to spend. The full answer, with what we’d do on an unusual day like a long hike, is in should you eat back exercise calories?

How do you use the number anyway?

• For your activity level, use steps. They’re the part the watch gets close. Your activity level turns them into a level.
• To compare days, use the total. If Saturday’s total is 400 higher than Tuesday’s, Saturday was probably more active. We’d trust the direction far more than the 400.
• To set your eating, use your scale. Four weeks of food logs and weigh-ins show your real maintenance. Measure your real TDEE has the method.
• Watch the trend over months. As you lose weight, your watch’s numbers fall partly because you weigh less, and the model knows it. That matches real life: by Kevin Hall’s model, each pound you lose lowers your needs by about 10 calories a day.

Say your watch averages 2,700 calories a day. The calculator gives a range of 2,000 to 2,300 for your size and steps. After four weeks of logging and weighing, your trend shows you maintain at about 2,250. Your real number is 2,250. The watch was about 450 high, and now you know which way it leans for you.