Free calculator
How many calories do you really burn?
Your watch and the formulas guess. Your scale doesn’t: what you ate, minus what your weight changed, is what you burned. Put in a few weeks of averages to see your real burn, how far off your watch is, and your real deficit.
Your real burn
Calibrated2 610kcal a day
Example numbers. Change them to yours.
Coming to iPhone
Realburn does this automatically, every day.
It reads your watch, your scale and your food log from Apple Health, works out your real burn every morning and corrects your watch for you. No averages to type in, and no food log needed once it’s calibrated.
The math
How it’s calculated
Your real burn
Energy doesn’t disappear. Whatever you ate and didn’t burn was stored, and whatever you burned beyond what you ate came out of your body. Over a few weeks, your weight shows which way it went:
real burn = calories eaten − weight change × 7 700 ÷ days
7 700 kcal is about one kilogram of body weight lost or gained (3 500 kcal per pound). In the example above, 81.7 kg became 80.0 kg in 28 days: −1.7 × 7 700 ÷ 28 = −468 kcal a day, the real deficit. With 2 140 kcal eaten a day, the real burn is 2 140 + 468 = 2 608, so 2 610 kcal.
How far off your watch is
Your real burn divided by what the watch reported is your watch’s factor: 2 608 ÷ 3 140 = ×0.83. This watch overestimates by 17%. By its count, the person above would be on a 1 000 kcal deficit a day, more than twice the real one. Watches are usually within ×0.6 to ×1.3; a bigger gap more often means a missing meal or a watch number without resting energy.
Without a food log: the formula
Without what you eat, the calculator estimates your burn from your body and your activity, the way Realburn does with just an iPhone:
| Part | How |
|---|---|
| Resting burn | Mifflin-St Jeor: 10 × kg + 6.25 × cm − 5 × age, then +5 for men or −161 for women |
| Background activity | 10% of the resting burn: standing, moving around, chores |
| Steps | steps × 0.0005 × kg, about 35 kcal per 1 000 steps at 70 kg |
| Workouts | (6 − 1) MET × kg × hours a week ÷ 7. The 1 is left out because the resting burn already counts it |
| Digestion | 10% of what you eat; without a food log, 10% of the total |
For the example (male, 35, 178 cm, 80.9 kg on average, 8 000 steps, 3 hours of workouts a week): 1 751 + 175 + 323 + 173 = 2 422, plus 269 for digestion, so 2 690 kcal. Formulas are built for the average person and can miss yours by a few hundred calories, so the estimate is shown as a range of ±12%: 2 370–3 010.
With your weight from a few weeks ago, the scale still tells your real balance without a food log, and from it roughly how much you eat: the formula’s burn plus the balance.
Questions
FAQ
Is my Apple Watch calorie count accurate?
Not very. Across 56 studies, Apple Watch calorie estimates were off by 28% on average, while its heart rate was off by 4% and its steps by 8%. It also varies from person to person, so the only way to know your own watch’s error is to check it against your scale, which is what this calculator does.
Why does my watch say I burn more than I do?
Watches turn heart rate and movement into calories with a model built for the average person. A higher heart rate from heat, stress or coffee, or arm movement that isn’t exercise, can add calories you never burned. Over a day that can be hundreds.
How long should I track before using the calculator?
At least two weeks; four is better. Your weight moves up and down by a kilo or more with water and salt, and over a longer period those swings matter less. Use averages for both the food and the weight if you can.
Does my food log have to be exact?
No, but it has to be complete. If you always underestimate by about the same amount, the real burn comes out lower by that amount, and it still works for planning in your own numbers. Days with meals left out are the problem: they make your burn look lower than it is.
Why 7,700 calories per kilogram?
It’s the usual energy value of a kilogram of body weight lost or gained, mostly fat with some water and protein. Early in a diet more of the loss is water, so over the first week or two the real deficit is smaller than this suggests.
Is this medical advice?
No. The calculator gives estimates for healthy adults. If you have a medical condition or an eating disorder, follow your doctor’s guidance.
Sources
- Mifflin et al., 1990: A new predictive equation for resting energy expenditure in healthy individuals (opens in a new tab)
- Hall, 2008: What is the required energy deficit per unit weight loss? (opens in a new tab)
- Westerterp, 2004: Diet induced thermogenesis (opens in a new tab)
- Ainsworth et al., 2011: Compendium of physical activities (opens in a new tab)
- Choe and Kang, 2025: Apple Watch accuracy in monitoring health metrics, Physiological Measurement (opens in a new tab)