LodestarYour Fitbit's calories burned figure is an estimate, and for any single day it can be off by a fair margin. It's still useful, as long as you know what goes into it and use it for trends rather than exact accounting.
How the estimate works
No wrist device measures calories directly. The gold-standard lab methods measure the oxygen you breathe in and the carbon dioxide you breathe out, which a watch can't do. Instead, your Fitbit combines two things:
- Your resting burn (basal metabolic rate), estimated from your age, sex, height and weight. This is the energy your body uses just to keep running, and it's the largest part of most people's daily total.
- Activity on top, estimated from movement and heart rate. Higher heart rate and more movement mean more calories.
Total calories include both. Active calories are just the part above resting. This is why your Fitbit shows calories burned even while you sleep: your body is always using energy.
How accurate is it?
Studies that compare wrist wearables with lab measurements have consistently found heart rate is measured fairly well, but calorie estimates are much less reliable. One lab study of seven wrist devices found none of them estimated energy expenditure accurately, with errors that were often large and varied by person and activity (Shcherbina and colleagues, Journal of Personalized Medicine, 2017).
That doesn't mean the numbers are useless. It means you should be cautious about using a single day's number as precise.
Why your number may be off
- Your profile settings. The resting part of the estimate depends on the weight, height, age and sex you entered. An old weight throws off every day.
- Individual differences. Two people with the same profile can have meaningfully different metabolisms. The formula can only estimate the average.
- Activity type. Running and walking are easiest to estimate. Cycling, rowing, strength training and activities where your wrist barely moves are harder.
- Heart rate glitches. A loose band or a bad reading during exercise can push the estimate up or down. Heart rate can also rise from heat, caffeine or stress without extra calories being burned.
- Strength training. Heart rate during lifting doesn't track the work well, so these sessions are often estimated poorly. See strength training and recovery.
How to use it sensibly
- Look at trends, not single days. Even if the absolute number is off, it's usually off in a similar way day to day. A week where your average is clearly higher than usual probably was more active.
- Compare yourself with yourself. Your number against someone else's tells you very little.
- Don't eat back every calorie. If you're managing weight, treating the active calories as exact can lead you to overeat. Many people find it more reliable to watch the scale trend over several weeks and adjust.
- Keep your profile up to date, especially your weight.
- Wear it snugly during workouts so heart rate is measured cleanly.
Total vs active: which to watch
Active calories are the better measure of how much you moved, since they strip out the resting part that barely changes. Total calories matter more if you're thinking about overall energy balance, but they carry the uncertainty of the resting estimate on top. If you want a sense of how active your days have been, active calories, steps and time in heart-rate zones together give a fuller picture than any one of them. See steps vs workouts.
How Lodestar helps
Lodestar is an iPhone app that reads your Fitbit total and active calories from Google Health and shows them on the Today screen and the Effort page, next to your steps. It sits alongside your Effort score, which uses time in heart-rate zones to show how hard each day was against your usual range.
Lodestar doesn't track food or count calories eaten. It shows what your Fitbit recorded, clearly, and works everything out on your phone.