When you work out with an Apple Watch or another smartwatch, it shows "calories burned" at the end. It's tempting to trust the number, but how reliable is it? This article explains why calorie estimates for strength training tend to be off, and how to make good use of the number anyway.
Calories burned is an estimate, not a measurement
A watch doesn't measure your body's energy use directly. It estimates calories from your heart rate, wrist motion, and profile details like age, weight, and height. Compared with running, where motion and heart rate move together, strength training is considered harder to estimate.
Why strength training is harder to estimate
- You rest between sets: heart rate drops between sets and rises during them. That stop-and-go pattern doesn't fit well with estimates built for continuous exercise.
- Heart rate doesn't always reflect intensity: a heavy set of a few reps can load your muscles hard without raising your heart rate much.
- Wrist motion doesn't capture lifting load: in exercises like the bench press, where the wrist barely moves, motion data says little about effort.
- Individual differences are large: the same workout costs different amounts of energy depending on body size, muscle mass, and training history.
How to use the number
Treat it as a rough guide, not a precise figure.
- Compare day to day: using values from the same device to see whether today felt harder than last week keeps the effect of absolute error small.
- Don't plan meals around a single number: avoid "eating back" the displayed calories.
- Track progress with other metrics: trends in weight, reps, sets, and volume show your growth more directly than calories do.
How LiFTY handles it
LiFTY doesn't calculate calories burned for strength training itself. When you log with Apple Watch, the Watch's own estimate is saved to Apple Health. For strength workouts logged on the iPhone alone, LiFTY writes only the duration to Health, not calories. The missing number isn't a bug; it's a deliberate choice not to make up a value with no basis.
Summary
Calories burned during strength training is one of the less accurate figures a watch gives you. Glance at it as a rough guide, and check your day-to-day progress through the records themselves: weight, reps, and volume.
Related articles
- Tips for Logging Strength Training with Apple Watch
- What Is Training Volume? How to Calculate It and Beat Plateaus
- What Is Progressive Overload? The Basic Principle Behind Every Gain
LiFTY is a workout log app that lets you track progress through weight, reps, and volume trends. It also works on Apple Watch.
Download on the App Store