A precise number is still an estimate
You finish a strength session and your watch shows fewer calories than yesterday’s walk. Was the session less useful? That number cannot answer the question. It estimates energy expenditure; it does not measure the strength you gained or the quality of your training plan.
Apple explains that its calorie calculations use personal details such as height, weight, age, and sex. The inputs used to measure activity also depend on the workout. [1] The number comes from a calculation, even when the display gives it no margin of error.
What the studies actually tested
A study published online in 2022 and in a 2023 journal issue tested Apple Watch 6, Polar Vantage V, and Fitbit Sense in 60 healthy young adults. It compared readings during sitting, walking, running, resistance exercise, and cycling with reference instruments. All three devices showed poor accuracy for energy expenditure across those activities. Heart rate accuracy was a separate result: Apple Watch 6 performed much better on that measure. [2]
Another 2023 study tested Apple Watch 6 and Polar A370 in 11 young men, at rest, during running, and afterward. Error varied with the conditions; the watches did not give interchangeable estimates. [3] A reliable pulse reading therefore does not automatically make the calorie estimate reliable.
These are tests of specific devices and protocols, not a ranking of watches sold today. Neither study validates your current model, software, or home workout. They also do not supply a universal correction: subtracting a fixed percentage from every reading would create another unverified number.
Check the setup before comparing
For Apple Watch, the manufacturer recommends keeping personal information current, wearing the watch with a suitable fit, choosing the workout type that matches the activity, and calibrating it. [1] Follow the instructions for your device. These steps improve the conditions for a reading; they do not prove that your calorie total is exact.
If you keep calorie estimates in your log, record the device and activity alongside them. Compare like with like. Changing watches, workout modes, or session length makes the interpretation harder. Even a consistent setup does not establish that small changes between sessions reflect real changes in energy expenditure.
Give the number a smaller job
Our practical suggestion: use the calorie estimate as optional context, not the target that decides when a strength session is finished. Avoid adding sets just to beat yesterday’s calorie total. That changes the workout to satisfy a measurement whose accuracy you have not established.
For strength training, record the exercise variation, load if used, repetitions, rest periods, and how demanding the sets felt. Those details describe the task you actually completed. A lower calorie estimate does not erase an improvement in a comparable task; a higher one does not establish muscle growth.
At your next review, choose two similar sessions and ask one question your log can answer: did you complete more repetitions at the same load and under similar conditions? Leave the calorie difference undecided if you cannot explain it. You do not need a verdict on every number to plan the next session.