Backstory: HP had an employee “wellness” program that provided employees with a (real cheap) pedometer. I found that using the pedometer did motivate me to go for longer walks. I bought a Fitbit One to replace the HP-provided pedometer and have been using Fitbits for 6-7 years. I had several One’s over the years. (I kept losing them. The One’s were a belt/pocket clip-on device). I am using a Fitbit Charge 4 currently.
Early in the COVID-19 pandemic, there were reports that one early-warning symptom was that blood oxygen readings (as measured by a “Pulse-Ox” device). I bought a Garmin Vivosmart 4. The Vivosmart will monitor blood oxygen while sleeping and can do an on-demand Pulse-Ox measurement.
The Charge and the Vivosmart both measure steps, floors climbed, and heart rate. The Fitbit and Garmin apps use this raw information to estimate the calories burned while walking or doing other exercise. I have been using these estimates for dieting and weight control.
For a period of two weeks, I have worn both devices on the same wrist. Ideally, all the raw measurements and the estimated calories burned would be the same, but that is not the case.
On average, the Fitbit step count is ~4.6% higher than Garmin. The floors climbed is ~2.3% higher and the calories burned is ~7.3% higher.
Which device is more accurate is difficult to know from this experiment. I do not have access to a medical-grade pedometer for comparison. The raw counts differ by less than 5%, and I consider that sufficiently accurate for a consumer-grade device.
An interesting implication is that if you rely on these devices as part of a dieting/exercise program to lose weight, you have to be aware that the calorie-burn estimate is just that — an estimate. Could be off by 5-10%.
Most weight-loss advice is to aim for a 500-1000 calorie per day deficit. This should in theory lead to a 1-2 pound per week weight loss.
What I have found is that the estimates provided by the device are a useful guide to calories burned. I also use the Fitbit app to track calories eaten. Both help achieve the calorie deficit target. BUT I find that I must “self-calibrate” the deficit to achieve consistent weight loss.