I have a Fitbit Surge and a Garmin 405cx with a heart rate strap that I've used for years. I always wear both when I work out. [Sitting/Walking] - Fitbit Surge matches my HR if I check my pulse with my hand [Running], even up to a 7 min/mi pace for 4-6 miles - the Fitbit Surge and Garmin are within 1-2 beats of each other [Weight lifting] - Fitbit is HORRIBLY inaccurate. Typically 30 beats off, but oftentimes up to 50-70 beats off right after a tough set. I'll regularly walk around between sets and see the Fitbit at 95-105 and the Garmin at 130-150. When I check my pulse, it is always within 5 beats of the Garmin's HR. I once read that it was a design choice to make a smaller HR monitor that uses less battery in exchange for some accuracy. Don't know how true that is, but the differences are there.

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