Why does Garmin Connect mobile app estimate 7.5km for Fartlek but 8.1km for Zone 2 run at same 47min duration?

7712098 7 days ago

I'm confused about Garmin's distance estimates for two similar workouts and hoping someone can explain the logic.

**Workout 1 - Easy Aerobic Run:**

- Duration: 47 minutes

- All in HR Zone 2

- Estimated distance: 8.1 km

**Workout 2 - Fartlek:**

- Duration: 47 minutes total

- 12 min warmup (Zone 2)

- 6x [2 min run (Zone 3) + 2 min recovery (Zone 2)]

- 11 min cooldown (Zone 2)

- Estimated distance: 7.5 km

My confusion:

The fartlek has 12 minutes in Zone 3 (faster pace) and 35 minutes in Zone 2 (same as easy run). Logically, it should cover MORE distance than the easy run, not 0.6 km less.

Both workouts have their low-intensity portions in the same HR Zone 2, so the pace should be identical during those segments. Adding high-intensity intervals should only increase total distance.

Is this a known quirk in Garmin's estimation algorithm? Does it account for acceleration/deceleration transitions? Or am I missing something obvious?

Thanks for any insight!

0 trux 7 days ago

Unfortunately, the estimation algorithm is rather rudimentary, and not really anywhere close to be smart (or accurate). As long as you do not specify any target pace in the workout step, your average running pace is used for the estimation (regardless of the HR target). It seems to be 6 min/km or perhaps somewhere slightly below it in your case. The difference in the total distance comes from the rounding errors - for example at the 6 series of 2 min run + 2 min recovery the pace of 6 min/km gives 0.3 km when rounded, although it is in fact 0.33333333 km, hence 4 km (6×2× 1/3) in total (perhaps even slightly more if the avg. running pace is faster than 6 min/km), instead of the 3.6 km used.