Heart-rate zone calculator.

Test rather than guess. The zones here are built on your lactate threshold heart rate (LTHR), the highest heart rate you can hold for about an hour. The simplest field test: warm up well, then run or ride 30 minutes alone, as hard as you can hold evenly, and take the average heart rate of the final 20 minutes. Do it separately for running and cycling; most people's bike threshold is 5 to 10 beats lower than their run threshold. If all you know is your max, the calculator estimates LTHR from it and says so; replace the estimate with a test within a couple of weeks.

Spend most of your time in zone 2. The best-studied pattern in endurance training is roughly 80% of sessions easy and 20% hard, and the mistake almost everyone makes is drifting into zone 3 on easy days. It feels productive and it costs you the quality of the hard days. If talking in full sentences is difficult, you are above zone 2. Zone 4 is where tempo runs and cruise intervals live; zone 5 is short repeats, a few minutes at a time, and a little goes a long way.

Heart rate lags and drifts. It takes a minute or two to respond to a change in effort, so on short intervals judge the first repeats by pace or power and let heart rate confirm later ones. On long sessions it rises at a steady effort as you heat up and dehydrate — that drift is itself a useful number, and a FORGE plan reads it from your watch to track your aerobic fitness. Retest every six to eight weeks, and any time a zone starts to feel wrong.