RPE (Rate of Perceived Exertion)
A subjective 1-10 rating of how hard an effort felt, no hardware required, and it catches things devices sometimes miss.
Rate of Perceived Exertion (RPE) is a subjective rating of how hard an effort felt, reported on a simple numeric scale. The original Borg scale ran from 6 to 20, chosen so that a young adult's rating would roughly match their heart rate divided by ten. Endurance sport mostly uses a simpler 1-10 scale today, sometimes called CR10.
Why it's personal, not universal
Across a population, RPE correlates reasonably well with objective measures like heart rate, power, and blood lactate. But the exact mapping is genuinely individual. An RPE of 7 might sit right at threshold pace for one athlete and only at a moderate tempo effort for another. That's why RPE works best tracked consistently for one athlete over time: watch how your own rating for a given pace or power shifts as fitness changes, rather than comparing your numbers against someone else's.
When it beats the numbers on a device
Heart rate can be skewed by heat, dehydration, caffeine, or incoming illness, in ways that make the number lie about how hard an effort actually is. A power meter can die mid-ride. Swimming rarely has a reliable direct power measurement at all. In these situations, RPE is picking up something real that the instrumented data is currently getting wrong. Think of it less as a backup for "real" data and more as a different kind of sensor, one that's always running and never needs batteries.
Using it well
The real failure mode is using RPE carelessly: rating everything as either "easy" or "hard" without enough resolution to tell a true recovery jog from a moderate aerobic effort. A useful RPE habit takes the scale seriously enough to separate close intensities. It also looks at the trend over weeks. A single session's rating isn't a verdict on the whole plan.