RPE (Rate of Perceived Exertion)
A subjective 1-10 rating of how hard an effort felt — a zero-hardware intensity measure that catches what 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, deliberately chosen so a young adult's rating roughly tracked their heart rate divided by ten; a simpler modified 1–10 scale (sometimes called CR10) is more common in endurance sport today.
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 makes RPE most useful tracked consistently for one athlete over time, watching how their own rating for a given pace or power shifts as fitness changes, rather than compared directly between different people.
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 all of these situations, RPE is catching something real that the instrumented data is currently getting wrong — it's not a lesser substitute for "real" data, it's a different sensor that happens to be running all the time and never needs batteries.
Using it well
The failure mode isn't using RPE — it's using it carelessly, rating everything as either "easy" or "hard" without the resolution to distinguish a true recovery jog from a moderate aerobic effort. A useful RPE habit treats the scale seriously enough to separate close intensities, and reads its trend over weeks rather than treating any single session's rating as a verdict on the whole plan.