RPE stands for rate of perceived exertion, a subjective sense of how hard a workout feels. Because it reflects how taxing the activity is on your body, it often tracks with physiological stress like heart rate and lactate buildup even though you’re not measuring those markers directly. This is why RPE is a practical tool for guiding intensity when heart-rate monitoring isn’t available or reliable—in the field, in heat, or with devices not worn. To put it into training prescription, you use a simple scale (commonly 6–20 or 0–10) to set target effort levels for different sessions. For example, easy aerobic work might be around a low to moderate RPE, tempo work around a moderate-to-high RPE, and interval bouts around high to very high RPE. A useful way to apply RPE is session-based: multiply the RPE you aim for by the duration of the workout to get a training load (RPE × minutes), and adjust future sessions based on how you felt. This approach allows autoregulation—you push or pull back based on daily readiness without relying solely on a device. RPE isn’t a direct measure of oxygen consumption, hydration status, or a tool that replaces all physiological monitoring. Rather, it’s a practical, athlete-centered gauge of effort that should be used alongside other measures (like heart rate, pace, and performance) to tailor training to how you feel on a given day.