Rate of Perceived Exertion Scale (RPE)

Rate of Perceived Exertion (often shortened to RPE) is a simple tool that can help you tune into your body more and still reach your fitness and running goals.

At its simplest, RPE is a scale of 1 to 10, measuring the intensity of your effort – 1 being extremely light activity like a slow stroll, 10 being an all-out sprint which you can only maintain for a few seconds.

Taken from https://marathonhandbook.com/rate-of-perceived-exertion/