RPE — what it is and how to use the scale
A rating of how hard a set was on a 6–10 scale, where 10 is not one more rep, 9 is one left, 8 is two.
RPE stands for rating of perceived exertion. In strength training the scale is tied to reps in reserve (RIR): RPE 10 is failure, RPE 9 means one more rep was there, RPE 8 two, RPE 7 three. Halves exist too: 8.5 means "maybe two, definitely one". That's how Zourdos and colleagues described it in 2016, and that's how programs use it.
Why bother: the same weight feels different on different days. "3×5 at RPE 8" adapts to the day — on a good one you'll load more, on a bad one less, but the effort stays the same. Beginners rate less accurately; it gets better with experience. An honest question after the set helps: "how many more did I have?"
In the diary you can add an RPE to a set, and I'll factor the reps in reserve into your estimated max. To turn an RPE into a weight for your next set, use the RPE chart.
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In the diary I work out all of these numbers myself — you just log the set.