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Estimated 1RM from RPE: Why Effort Changes the Math
Every rep-based 1RM formula has the same blind spot: it doesn't know how hard the set was. Five reps at RPE 7 and five reps at RPE 10 produce identical estimates, and they shouldn't. Adding effort to the math fixes it.
The classic approach: reps-only formulas
The best-known estimate is Epley's formula (Epley, 1985):
Lift 300 for 5 and it says your max is about 350. It's simple, it's decent in the 1 to 10 rep range, and it degrades as reps climb (past about 10 reps, all the classic formulas start guessing). But its deeper limitation is baked into the inputs: weight and reps are all it sees. Whether those 5 reps were smooth with 3 left in the tank, or an all-out grind, the formula returns the same number.
What RPE adds
RPE in strength training is effectively a reps-in-reserve scale: RPE 10 means nothing left, RPE 9 means one rep left, RPE 8 means two, and so on (the formalization comes from Zourdos and colleagues, 2016, building on Mike Tuchscherer's work, with the practical application laid out by Helms and colleagues, 2016). That extra input pins down what a set actually says about your max. The standard chart maps any combination of reps and RPE to a percentage of 1RM, and the estimate becomes:
Worked example at 300 pounds for 5 reps:
- 5 reps at RPE 10 (a true 5RM): the chart says that's about 86.3% of max, so the estimate is 300 ÷ 0.863 ≈ 348.
- 5 reps at RPE 7 (3 reps in reserve): about 78.6% of max, so the estimate is 300 ÷ 0.786 ≈ 382.
- Epley, either way: 350. Right in one case, 30 pounds low in the other.
The easier set implies the higher max, which surprises people at first. Reps in reserve are strength you demonstrated by not needing them.
When reps-only is still the right tool
Two honest limits. First, the RPE chart's domain is roughly 1 to 10 reps at RPE 6 to 10; outside that range, stretching the chart inflates estimates, so a reps-only formula is the safer fallback. Second, an RPE-aware estimate is only as good as the RPE you log; if you don't rate the set, there's nothing to work with. Either way, every estimate is an estimate. Bar speed, sleep, and bodyweight all move your real max around day to day.
How Ægir Iron does it
When a logged set carries an RPE, Ægir Iron estimates your 1RM through the chart; when it doesn't, or the set falls outside the chart's domain, it falls back to Epley. The same chart also runs the other direction to translate RPE prescriptions into percentages in the program builder, so a set's implied intensity means the same thing whether you're reading a plan forward or a logged set backward. The estimate shows live as you log, drives your progress charts and strength standards, and feeds your relative intensity distribution.