How the estimate works
Steelheft uses the Epley formula: max = weight × (1 + reps ÷ 30). One rep to failure counts as the max itself, not a few percent above it, so a single you actually lifted is never inflated.
RPE tells the formula how many reps you had left. RPE 8 means two in reserve, so 100 kg × 5 at RPE 8 is treated as 100 kg × 7: an estimated max of 123.3 kg. Without RPE the set counts as taken to failure, and the estimate comes out on the careful side.
Why sets over 8 reps are less reliable
The formula stretches worse the longer the set. Around 15 reps it can put your max noticeably too high, and weights planned from that number will not move. Steelheft does not use sets over 8 reps to set working weights for heavy lifts; exercises trained in higher rep ranges are compared with sets in their own range.
From max to working weight
To plan a set, the math runs backwards: the weight you could lift for your reps plus your reserve. Five reps with two in reserve is the weight you could lift seven times, 81% of your max. A set of 115 kg × 3 gives an estimated max of 126.5 kg; 81% of it is 102.6 kg, which rounds to 102.5 kg: 25 + 15 + 1.25 per side on a 20 kg bar.
What Steelheft does with this
In the app this happens on every set without a calculator. Steelheft keeps your best estimate from the last six weeks, reads the RPE you log and puts the working weight and the plates per side on the set screen before you lift.