RPE to Percentage Calculator: What Your Set Was Worth

How to use it

1. Choose kilograms or pounds.
2. Enter the weight you lifted on one working set.
3. Enter the reps you completed on that set.
4. Enter how many more reps you believe you could have completed with the same technique.

The calculator returns four things: the percentage of your one-rep max that set represented, the RPE value it corresponds to, the reps-to-failure figure the arithmetic actually runs on, and an estimated one-rep max.

Use a real working set, not a warm-up and not your best set ever. The answer describes the set you entered.

What the inputs and outputs mean

Reps plus reps in reserve cannot exceed 12. Past that the estimate stops being reliable, and the calculator says so rather than returning a number it cannot stand behind.

The formula

Everything runs on one idea: a set of 5 with 2 left in reserve is the same effort as a set of 7 taken to failure. So the first step is not a formula at all.

reps to failure = reps completed + reps in reserve

Then the Brzycki equation, the same one Lift's one-rep max calculator uses, read at that reps-to-failure figure:

estimated 1RM = weight / (1.0278 - 0.0278 x reps to failure)

Rearranging gives the percentage directly, and the weight cancels out:

percentage of 1RM = (1.0278 - 0.0278 x reps to failure) x 100

That is worth noticing. The percentage does not depend on how much you lifted, only on how many reps you had in you. Two lifters at completely different strength levels doing a set of 5 at 2 RIR were both working at about 83% of their own max.

RPE is a lookup, not a calculation: RPE = 10 - reps in reserve.

The conversion table

Since the percentage does not depend on the weight, the whole tool fits in one table. This is the same table published in how to measure training intensity, computed the same way.

Read down a column and you can see how much a single rep is worth: about 2.8 percentage points, every time. That is a property of the formula, not a finding about muscle.

Worked examples

100 kg for 5 reps, 2 reps in reserve

Reps to failure is 7. The percentage is (1.0278 - 0.0278 x 7) x 100 = 83.3%, and the estimated max is 100 / 0.8332 = 120.0 kg. RPE 8.

140 kg for 1 rep, 0 reps in reserve

Reps to failure is 1, which makes the bracket 1.0278 - 0.0278 = 1.0 exactly. The percentage is 100% and the estimated max is the 140 kg you lifted. A true single is your max by definition, and the formula agrees, which is a reasonable sanity check on it.

60 kg for 10 reps, 2 reps in reserve

Reps to failure is 12, the edge of the supported range. The percentage is 69.4% and the estimated max is 86.4 kg. This is the case to be most sceptical of: rep estimates get less accurate the further you are from failure and the longer the set.

Assumptions and limitations

• It is one formula's estimate, not a measurement. Formulas disagree with each other about the same set. The one-rep max calculator offers three, and they return different numbers.
• The output is only as good as your reps-in-reserve call. Research pooling a dozen studies found people underestimate what they have left by about one rep on average, and that the guess does not get better with training experience. One rep is roughly 2.8 percentage points here.
• Treat the percentage as a band. If you called 2 RIR, the honest reading is somewhere between 80% and 86%, not exactly 83.3%.
• Long sets are the weakest case. Both the formula and your own judgment are least reliable above about 10 reps to failure, which is why the tool stops at 12.
• It assumes consistent technique and full range of motion. A rep you would not count is not a rep the formula should count either.
• It says nothing about whether the load is appropriate for you. It describes a set you already did. If a lift hurts, or an injury or medical condition could change the answer, ask a qualified professional rather than a calculator.

What to do with the number

The most useful application is checking whether your training matches your goal. Loads at or above 80% of 1RM are what the 2026 ACSM position stand associates with gains in maximal strength, while muscle growth happens across a much wider span of loads as long as sets end close to failure. If you are training for strength and every set converts to 72%, that is worth knowing. Hypertrophy vs. strength covers what follows from it.

The second use is translating a program. If a plan prescribes "3 sets of 5 at 80%" and you have no current max, this table tells you 80% is roughly a six-rep-to-failure load, so sets of 5 at 1 RIR gets you there without testing anything.

The third is calibration. Take one set to genuine failure occasionally and compare the reps you actually got with what you predicted. That is the only way to find out whether your 2 RIR is really 2.

Recording effort, not just weight

Weight and reps cannot tell a set of 8 that nearly ended you from a set of 8 with three left. Those are different sets at different percentages, and next session's decision depends on which one happened.

Lift stores an RPE value alongside weight and reps on every set, shows last session's numbers on the set you are about to log, and keeps an estimated one-rep max across your whole history, so the conversion above happens without you doing it by hand. Full progress charts require Premium. See how the Lift app handles progressive overload, or read how to measure training intensity for the difference between intensity of load and intensity of effort.