An estimated 1RM (e1RM) is a prediction of the most weight you could lift for one rep, calculated from a set you actually did at a lighter weight for more reps. You never attempt the single. You lift 100 kg × 8, a formula turns that into "about 124 kg for one", and that number becomes a way to compare today's set with last month's even when the weights and reps were different.
Why estimate instead of test
A true one-rep max is a measurement; an e1RM is arithmetic. Testing is precise but costly — it needs a fresh day, a spotter, and a willingness to miss — so nobody does it weekly. Estimating costs nothing and can be done for every working set of every session, which is exactly what a progress chart needs: many points, taken the same way, over months.
The trade is precision for frequency. A single e1RM can be a few kilos off in either direction. A series of them, from sets logged the same way, shows a slope that a quarterly max test never could. That is why Zyra Workouts charts e1RM per session rather than asking you to test, and why its plateau detection reads the e1RM trend rather than your heaviest weight (see how to track strength progress).
Two rules keep the estimate honest in the app. Only working sets count — warm-ups are excluded before the formula runs, because a light set at RPE 4 predicts nothing. And the rep count is full reps, so a set logged as 5 + 2 partials is estimated from 5.
Brzycki, Epley and the 12-rep switch
The app uses two formulas and picks by rep count:
e1RM = weight × 36 / (37 − reps) — Brzycki, for sets of 1 to 12 reps e1RM = weight × (1 + reps / 30) — Epley, for sets of 13 reps and more
Both take the same two inputs and both agree on one thing: more reps at a weight implies a higher max. They disagree on the shape of the curve, and they cross at exactly 10 reps (36/27 = 1 + 10/30 = 1.333). Below 10 Brzycki is the more conservative of the two; at 11–12 it edges higher; beyond 12 it climbs steeply (at 37 reps it divides by zero), which is why the app hands over to Epley's straight line there. The pair are compared side by side in Brzycki vs Epley.
Worked example, barbell bench press, 100 kg × 8: 100 × 36 / 29 = 124.1 kg. The same lifter doing 100 kg × 5: 100 × 36 / 32 = 112.5 kg. If both sets were logged in the same week, the 8-rep set is the higher estimate — either the 5-rep set was cut short, or the 8 was an unusually good day. The chart shows the maximum e1RM per session, so one strong set sets that session's point.
In plain terms: the formula converts "this much for this many" into "this much for one", and the app keeps whichever set gave the highest answer that day.
Rep-max table
The formula runs backwards too. Given an e1RM, dividing by the Brzycki factor tells you the weight the formula expects you to manage for a given rep count. That is how the per-exercise pages such as bench press 1RM build their tables. For an e1RM of 120 kg:
| Reps | Brzycki factor 36 / (37 − reps) | Weight for that rep count | % of e1RM |
|---|---|---|---|
| 1 | 1.000 | 120.0 kg | 100% |
| 2 | 1.029 | 116.7 kg | 97% |
| 3 | 1.059 | 113.3 kg | 94% |
| 4 | 1.091 | 110.0 kg | 92% |
| 5 | 1.125 | 106.7 kg | 89% |
| 6 | 1.161 | 103.3 kg | 86% |
| 8 | 1.241 | 96.7 kg | 81% |
| 10 | 1.333 | 90.0 kg | 75% |
| 12 | 1.440 | 83.3 kg | 69% |
Read it as a map, not an instruction. If your logged 5 × 106.7 kg was a grind at RPE 9.5, the table says your e1RM is about 120 kg and that 90 kg is the weight the formula pairs with a set of ten. Whether that ten is easy or hard on the day depends on you, the exercise and how rested you are — which is why the app also asks for RIR on every set.
The 1RM calculator produces this table for any weight and rep count you type in, using the same two formulas.
Why high-rep estimates are fuzzy
Every 1RM formula is a curve fitted to a population, and the fit is tightest where the data is densest: heavy sets of a few reps. Move out to sets of 15 or 20 and two things happen.
First, the formulas disagree with each other more. Brzycki and Epley are within a kilo or two of each other around 8–12 reps at 100 kg; by 15 reps Brzycki says 163.6 kg and Epley says 150 kg, and by 20 reps Brzycki says 211.8 kg against Epley's 166.7 kg. When two reasonable formulas differ by 45 kg, neither is a number to plan a max attempt around.
Second, a long set stops being limited only by strength. Breathing, grip, local fatigue and pacing all decide where a 20-rep set ends, and none of those are what a one-rep max measures. The individual spread around the formula grows with the rep count. This is arithmetic and mechanism, not a trial result — the exact size of the error for you is unknown, and the honest reading is: the fewer the reps, the more the estimate means.
The practical consequence is the 12-rep switch. Above 12 the app deliberately uses the flatter formula, so a 20-rep set of a warm-up weight cannot produce an e1RM that beats your real heavy sets and paints a false record on the chart.
In plain terms: trust e1RMs from sets of 1–8, use 9–12 for the trend, and treat anything longer as a rough guide.
The short version
An e1RM turns a set you did into the single you probably could do: Brzycki (weight × 36 / (37 − reps)) up to 12 reps, Epley (weight × (1 + reps / 30)) above. Working sets only, full reps only, highest estimate of the session wins. It is a population formula, so read the trend across sessions, not the decimals of one set, and use the calculator to run the table for your own numbers. Zyra Workouts is in development and not yet on the App Store; the rules quoted here are read from its source code and may change before release. Training guidance, never medical advice. Educational overview only — not medical advice.