ALGORITHM · 1RM ESTIMATION
Estimated one-rep max is how Calyber tells whether you are getting stronger, even when the weight on the bar looks the same, and how it sets your next target.
You lifted 145 for 10. You are stronger than that looks: a 193 lb single, and tomorrow's target is 150.
New to Calyber? See how it works →From a set to a prescription
The estimate is not the goal. The next prescription is the goal.
§1 · Calculator
The calculator below is the live engine. It opens on the worked example, 145 lb for 10 reps at RIR 2, which estimates a 193 lb single. Change any field to see your own estimate.
Trend anchor, not true max.
§2 · The formula
Calyber estimates current strength from the set you actually performed: load, reps, and reps in reserve. Here is the same calculation in plain terms, with the raw formula one click away.
e1RM = W × (1 + R / 36)
You lifted 145 for 10 reps with 2 left in reserve, so Calyber treats it as a 12-rep performance and estimates a 193 lb single.
e1RM = W x (1 + R / C) W = weight lifted (lb) R = reps completed + RIR (reps in reserve) C = 36 (round trip: estimating from a prior session) C = 40 (prediction: first estimate of a heavier weight)
Terms:
§3 · Why there are two constants
Calyber uses two constants. The choice is mechanical: it depends only on whether the weight you entered matches a prior session or projects into a heavier, less-tested load.
Is this weight already tested?
Used when the weight matches a prior session. Tighter estimate.
Used when projecting into a less-tested load. More conservative.
Prevents over-creditingC=40 makes new-load predictions more conservative, so one unusually strong set does not inflate your next target.
§4 · Formula variants
Different formulas estimate strength differently. Calyber uses a modified Epley form because it supports reps in reserve and works cleanly inside a session-to-session prescription system.
Modified Epley. Supports RIR and a conservative heavier-load projection. Best fit for session-to-session prescription.
Accurate at lower rep counts (10 or fewer). Less reliable above 15 reps.
A simple power curve. Can overestimate at high rep ranges.
Calyber uses a modified Epley form because it accepts reps in reserve and stays stable across the 6 to 15 rep range a prescription engine actually works in.
| FORMULA | CONSTANT | EQUATION | NOTES |
|---|---|---|---|
| Epley (1985) | C = 30-40 | W x (1 + R/C) | Most widely used; implementations commonly use a constant in this range |
| Calyber round trip | C = 36 | W x (1 + R/36) | Used when the weight matches a prior session (rep ranges 6-15) |
| Calyber prediction | C = 40 | W x (1 + R/40) | Used when the weight is heavier than the prior session |
| Brzycki (1993) | n/a | W / (1.0278 - 0.0278 x R) | Accurate at low rep counts (10 or fewer); diverges above 15 |
| Lombardi (1989) | n/a | W x R^0.10 | Overestimates at high rep ranges |
§5 · Accuracy and limitations
An estimated one-rep max is most useful as a trend and a prescription anchor, less so as a prediction of a true max single.
Best used for strength trend and next-session prescription.
It means
It does not mean
Calyber treats e1RM as a trend and prescription anchor, not a trophy number.
§6 · In the app
Every prescription in the app shows this Basis line, so you can check the math on any number the engine gives you.
§7 · FAQ
It is the single number that shows whether you are getting stronger, even when the weight on the bar looks the same, because more reps or lower reps in reserve raise it. Calyber also uses it to set your next weight, so you are not guessing whether to add load.
It reads the set you actually performed, your weight, reps, and reps in reserve, and converts it to effective reps (reps plus RIR), then applies a modified Epley-form estimate. No true max attempt is required.
C=36 is used when the weight matches a prior session, giving a tighter estimate. C=40 is used when you enter a heavier, less-tested weight, giving a more conservative estimate so a single strong set does not inflate your next target.
No. It is a working estimate used as a trend and a prescription anchor. It is not a guaranteed single and is not a command to test a true max.
The estimate becomes the anchor for your next target. In the worked example, a 193 lb e1RM produces a next prescription of 150 lb for 8 to 12 reps at RIR 1, shown with its Basis line.
References
Epley, B. (1985). Poundage Chart. Boyd Epley Workout. Lincoln, NE: Body Enterprises.
Mayhew, J.L. et al. (1992). Muscular endurance repetitions to predict bench press strength in men. Perceptual and Motor Skills. doi:10.2466/pms.1992.75.3.1389
Reynolds, J.M. et al. (2006). Prediction of one repetition maximum strength from multiple repetition maximum testing and anthropometry. Journal of Strength and Conditioning Research. doi:10.1519/R-15304.1
These references support repetition-based strength prediction. Calyber's exact constants (36 and 40) are product-specific calibration choices.
Log a set, get your e1RM, and see how it sets your next prescription.
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