§1 · The formula.
e1RM = W × (1 + R / C) W = weight lifted (lbs) R = reps completed + RIR (reps to failure) C = 36 (round-trip: predicting from prior session) C = 40 (prediction: first estimate of new weight)
Calyber uses two constants: C=36 for same-weight round-trip calibration (prior session weight reused), and C=40 for cross-weight prediction (new weight entered). The dual-constant corrects for underestimation bias at lower rep counts when predicting into an untested weight.
§2 · Calculator.
e1RM = 180 lbs
§3 · Worked example.
Session input: 145 lbs × 10 reps @ RIR 2 Reps to failure: 10 + 2 = 12 e1RM: 145 × (1 + 12/36) = 145 × 1.333 = 193 lbs Next prescription basis: 193 lbs e1RM → 150 lbs target @ 8-12 reps, RIR 1
§4 · Formula variants.
| FORMULA | CONSTANT | EQUATION | NOTES |
|---|---|---|---|
| Epley (1985) | C = 30-40 | W × (1 + R/C) | Most widely used; constant varies by implementation |
| Calyber round-trip | C = 36 | W × (1 + R/36) | Calibrated for submaximal rep ranges 6-15 |
| Calyber prediction | C = 40 | W × (1 + R/40) | Used when weight differs from prior session |
| Brzycki (1993) | — | W / (1.0278 − 0.0278 × R) | Accurate at low rep counts (≤10); diverges above 15 |
| Lombardi (1989) | — | W × R^0.10 | Overestimates at high rep ranges |
§ Citations
Epley, B. (1985). Poundage Chart. Boyd Epley Workout. source
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