Every serious strength programme — 5/3/1, Westside, GZCLP, powerlifting peaking blocks — is built on one number: your one-rep max. But actually lifting your true 1RM carries a non-trivial injury risk, particularly for newer lifters or during a high-volume training phase. The practical solution is a submaximal set: lift a weight you can move for 2–10 clean reps, then run the result through one of the validated 1RM estimation formulas.
The Six Main Estimation Formulas
Epley (1985): 1RM = w × (1 + r / 30) Brzycki (1993): 1RM = w / (1.0278 − 0.0278 × r) Lombardi (1989): 1RM = w × r^0.10 Mayhew et al. (1992): 1RM = 100w / (52.2 + 41.9 × e^(−0.055r)) O'Conner et al. (1989): 1RM = w × (1 + 0.025 × r) Wathan (1994): 1RM = 100w / (48.8 + 53.8 × e^(−0.075r)) Where w = load used (kg or lbs), r = reps completed to near-failure
Which Formula Should You Use?
A 2006 study by Reynolds, Gordon and Robergs comparing multiple formulas found no single formula was most accurate across all populations and rep ranges. The Epley formula is most commonly cited in peer-reviewed literature and is a reliable general-purpose choice. Brzycki tends to be slightly more accurate for low-rep sets (1–5 reps) because its linear model tracks closely with actual performance in that range. For 6–10 reps, Epley and Wathan both perform well. The most conservative and reliable approach is to average all six formulas — this is what the CostNest 1RM calculator presents alongside the individual estimates.
Why Accuracy Drops Above 10 Reps
All 1RM formulas assume that the relationship between load and maximum possible reps is predictable. Below 10 reps, this relationship is fairly consistent across trained individuals. Above 10 reps, muscular endurance and fatigue-resistance start contributing significantly to how many reps you can complete. A high-rep specialist might push out 20 reps at 60% of 1RM while a pure strength athlete struggles past 15. The formulas cannot account for this individual difference, so estimates diverge from reality. Use 2–8 rep sets for your estimation input whenever possible.
Using % of 1RM for Training Zones
NSCA-recommended training intensity zones (Haff & Triplett, 2016): 95–100% 1RM → Maximal strength: 1–2 reps 85–95% 1RM → Near-maximal strength: 2–4 reps 80–85% 1RM → Strength + some hypertrophy: 4–6 reps 67–80% 1RM → Hypertrophy: 6–12 reps 50–67% 1RM → Muscular endurance: 15+ reps Example: Deadlift 1RM estimate = 150 kg • Strength day (4×4 at 85%): 4 × 4 × 127.5 kg • Hypertrophy day (4×8 at 72%): 4 × 8 × 108 kg • Deload week (3×10 at 60%): 3 × 10 × 90 kg
Tip
Re-test your estimated 1RM every 6–8 weeks, or after completing a dedicated strength block. Percentage-based programming becomes inaccurate if your actual 1RM has shifted and you are still using the old number — either training too light (wasting a strength block) or too heavy (accumulating excessive fatigue). A fresh 3–5 rep test set at the start of each new block takes under 5 minutes and keeps your percentages honest.