How it is estimated
This uses the Epley formula: 1RM = weight × (1 + reps ÷ 30).
Worked example: 100 kg lifted for 5 reps gives 100 × (1 + 5/30) = 100 × 1.167 ≈ 117 kg. A second: 60 kg for 8 reps gives 60 × (1 + 8/30) = 60 × 1.267 ≈ 76 kg.
Several other equations exist and disagree slightly. Brzycki uses weight ÷ (1.0278 − 0.0278 × reps), which on the same 100 kg × 5 input returns about 112.5 kg — roughly 4% below Epley. Lombardi and Wathen produce other figures again. None is definitively correct; they are curve fits to different datasets, and the spread between them is a fair indication of the method's real precision.
Accuracy, honestly
Prediction equations work best close to a true single. At three to five reps, estimates typically land within a few per cent. Accuracy falls away as reps rise, because the number of repetitions someone can perform at a given percentage of their maximum varies substantially between individuals — muscle fibre composition, training history, and the lift itself all matter.
Research comparing prediction equations against measured maxima finds meaningful error, and finds it varies by exercise and by population. As a rule: estimate from a set of ten reps or fewer, prefer five or fewer if you can, and treat the output as a working figure to programme from rather than a number to put in your training log as fact.
The lift matters too. Estimates tend to be more reliable for the bench press than for the squat or deadlift, where fatigue accumulates differently across a set and technique degrades faster under load.
Using your 1RM to programme training
Strength programmes prescribe loads as percentages of 1RM. As a rough guide, based on ACSM progression guidance:
- Maximal strength: 85–100% of 1RM, 1–5 reps, longer rest periods of 3–5 minutes.
- Hypertrophy (muscle size): roughly 67–85%, 6–12 reps, 1–2 minutes rest.
- Muscular endurance: below 67%, 15+ reps, short rest.
These ranges overlap more than the neat categories suggest. Muscle grows across a wide span of loads provided sets are taken reasonably close to failure, and strength improves at moderate loads too. The percentages are a scaffold, not a law.
Autoregulation is often better
A fixed percentage assumes your capacity is constant, and it is not — sleep, stress, nutrition and accumulated fatigue all move it. Some days 85% feels like 75%; some days the reverse.
An increasingly common alternative is to programme by reps in reserve: rather than "5 reps at 85%," aim for "5 reps leaving about 2 in the tank." This self-adjusts to how you actually are on the day, and for most non-competitive lifters it is more practical than recalculating percentages from a max that may be weeks out of date.
Testing a true max, if you must
Rarely necessary outside competition, but if you are going to:
- Warm up thoroughly with progressively heavier, lower-rep sets.
- Use a spotter for bench press, and safety pins or a rack for squats.
- Rest three to five minutes between attempts.
- Make small jumps near the top, and stop when form deteriorates rather than pushing one more.
- Do not test when tired, ill, or under-slept.
For most people, estimating from a comfortable three-to-five-rep set gives nearly the same information with far less risk and disruption.