Your one-rep max (1RM) is the heaviest weight you can lift for a single clean repetition. It is the yardstick almost every strength program is built around: prescribing loads as a percentage of your 1RM keeps training intensity consistent as you get stronger. This one rep max calculator estimates your 1RM from a normal working set — no risky maximal attempt required — and builds a full percentage table so you know exactly what to load for any session.
How the one-rep max calculator works
Testing a true 1RM is risky and hard to repeat, so lifters estimate it from a submaximal set instead. Enter the weight lifted (in kg or lb) and the number of reps you completed with good form, and the tool averages two of the most established equations in strength training:
- Epley: 1RM = weight × (1 + reps ÷ 30)
- Brzycki: 1RM = weight × 36 ÷ (37 − reps)
Averaging the two smooths out their individual quirks — Epley reads slightly high at higher reps, Brzycki slightly low — for a more reliable single number.
Worked example
Suppose you bench press 100 kg for 5 reps. Epley gives 100 × (1 + 5 ÷ 30) ≈ 116.7 kg; Brzycki gives 100 × 36 ÷ (37 − 5) = 112.5 kg. The average — about 115 kg — is your estimated one-rep max. From there, a 5×5 session at 80% would load roughly 92 kg, while a heavy single at 95% would be about 109 kg.
Using your training percentages
Programs express working loads as a percentage of 1RM. As a rough guide: 90–100% builds maximal strength (1–3 reps), 80–90% is the strength sweet spot (3–6 reps), 70–80% drives muscle growth (6–12 reps), and 60–70% suits speed work and technique. The percentage table under your result gives the exact weight for each level in your chosen unit.
Getting the most accurate estimate
For the best result, use a set of 3–8 reps taken close to failure with strict form. Estimates get progressively less accurate above roughly 10 reps, where fatigue and pacing vary too much between lifters. Retest with a fresh submaximal set every few weeks — as your estimated 1RM climbs, your percentage-based loads rise with it, keeping the training stimulus honest.
Estimates are a training aid, not a guarantee. Warm up thoroughly, use a spotter for heavy attempts, and stop if form breaks down.