Powerlifting One-Rep Max Calculator

Find your max without risking a max attempt

Knowing your one-rep max (1RM) drives almost every percentage-based training program, but actually testing a true single is fatiguing and carries injury risk. Instead you can estimate it from a submaximal set you already performed. This calculator runs four established formulas at once so you can see both a best estimate and the spread between models.

How it works

Each formula maps the weight you lifted and the reps you completed onto a predicted single. The four implemented here are:

Epley     : 1RM = weight × (1 + reps / 30)
Brzycki   : 1RM = weight × 36 / (37 − reps)
Lombardi  : 1RM = weight × reps^0.1
O'Connor  : 1RM = weight × (1 + reps / 40)

At one rep, every formula returns the weight unchanged. As reps rise, Epley and O’Connor scale roughly linearly, Brzycki rises faster (and is undefined at 37 reps), and Lombardi uses a gentle power curve. The tool also reports the average of the four as a balanced estimate.

The four formulas compared

Epley (1985) is one of the oldest and most widely cited formulas. It scales linearly, making it simple and slightly optimistic at higher rep counts. Most standard in gym settings and online fitness tools.

Brzycki (1993) tends to give slightly lower estimates than Epley at moderate rep counts and becomes very aggressive above 15–20 reps. The denominator (37 − reps) means the formula is only valid up to 36 reps. For sets of 1–10 reps it tracks Epley closely.

Lombardi (1989) uses a power function (reps^0.1), which grows more slowly than the linear formulas. It tends to be slightly more conservative at low reps but diverges less dramatically at higher rep counts than Brzycki.

O’Connor et al. (1989) divides by 40 instead of 30, producing consistently lower estimates than Epley across all rep ranges. It is considered the most conservative of the common formulas and is sometimes preferred for safety planning.

Accuracy and rep range

All 1RM formulas lose accuracy as rep count rises:

• 1–3 reps: estimates are very close to your actual max and highly reliable.
• 4–6 reps: good accuracy; the typical range used in powerlifting programs.
• 7–10 reps: reasonable accuracy for planning purposes; expect ±5–10%.
• Above 10 reps: accuracy degrades meaningfully. At 15+ reps, strength-endurance and technique play large roles that the formulas cannot account for, and all four will tend to overestimate.

For powerlifting specifically — squat, bench, deadlift — the most reliable estimates come from sets of 3–5 reps performed at true near-failure effort.

Worked example

Suppose you completed 100 kg for 5 reps (genuinely close to failure):

The tight cluster (112–118 kg range) tells you your true single is very likely in the mid-110s. If the formulas diverged widely — say 105–130 kg — that would signal the rep count was high enough to reduce reliability and you should test a heavier set of 2–3 reps instead.

Practical use in programming

Once you have an estimated 1RM, most percentage-based programs use it directly:

• 85–90% 1RM — typical heavy single or top-set weight in a peaking block
• 75–82.5% 1RM — common working-set range in strength programs
• 65–75% 1RM — technique, volume, or speed work

Treat the estimated 1RM as a planning number. Verify it with a real top single only during a deliberate peak, when you are rested and the risk is managed.