One Rep Max and PR Calculator

How it works

Your one-rep max (1RM) is the heaviest weight you could lift for a single, full rep with good technique. Testing it directly means loading a bar to failure on one all-out attempt, which carries real injury risk and needs a spotter and a proper warm-up — so most lifters estimate it instead, from a lighter set taken close to failure.

This calculator uses the Epley formula, one of the most widely used 1RM estimators in strength training:

Estimated 1RM = Weight lifted × (1 + Reps ÷ 30)

If you enter exactly 1 rep, no adjustment is needed — the weight you lifted already is your 1RM, so the calculator returns it unchanged rather than inflating it.

The Epley formula, like every rep-based 1RM formula, produces an estimate, not a guarantee. It assumes the set was taken close to failure with consistent bar speed and technique, so your real 1RM can vary from the estimate with fatigue, sleep, bar speed and even the time of day you trained. Estimates are most reliable at lower rep counts — roughly 1 to 10 reps — because the relationship between load and reps-to-failure gets less predictable as sets get longer, and factors like pacing and grip endurance start to matter more than raw strength.

Below the estimated 1RM, this calculator also shows a percentage-based training table. Strength programs are usually written as a percentage of 1RM — “work up to 3 sets of 5 at 80%” — rather than an absolute weight, because a percentage scales automatically to each lifter and stays useful as your strength changes, without needing to rewrite the whole program every time your max moves.

Worked example

Jack, training at a gym in Newcastle, benches 100kg for 5 reps on the last set of his session — a genuine rep PR for him at that weight. Plugging those numbers into the Epley formula:

100 × (1 + 5 ÷ 30) = 100 × 1.1667 = 116.7kg estimated 1RM

Jack isn’t about to load 116.7kg onto the bar and attempt it cold — the estimate is there to guide his next block of training, not to be tested directly. Using the percentage table, he can plan future sessions straight off that number:

% of 1RM	Weight	Typical reps
90%	105.0kg	~4 reps
80%	93.3kg	~8 reps
70%	81.7kg	~12 reps

So for a heavy strength session, 105.0kg for sets of around 4 reps sits at 90% of his estimated max. For a hypertrophy-focused session, 93.3kg for sets of around 8 reps sits at 80%. And for a lighter technique or deload session, 81.7kg for sets of around 12 reps sits at 70% — all planned from that single 100kg × 5 rep set, without Jack ever attempting the 116.7kg directly.