One-rep max calculator

How estimation formulas work

A one rep max estimate comes from a simple idea: if you know how much weight you moved and how many times, you can work backward to the weight you could have moved exactly once. Every rep past the first costs you strength — fatigue builds set by set, so the fifth rep at a given weight is harder than the first. Formulas like Epley and Brzycki model that cost as a relationship between weight and reps. That curve is fitted to data from lifters testing both submaximal sets and true maxes. Feed in what you actually lifted for reps, and the formula projects what that same effort would look like at a single rep.

Both formulas assume the reps were taken close to failure — the last rep in the set should have been genuinely hard, not stopped with three left in the tank. A submaximal set that stops early isn't testing anything close to a limit. A calculator can't tell "I could have done two more" from "that was as far as I could go." The estimate is only as honest as the set it's built from.

Why accuracy drops above about 10 reps

The formulas were built from data mostly clustered under about 10 reps, and the relationship between reps and strength loss isn't perfectly linear — it just behaves close enough to linear in that range for the math to hold up. Past 10 reps, technique breakdown, cardiovascular fatigue, and grip endurance start deciding when a set ends more than raw strength does, and none of the formulas account for any of that. A set of 20 reps is testing muscular endurance as much as it's testing strength, and pulling a one rep max out of it stretches the model well past where it was ever checked against real lifters.

There's a second, sharper problem with Brzycki specifically: the formula divides by (37 − reps), so as reps climbs toward 37 the denominator shrinks toward zero and the estimate balloons toward infinity. That's not a rounding error — it's the formula breaking down entirely. This calculator caps input at 30 reps for that reason: past 10 the number is already a rough guide, and past 30 neither formula is describing anything real.

Epley vs Brzycki: the same set, two numbers

Take a lifter who does 225 lb for 5 reps. Epley's formula is 1RM = weight × (1 + reps ÷ 30), which gives 225 × (1 + 5/30) = 225 × 1.167 = 262.5. Brzycki's formula is 1RM = weight × 36 ÷ (37 − reps), which gives 225 × 36 ÷ 32 = 225 × 1.125 = 253.1. Same set, same lifter, and the two formulas land about 9 lb apart. Under 10 reps, Epley's figure is the larger one, as it is here. At exactly 10 reps the two agree. Above 10, Brzycki gives the higher estimate and the gap keeps growing: 225 lb for 15 reps works out to 338 lb by Epley and 368 lb by Brzycki.

At a single rep there is nothing to extrapolate: the weight you lifted is your one rep max, and both formulas are made to return it unchanged. Brzycki does that on its own, since 36 ÷ (37 − 1) is exactly 1. Epley's curve does not: written out, it would give 225 × (1 + 1/30) = 232.5. So at 1 rep this calculator returns the weight lifted, rather than a figure the formula was never meant to produce.

Neither number is more correct in any absolute sense — they're two different curve fits to overlapping but not identical data, and the gap between them is a rough measure of how much uncertainty sits inside any estimate taken from 5 reps. If a program specifies a formula (Brzycki shows up in a lot of older strength literature), use that one for consistency. If it doesn't, Epley is the more common default, and the one this calculator opens on — but the honest answer to "what's my 1RM" from a 5-rep set is a range in the low-to-mid 250s to low 260s, not a single decimal point.

Using percentages to program from your 1RM

Once you have an estimated max, the number that actually gets used in a program is almost never the max itself — it's a percentage of it. The table above converts your result into working weights for specific goals. Heavy singles and doubles at 90–95% build peak strength. Moderate sets around 70–80% build a mix of strength and size. Lighter sets below 65% are usually for volume, technique work, or a deload week. A 262 lb estimated max shows roughly 223 lb for a set of 5–6 at 85%, or 158 lb for higher-rep work at 60%.

Programs use a percentage rather than a fixed weight because the percentage travels with you as you get stronger. "Work up to a set of 5 at 80%" applies whether your max is 200 lb or 400 lb, without anyone rewriting the plan. That's also why it's worth re-estimating or re-testing a 1RM periodically: percentages calculated off a max from six months ago undersell where you actually are now.

When an actual max attempt beats an estimate

An estimate is a starting point, not a ceiling. It earns its keep when you don't want to spend a training session finding your actual max, don't have a spotter, or are working with a lift where a true single carries real technical risk. A heavy squat or bench attempt with breaking form has a different injury profile than a controlled set of 5. For programming ordinary training weeks, an estimate from a recent, honestly-pushed set is close enough that the difference from a tested number rarely changes what actually goes on the bar.

A real test is worth doing when the estimate is about to drive something that matters — setting weights for a meet, checking whether a program actually worked, or when two formulas disagree by enough that you're not sure which working weight to trust. It's also the only way to catch a lift where technique, not raw strength, is the limiting factor: weak grip or a poor bar path can grind out reps at a submaximal weight in a way that doesn't extrapolate cleanly to a heavier single. When it's safe to do, a tested max always beats a formula — that's the whole reason the formulas exist, as a stand-in for testing, not a replacement for it.

This calculator estimates a one rep max from a formula applied to the weight and reps you enter — it doesn't know your technique, your recovery, or whether that last rep was really your limit. Epley and Brzycki are curve fits built from other lifters' data, not a measurement of your body, and they can be meaningfully wrong outside the rep ranges they were built from. Treat the number as a planning tool, not a target to load onto a bar for a first attempt.