One Rep Max Calculator

What a one rep max measures

A one rep max, usually written 1RM, is the heaviest load you can lift once with the same form you use on a working set. It is the anchor for percentage training: when a program says three sets of five at 85 percent, that 85 percent is a slice of this number.

Testing a true max takes a rack, a spotter, several warm-up sets and a willingness to fail a lift. Most lifters get a close enough number from a set they already finished. The equations on this page take the weight you used and the reps you completed, then project the single you did not attempt.

The CDC activity guidance puts muscle strengthening at two or more days a week, with 8 to 12 reps per set for general health. A 1RM estimate is a training tool, not a medical test, and it is not a reason to lift through pain.

One detail matters before any formula: the rep count has to come from a set taken close to failure. A set of five that felt easy is not a five rep max, and feeding it into an equation will understate your strength by a wide margin.

The Epley formula, step by step

Epley's equation is the one most gym apps use, because it needs nothing more than a weight and a rep count:

1RM = weight x (1 + reps / 30)

Lift 185 lb for 6 reps and the estimate is 185 x 1.2 = 222 lb. Each rep past the first adds one thirtieth of the bar weight, so six reps add 20 percent.

The same arithmetic on a heavier set: 275 lb for 3 reps gives 275 x 1.1 = 302.5 lb. Note how the formula treats every rep as equal. That is a simplification, and it is why the estimate gets shaky as the rep count climbs.

At a single rep the equation returns 3.3 percent more than the weight on the bar. If you truly completed a single, that weight is your 1RM and you do not need the projection.

The Brzycki formula and where it lands

Brzycki's equation uses a ratio instead of a fixed bonus per rep:

1RM = weight x 36 / (37 - reps)

The same 185 lb for 6 reps becomes 185 x 36 / 31 = 214.8 lb, about 7 lb below the Epley answer. The two equations meet at 10 reps, where both multiply the weight by 1.333. Below 10 reps Epley runs higher, and above 10 reps Brzycki climbs faster and eventually breaks down: at 37 reps the denominator reaches zero.

Brzycki built the equation from a percent-of-max chart and put its useful range at 10 reps or less. A University of New Mexico review of prediction equations reaches the same limit, noting that no more than 10 repetitions should be used in the linear equations for leg press and bench press.

Four formulas on the same set

Two more equations show how much the choice matters. Lander's equation is 100 x weight / (101.3 - 2.67123 x reps), and O'Conner's is weight x (1 + reps / 40). Both take the same inputs and return a different single.

At 225 lb for 5 reps the four answers run from 253.1 to 262.5 lb, a spread of 9.4 lb. At 12 reps the spread reaches 32.4 lb, which is wider than a pair of 10 lb plates. The average is not a fifth formula; it is a way to stop one equation's quirk from setting your whole program.

For a working estimate, the average of four is the safest single number to carry into a percentage table. If you want one formula, use Brzycki for sets of 1 to 5 and Epley for sets of 6 to 10.

Why the rep count decides the error

All four equations were fitted to sets of roughly 1 to 10 reps. Past 10 reps the limit on performance stops being maximal force and starts being muscular endurance, breathing and pain tolerance. The curves were not built for that, so they fan out.

The percent chart below is the other direction of the same relationship. It comes from the strength testing chapter of Essentials of Strength Training and Conditioning (Baechle and Earle, 2008), as reproduced in the New Mexico review. Use it to read a set backwards: eight reps is about 80 percent of max.

Read that row again with a real bar: 225 lb for 8 reps implies a 281 lb single, since 225 / 0.80 = 281.25. The same 225 lb for 5 reps implies 259 lb, because five reps sits at about 87 percent. Two sets, one bar, estimates 22 lb apart.

Turning the estimate into working weights

A 1RM is not a workout. It is a reference point that lets you pick loads without guessing. Take the averaged 5 rep estimate of 256 lb from the table above and run it through the common percentages.

At 80 percent the arithmetic is 256 x 0.80 = 204.8, so the bar gets 205 lb. At 70 percent it is 256 x 0.70 = 179.2, which rounds to 180 lb. Round to the smallest pair of plates you own: 5 lb steps in a commercial gym, 2.5 kg steps in a metric gym.

If your plates are in kilograms, divide the pound figure by 2.20462. A 256 lb max is 116.1 kg, and 80 percent of that is 92.9 kg. Do the conversion once, then stay in one unit for the whole program.

How to test without a true max

A submaximal test is safer than a true single and accurate enough for programming. Warm up for five minutes, then work through the bar, 50 percent for five, 70 percent for three and 85 percent for one, resting two to three minutes between sets.

Pick a weight you can lift for three to six reps with two reps left in reserve. Stop the set when bar speed drops, not when the bar stops. A set stopped two reps short tends to produce an estimate slightly below your true max, which is the safe direction to miss in.

Use the same equipment you train on. A bench press on a different bench, with a different bar and a different grip width, is a different lift. Record the weight, the reps and the date; a number without a date is a rumor.

Safety is not optional. Use a spotter or safety pins for bench and squat, keep your hips and feet set, and end the set if form breaks. The CDC strength guide for older adults is a sensible starting point for anyone returning after a long break, and it recommends working up over weeks rather than testing a max in the first session.

Bodyweight ratios and four common mistakes

Dividing a 1RM by bodyweight gives a ratio that travels better than the raw number. A 180 lb lifter with a 256 lb bench has a ratio of 256 / 180 = 1.42. The same lifter at 170 lb would show 1.51 without adding a pound to the bar, which is why ratios are useful for tracking and useless for bragging.

The largest error source is the rep count, not the formula. A set of 15 reps fed into Epley returns 225 x 1.5 = 337.5 lb from a 225 lb set, while Brzycki returns 225 x 36 / 22 = 368.2 lb. Both numbers are fiction: the set was an endurance set, and the equations were fitted to strength sets.

Treat the output as a starting load, not a verdict. Run the first week at 90 percent of the calculated working weight, add load when all reps are clean, and retest the rep set every four to six weeks. The estimate follows the training, not the other way around.