One Rep Max Calculator

How a one-rep max is estimated

Your one-repetition maximum (1RM) is the heaviest load you can lift once with good form. It is the standard way to express strength for a given exercise, and many programs set training weights as a percentage of it. Testing it directly means working up to a true maximal attempt, which takes time, needs good technique and usually a spotter.

The alternative is to lift a lighter load for as many clean reps as you can, then estimate the 1RM from the weight and the rep count. The fewer reps you can do with a weight, the closer that weight is to your maximum. Each equation on this page describes that relationship a little differently, which is why they don’t agree exactly.

How to get a good number: warm up, choose a weight you expect to manage for roughly 3–10 reps, and count only reps with full range of motion. Stop when the next rep would break form. If you manage exactly one rep, that weight is your 1RM and the calculator shows it as measured.

The six equations

In each equation, w is the weight lifted and r is the number of reps completed. The result is in the same unit as the weight, so pounds and kilograms work the same way.

w

weight lifted (lb or kg)

r

repetitions completed to failure

e

the constant 2.71828…

Epley is the headline figure because it is simple and widely used; Brzycki is the other common choice. Their curves cross at exactly 10 reps, where both give w × 4 ÷ 3. Below 10 reps Epley gives the higher estimate, above 10 Brzycki does.

Brzycki’s equation is often written as w ÷ (1.0278 − 0.0278 × r), which is the same thing. Its denominator reaches zero at 37 reps, so it has no answer there; Lander’s has the same problem just below 38 reps. This calculator stops at 15 reps, well before either.

One rep. Most equations don’t return the weight itself at r = 1 (Epley adds 3.3%). A single completed rep is already a measured maximum, so the calculator shows the weight lifted for every equation.

The percentage table multiplies the Epley estimate by 100%, 95% and so on down to 50%, giving loads you can use when a program prescribes “3 sets at 80%”.

Worked example

You bench press 185 lb for 5 reps and can’t complete a sixth.

1. Epley: 185 × (1 + 5 ÷ 30) = 185 × 1.1667 = 215.8 lb
2. Brzycki: 185 × 36 ÷ 32 = 208.1 lb
3. Lander: 18,500 ÷ (101.3 − 13.356) = 210.4 lb
4. Lombardi: 185 × 5^0.10 = 217.3 lb
5. Mayhew et al.: 18,500 ÷ (52.2 + 41.9 × 0.7596) = 220.2 lb
6. O’Conner et al.: 185 × 1.125 = 208.1 lb

The estimates span 208.1 to 220.2 lb, a spread of about 12 lb. From the Epley figure, 80% of 1RM is 172.7 lb and 70% is 151.1 lb.

The same set in kilograms, 85 kg for 5 reps, gives an Epley estimate of 99.2 kg.

How accurate are 1RM estimates?

• Fewer reps, better estimates. Reynolds and colleagues (2006) tested 70 adults on the bench press and leg press. Estimates from a 5-rep max were more accurate than from a 10-rep max, and they advised using no more than 10 reps with linear equations such as Epley and Brzycki.
• The exercise matters. LeSuer and colleagues (1997) compared seven equations in 67 untrained college students. Predicted and actual maxes were closely correlated, but most equations were significantly off on average for the bench press and squat, and every equation underestimated the deadlift.
• People differ. Training history, muscle fiber make-up and how close you really go to failure all change how many reps you can do at a given percentage. Mayhew and colleagues (2008) noted that most equations give no details of the people they were built on, and that few included women.
• Machines and free weights differ. Most of the research used barbell lifts or specific machines. An estimate from one exercise does not transfer to another.