How is 1RM calculated?

Updated October 25, 2026

Short answer: a 1RM is calculated by putting a set you lifted for a few reps into a formula that estimates your one-rep max. The four common formulas are Epley, Brzycki, Lombardi and O'Conner. For 100 kg lifted 5 times they give 116.67, 112.5, 117.46 and 112.5 kg. They agree closely at low reps and drift apart as the reps go up.

Example: one set of 5, four formulas

Say you squat 100 kg for 5 reps. Here is each formula worked through for that set:

So the estimate for this one set lands between 112.5 and 117.46 kg, depending on the formula. None of them is wrong: each is a different fit to how many reps people manage as the weight gets heavier. To run your own set through all four at once, use the 1RM calculator.

Why the formulas split apart above about 10 reps

Here is the same 100 kg lifted for 10 and 15 reps:

At 5 reps the four answers stay between 112.5 and 117.46 kg. At 15 reps they run from 131.1 kg (Lombardi) to 163.64 kg (Brzycki). Brzycki climbs fastest because its divisor, 37 minus the reps, shrinks as the reps rise. Lombardi climbs slowest because a power of 0.1 grows very little. Epley and O'Conner add a fixed share per rep, one thirtieth and one fortieth.

The bigger reason is the lifter. A long set tests endurance as much as strength, and two people with the same max can get very different rep counts with the same weight. Every formula is an average, and the further a set is from a single, the less any average says about you.

Picking one formula and keeping it

Epley and Brzycki are the most widely used, and at 10 reps they give the same answer: 133.33 kg for 100 kg. Below 10 reps Brzycki is the more cautious of the two; above 10 it is the more generous. Whichever you choose, the habits below matter more than the choice:

• Use a hard set of about 3 to 5 reps. Low-rep sets give the closest estimates.
• Count only full reps. A rep cut short or helped by a spotter inflates the result.
• Stay with the same formula. Your estimates then compare from month to month, even if the formula is a little high or low for you.

The formulas run backwards: rep max charts

A 1RM formula also works the other way. Instead of turning a set into a max, it turns a max into the share of it you can lift for each rep count. By Epley, 5 reps is about 85.7% of a 1RM. That is what a rep max chart lists, from 1 to 12 reps.

Estimate or test?

An estimate costs nothing: it comes from a set you were doing anyway. A test gives the real number for that day, but it takes a full session, careful warm-ups and a spotter or safety arms. Most lifters estimate between tests and test now and then, for example at the end of a training block. When you do test, how to test your 1RM safely walks through the attempts.

How LoadPR estimates your 1RM

The LoadPR app estimates a 1RM from your logged sets with the Epley formula by default. With LoadPR+ you can pick another formula, such as Brzycki, and the estimate appears as its own line on the full progress chart. The app keeps your current PR separate from the estimate: the current PR is the heaviest weight you have logged, not a formula result. More in PR tracking.

Mistakes when calculating a 1RM

• Estimating from a set of 12 or more. At 15 reps the formulas are more than 30 kg apart for a 100 kg set.
• Using a set that was not close to failure. If you could have done 3 more reps, the formula underestimates you.
• Switching formulas between months. A jump from 112.5 to 117.46 kg can be the formula, not your strength.
• Mixing units. Estimate in the unit you lifted in, then convert if you need to.
• Treating the estimate as a weight to hit. It is a planning number for setting training loads.