How to estimate your one-rep max without testing it

2026-10-10 · Zefro

A true one-rep max is a risk and a bad day away from a missed lift. A good set of five to ten gets you almost the same answer.

Your one-rep max, or 1RM, is the most weight you can lift for one clean rep. Many programmes set their loads as a share of it. You can estimate it from a lighter set instead of testing it. This guide shows the formula Zefro uses, works through real examples, and explains when the number holds up and when it does not.

Why estimate instead of test

A real max test means a long warm-up, several heavy singles and a day when everything goes right. Researchers who study strength testing also point out that for some people, age and existing medical conditions can stand in the way of a safe 1RM test. That is one reason prediction equations exist.

An estimate costs you nothing extra. You already do hard sets in training. Log one with honest reps and the maths does the rest. You can repeat it every week without a separate test day.

The formula

Zefro uses the Epley formula: weight × (1 + reps ÷ 30). Lift 80 kg for 8 clean reps and your estimated one-rep max is 80 × (1 + 8 ÷ 30), which is about 101 kg.

Step by step: 8 ÷ 30 is 0.267. Add 1 and you get 1.267. Multiply by 80 and you get 101.3 kg.

The formula treats each rep as worth one thirtieth of the weight on the bar. At 80 kg, one more rep adds 80 ÷ 30, about 2.7 kg, to the estimate. That is why honest rep counts matter so much.

Two more examples:

• 100 kg for 5 reps: 100 × (1 + 5 ÷ 30) = 100 × 1.167, about 117 kg.
• 60 kg for 10 reps: 60 × (1 + 10 ÷ 30) = 60 × 1.333, which is 80 kg.

When to trust it

It is most accurate between 2 and 10 reps. Past that, fatigue and not strength decides when you stop, and the estimate drifts. Use a set taken close to failure, with the same form you would use on a heavy single.

Research backs the low-rep range. In a 2006 study in the Journal of Strength and Conditioning Research, 70 adults did 1, 5, 10 and 20 rep maximums on the bench press and the leg press, each set taken to failure. Predictions from the 5-rep test were the most accurate, and accuracy got worse as the reps went up. The authors concluded that no more than 10 reps should be used in linear equations like Epley's.

The same study compared published formulas. On the leg press, Epley's was among those with the smallest average error when 5-rep data was used. Every formula was less accurate with 10-rep data than with 5-rep data.

Zefro builds a limit in. When the app works out an estimated one-rep max for your records, it counts any set above 12 reps as 12. A set of 25 light reps cannot produce a silly number.

How close to failure

The formula assumes the set ended because you could not do another good rep. If you stop with reps left, the estimate comes out low. Say your true limit at 80 kg is 10 reps but you stop at 8. The formula gives 101 kg instead of 107 kg.

Most people do not judge this perfectly. A 2022 review in Sports Medicine pooled 12 studies and found that people underpredicted how many reps they had left by about one rep on average. Predictions got better closer to failure, with heavier loads and in later sets.

Reps in reserve, or RIR, is how many more reps you could have done. On the RPE scale built on reps in reserve, RPE 10 means none left and RPE 9 means one left. For an estimate, use a set that ends at RPE 9 or 10.

Common mistakes

• Using a high-rep set. 60 kg for 20 reps gives 100 kg by the raw formula. In Zefro's records the same set counts as 12 reps: 60 × 1.4 = 84 kg. Neither is a reliable guess at a single. Use a heavier set.
• Using an easy set. A set you stopped well short of failure tells you little. Zefro does not let warm-up sets or drop sets claim a record.
• Changing the form. The authors of the 2006 study stressed the quality of each rep and used a fixed tempo of one second up and one second down. Keep the same depth, grip and tempo from week to week, or the numbers are not comparable.
• Using one percentage table for every lift. Another 2006 study, of trained and untrained men, found more reps at 60 percent of 1RM on the back squat than on the bench press or the arm curl. The number of reps at a given percentage depends on the lift, so treat percentages as a starting point.
• Mixing equipment. The bench and leg press study warned against using its equations on other machines. Track a barbell bench press and a machine press as separate lifts.

What to do with the number

• Pick working weights as a share of it. A set of 8 sits around 75 to 80 percent.
• Track it over weeks. A rising estimate on the same lift means you are getting stronger, even if you never test a max.
• Re-estimate when your reps change, not on a schedule.

To turn the formula around, divide 1 by (1 + reps ÷ 30). For 5 reps that is about 86 percent of your max, for 8 reps about 79 percent and for 10 reps 75 percent. With an estimated max of 101 kg, a set of 5 to failure lands near 101 × 0.86, about 87 kg.

Most lifters leave a rep or two in reserve on working sets. Add those reps to the count before you divide. A set of 5 with 2 reps in reserve is treated like 7 reps: 1 ÷ (1 + 7 ÷ 30) is 81 percent, about 82 kg from a 101 kg max.

How you pick the reps depends on your goal. The American College of Sports Medicine recommends loads of an 8 to 12 rep maximum for beginners. For intermediate and advanced lifters it recommends a wider range of 1 to 12 rep maximums over time, with an eventual emphasis on heavy loads of 1 to 6. When you can do one or two reps more than your target, it recommends adding 2 to 10 percent to the load.

Track progress with it

The estimate lets you compare sets with different weights and reps. Week 1: 80 kg for 8 gives 101 kg. Week 4: 80 kg for 10 gives 107 kg. Week 6: 85 kg for 8 gives 85 × 1.267, about 108 kg. Three different sets, one clear trend.

Look at the trend over several weeks, not one session. At 80 kg, a single rep moves the estimate by about 2.7 kg, and a rep is easy to miss on any given day.

Who it suits and who it does not

It suits you if you train with sets of about 2 to 10 reps, want a training max without a test day, or want one number to compare progress across different sets and reps.

It suits you less if you only train with sets above 12 reps, where the estimate is weakest, or if your form on a lift still changes from week to week. If you need your exact single, for example to compete, the estimate tells you where to start, not what you will lift on the day.

How Zefro uses it

When you finish a workout, Zefro compares each exercise's best estimated one-rep max with your earlier best. If the new number is higher, the workout summary flags a new personal record. A tie is not a record, and your first session on an exercise sets none, because there is nothing to beat yet.

When a routine in Zefro sets a load as a percentage of your estimated max or by a target RPE, the app takes the best estimate from your last session's working sets, leaves out sets above 12 reps, and rounds the load to the nearest 1.25 kg. For example, 75 percent of a 101 kg estimate is 75.75 kg, which rounds to 76.25 kg. For a target RPE it uses the reps in reserve sum above, so 5 reps at RPE 8 from a 101 kg estimate comes to 82.5 kg.

Zefro+ also follows your estimated one-rep max for each exercise over time.

Practical steps

1. Warm up the way you normally would.
2. Pick a weight you can lift about 5 times. That is where the 2006 bench and leg press study got its most accurate predictions.
3. Do as many clean reps as you can and stop when the next rep would break your form.
4. Log the weight and the reps.
5. Run the numbers in the calculator, or log the set in Zefro and let it check for a new record when you finish.
6. Use the result to pick working weights, and check again whenever your reps at a given weight change.

Questions

Is the estimate my real max? No. It is a prediction. In the 2006 bench and leg press study, the authors' own 5-rep equation was off by about half a kilo on average in a separate test group, but individuals still varied. On the bench press, most fell within about 3.5 kg either side. Treat it as a guide.

Which formula is best? There is no single winner. In the 2006 comparison, Epley's was among the most accurate on the leg press, while other formulas did best on the chest press. Every formula was less accurate with 10-rep sets than with 5-rep sets, so pick your set with care.

Does the site calculator cap reps at 12 like the app? No. The calculator applies the plain Epley formula to whatever reps you enter, in kilograms or pounds. Keep your input between 2 and 10 reps for a useful answer.

Do I need to test a real max to follow a programme? Not for the recommendations above. The ACSM writes its loads as rep-maximum ranges, not as a share of a tested single. A hard set in the right rep range already tells you the load.

Try it with your own numbers in the 1RM calculator. See the bench press, squat and deadlift pages for how each lift is done, and the glossary for the terms, including estimated 1RM. To keep the estimate rising, read about progressive overload or pick one of the workout plans.