One Rep Max (1RM) Calculator
Last updated: 27 June 2026
Reviewed by Gavin Meiring, Lead research and primary author · Doctoral Candidate (Corporate Governance) · Research and drafting assisted by AI
- The Epley formula behind many 1RM calculators was created in 1985 by Boyd Epley, the strength coach who built the University of Nebraska's pioneering weight-training programme.
- One-rep-max prediction is an old idea in new clothes: the Greek wrestler Milo of Croton reportedly built strength in the 6th century BC by carrying a calf every day until it grew into a full-grown bull.
- A true 1RM is the heaviest weight you can lift exactly once with good form — most calculators instead estimate it from a lighter set, which is safer and needs no spotter for a maximal lift.
One Rep Max Calculator
A one rep max (1RM) calculator estimates the maximum weight you could lift for a single repetition of any exercise based on a lighter weight and how many reps you complete. It is used by strength athletes, powerlifters, and gym-goers to set training loads without the risk of a true maximal lift.
How to Use the One Rep Max Calculator
- Enter the weight you lifted, in kilograms or pounds.
- Enter the number of repetitions you completed with that weight.
- Click Calculate to see your estimated 1RM.
- Use the percentage table shown to plan your training loads at different intensities.
- For best accuracy, use a set where you lifted between 3 and 10 reps to near failure.
The Formula
Several formulas exist; the Epley formula is the most widely used. 1RM = weight multiplied by (1 plus (reps divided by 30)). The Brzycki formula is also common: 1RM = weight multiplied by (36 divided by (37 minus reps)). Both are accurate up to about 10 repetitions; accuracy decreases at higher rep counts. The calculator typically averages two or more formulas for a more reliable estimate.
Real-World Example
A lifter squats 100 kg for 8 repetitions to near failure. Using the Epley formula: 1RM = 100 multiplied by (1 plus (8 divided by 30)) = 100 multiplied by (1 plus 0.267) = 100 multiplied by 1.267 = 126.7 kg. Using Brzycki: 1RM = 100 multiplied by (36 divided by (37 minus 8)) = 100 multiplied by (36 divided by 29) = 100 multiplied by 1.241 = 124.1 kg. The estimated 1RM is approximately 125 kg.
Using Percentages to Programme Training
Once you know your 1RM, you can structure training with precision. 90 to 95 percent of 1RM (1 to 3 reps) develops maximum strength. 80 to 85 percent (4 to 6 reps) builds strength and some hypertrophy. 70 to 80 percent (6 to 12 reps) is the primary hypertrophy range. 60 to 70 percent (12 to 20 reps) develops muscular endurance. Many structured programmes such as 5/3/1 and Starting Strength use 1RM percentages as their loading framework. Reassess your 1RM every four to eight weeks as strength improves.
Frequently Asked Questions
Should I attempt a true 1RM lift? True 1RM testing carries injury risk, particularly for novice lifters or on technical movements such as the snatch or clean. Estimated 1RM from a calculator is safer and sufficiently accurate for programming purposes. True 1RM testing is best reserved for competition or under supervision.
Which exercises work best with a 1RM calculator? The formula works best for compound barbell movements such as the squat, deadlift, bench press, and overhead press. It is less reliable for isolation exercises or cable machines because the rep-to-strength relationship differs from free-weight compound lifts.
Why does my estimated 1RM vary between formulas? Different formulas were derived from different populations and use different mathematical relationships between reps and weight. Variation of 2 to 5 percent between formulas is normal. Using an average of several formulas gives a more reliable estimate.
How often should I retest my 1RM? For most trainees, retesting every four to eight weeks is appropriate. Testing too frequently reduces training time and the accuracy of the estimate, as your true 1RM changes slowly. If you are following a periodised programme, retest at the end of each training block.
The same lift through four formulas
The Epley and Brzycki formulas above cover two of the published estimates. Two more appear often enough to be worth running on the same set, because the spread between them is the error bar a lifter is actually working with.
Running 100 kg for 8 reps through each:
| Formula | Estimate for 100 kg at 8 reps | Form of the relationship |
|---|---|---|
| Epley | 126.67 kg | Linear in reps, so it rises steadily and overstates at high reps |
| Brzycki | 124.14 kg | Approaches a limit at 37 reps, so it rises steeply near that limit |
| Lombardi | 123.11 kg | Uses reps raised to the power 0.10, so it flattens as reps rise |
| Lander | 125.11 kg | A separate linear regression fitted to rep-max data |
| Mean of the four | 124.76 kg |
The four estimates sit within 3.55 kg of each other, a spread of 2.85% of their mean, and the page's rounded answer of about 125 kg sits inside that band. Averaging is the reason the calculator described on this page does not depend on a single formula. Each formula was fitted to a different group of lifters on a different set of exercises, and the errors are partly independent, so the mean of several is more stable than any one of them.
The mean is not a correction for bad data. It is a way of admitting that the relationship between reps and maximum strength differs between people, and that a formula derived from one population carries that population's habits with it.
Epley and Brzycki diverge as reps rise
The two most common formulas agree at one rep count and nowhere else, which is worth knowing before choosing one for a high-rep set.
| Reps at 100 kg | Epley | Brzycki | Difference |
|---|---|---|---|
| 1 | 103.33 kg | 100.00 kg | +3.33% |
| 3 | 110.00 kg | 105.88 kg | +3.89% |
| 5 | 116.67 kg | 112.50 kg | +3.70% |
| 8 | 126.67 kg | 124.14 kg | +2.04% |
| 10 | 133.33 kg | 133.33 kg | 0.00% |
| 12 | 140.00 kg | 144.00 kg | -2.78% |
| 15 | 150.00 kg | 163.64 kg | -8.33% |
| 20 | 166.67 kg | 211.76 kg | -21.30% |
| 30 | 200.00 kg | 514.29 kg | -61.11% |
| 36 | 220.00 kg | 3,600.00 kg | -93.89% |
The two formulas return identical figures at exactly ten reps, and that coincidence is a property of the algebra rather than a claim about the physiology. Below ten reps Epley reads higher; above ten reps Brzycki climbs away and eventually leaves the range of anything a person can lift. At 36 reps it returns 3,600 kg, and at 37 reps it divides by zero. Brzycki's curve has a vertical asymptote at 37 because the denominator is 37 minus the rep count, and that limit is a feature of the fitted shape rather than a claim that a maximum exists there.
The practical reading is a rep range. Both formulas were fitted on sets between roughly one and ten reps, they agree closely from one to eight reps, and they part company above ten. A set of eight to failure is a sound input for either. A set of twenty is not a sound input for Brzycki, and Epley tends to overstate it.
A full percentage ladder off a 125 kg estimate
An estimate is only useful once it is turned into loads, and the steps below use a 1RM of 125 kg with a loadable increment of 2.5 kg.
| Share of 1RM | Exact load | Load on 2.5 kg steps | Typical target reps | What the band trains |
|---|---|---|---|---|
| 95% | 118.75 kg | 120.0 kg | 1 to 3 | Maximum strength |
| 90% | 112.50 kg | 112.5 kg | 3 to 5 | Maximum strength |
| 85% | 106.25 kg | 105.0 kg | 4 to 6 | Strength with some size |
| 80% | 100.00 kg | 100.0 kg | 6 to 8 | Size and strength |
| 75% | 93.75 kg | 95.0 kg | 8 to 10 | Size |
| 70% | 87.50 kg | 87.5 kg | 10 to 12 | Size |
| 65% | 81.25 kg | 80.0 kg | 12 to 15 | Size and endurance |
| 60% | 75.00 kg | 75.0 kg | 15 to 20 | Endurance |
Rounding to a loadable weight changes the true intensity, and the change is small in the middle of the table and larger at the edges. The 95% row moves up to 120 kg, which is 96% of the estimate, and the 75% row moves up to 95 kg, which is 76%. Both moves push the set a fraction harder than the label says. Lifters on a strict programme often round down for the heavy rows and round up for the light ones, so that a hard set is never harder than planned.
The percentage table is also the reason the estimate's precision matters less than its consistency. A 2.85% spread between formulas shifts every load in the table by that same margin, which is a kilogram or two at most. Comparing the same formula's output from month to month, on the same exercise and a similar rep count, carries more information about progress than the absolute figure does.
Where the formulas were published and where they break
The Epley estimate comes from Boyd Epley's 1985 poundage chart, produced in a strength coaching context and still the most widely used of the group. Brzycki's came from Matt Brzycki's work on strength testing published in 1993, and it is the more conservative of the two in the low rep ranges that matter for strength work.
Three failure modes apply to all of them. Sets not taken near failure produce estimates that are too low, because a set stopped short does not reveal how many more reps were available. Sets with technical breakdown in the last reps produce estimates that are too high, because a rep completed with poor form is not the same stimulus as a clean one. And single-joint movements fit these formulas less well than compound barbell lifts, because the relationship between reps and load differs between a squat and a curl.
The individual variation around any of these estimates runs at a few percent, and that variation is larger than the disagreement between the formulas in the one to eight rep range. A lifter whose true maximum sits above the estimate will find every working weight in the percentage table a little light, and the fix is to adjust the loads rather than to distrust the formula.
Sources: Epley, B., poundage chart, 1985, and Brzycki, M., Strength Testing: Predicting a One Rep Max from Reps to Fatigue, Journal of Physical Education, Recreation and Dance, 1993, for the two formulas already printed on this page.
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