A 1RM calculator will give you a max to one decimal place from any set you type in. Feed it a hard set of 5 reps or fewer and the number is usually close enough to program from; feed it a set of 10 or more and it starts to drift, by a different amount on every lift. And whatever it tells you, an estimated 1RM is only comparable to other estimates made the same way.
How does a 1RM calculator estimate your max?
It takes a weight you lifted for a given number of reps and extrapolates what you could lift once. The output is an estimated 1RM, usually written e1RM. Two formulas sit behind most of the calculators you’ll find.
The Epley formula
1RM = w × (1 + r / 30), where w is the weight and r is the reps. Each rep adds one thirtieth of the load.
A 100 kg (220 lb) set of 5 gives 100 × (1 + 5 / 30) = 116.7 kg. One quirk: Epley never hands back the weight you lifted for a single. Put in 100 kg × 1 and you get 103.3 kg.
The Brzycki formula
1RM = w / (1.0278 − 0.0278 × r). Every extra rep shrinks the denominator, which pushes the answer up.
Same set: 100 / (1.0278 − 0.139) = 100 / 0.8888 = 112.5 kg. For a single, the denominator is exactly 1, so Brzycki returns the weight on the bar.
Same set, two formulas: how far apart are they?
Here is 100 kg (220 lb) run through both, rounded to one decimal:
| Reps | Epley | Brzycki | Gap (Epley − Brzycki) |
|---|---|---|---|
| 3 | 110.0 kg | 105.9 kg | +4.1 kg |
| 5 | 116.7 kg | 112.5 kg | +4.2 kg |
| 8 | 126.7 kg | 124.2 kg | +2.5 kg |
| 10 | 133.3 kg | 133.4 kg | ≈ 0 kg |
| 12 | 140.0 kg | 144.1 kg | −4.1 kg |
Up to 8 reps, Epley reads 2.5 to 4 kg (roughly 5 to 9 lb) higher. The two lines cross around 10 reps, then Brzycki pulls away fast: 163.7 kg against 150.0 kg at 15 reps, and 212.0 kg against 166.7 kg at 20. That’s roughly 45 kg (100 lb) of disagreement about the same set.
Blame Brzycki’s denominator. 1.0278 − 0.0278 × r heads for zero as r approaches 37. A set of 36 with 100 kg “predicts” a max of over 3,700 kg, and at 37 the answer turns negative. That’s arithmetic, not a study finding, but the warning is clear: never feed Brzycki a set of 20.
How accurate are 1RM formulas, really?
What LeSuer found when they checked
In 1997, LeSuer and colleagues put seven prediction equations up against measured 1RMs on the bench press, squat and deadlift. The subjects were 67 untrained college students (40 men, 27 women) taking weight-training classes.
Every correlation between predicted and actual 1RM was above 0.95. Sounds like a clean win, until you remember what correlation measures: that two numbers rise and fall together, not that they land in the same place.
On the bench, the average gap between predicted and actual was significantly different from zero for five of the seven equations. On the squat, six of seven. On the deadlift, every equation significantly underestimated the 1RM. The error is lift-specific: an equation that nails your bench tells you nothing about your pull. And these were beginners, not trained lifters.
Is a real 1RM test any better?
Yes, but it isn’t a fixed number either. A 2020 systematic review by Grgic and colleagues pooled 32 studies and 1,595 participants. Test–retest reliability came out good to excellent: ICCs from 0.64 to 0.99, median 0.97, with 92% at 0.90 or higher, regardless of training experience, exercise, sex or age.
The number worth remembering is the coefficient of variation: 0.5% to 12.1% across studies, median 4.2%. Even a max you actually lift moves by a few percent from one test to the next. So if your estimate climbs 3% in a week, that’s noise, not a PR.
Your RPE is part of the error
Every formula assumes the set went to failure, or close to it. Most working sets don’t, and the 2016 paper by Helms and colleagues on the RIR-based RPE scale cautions against training to failure on a regular basis anyway. The scale gives the conversion: RPE 10 = 0 reps in reserve, RPE 9 = 1, RPE 8 = 2, RPE 7 = 3.
So a set of 5 at RPE 8 was really a 7-rep set you stopped early, and plugging in 5 drags the estimate down. A common convention is to add reps and RIR before running the formula, but your RIR call is itself a guess. To sharpen it, see how to choose your working weight.
Worked example: from Monday’s deadlift to next week’s plan
Say you pull 140 kg (309 lb) for 5 on Monday, last set at RPE 8.
1. Run both formulas. Epley: 140 × (1 + 5 / 30) = 163.3 kg (360 lb). Brzycki: 140 / 0.8888 = 157.5 kg (347 lb). Count the 2 reps in reserve as if you’d done 7 and you get 172.7 kg (381 lb) and 168.0 kg (370 lb). Your plausible max spans about 15 kg (33 lb). And in LeSuer’s data every equation came in low on the deadlift, so don’t read the bottom of that range as your ceiling.
2. Pick one method and stick to it. Say Epley on the reps you actually did: 163.3 kg. It isn’t your “true” max. It’s a reference point.
3. Turn a percentage into a load. Next week calls for 3 × 3 at 80%, the bottom of the 80–100% range Helms and colleagues discuss for maximal strength work. That’s 0.80 × 163.3 = 130.7 kg (288 lb). From the Brzycki number it would be 126.0 kg (278 lb), almost 5 kg (10 lb) lighter for the same prescription. Load 130 kg (287 lb) and let the RPE of your first set decide whether it moves.
4. Progress the load, not the estimate. The ACSM position stand (2009) suggests adding 2–10% to the load once you beat your target by 1–2 reps on two consecutive sessions. Target: 5 reps at 140 kg. You get 6 two sessions running, so you go to 145 kg (320 lb), a 3.6% jump. That’s double progression in one line.
5. Read the trend, not the dot. A few weeks later, 145 kg × 5 at RPE 8 gives an Epley of 169.2 kg (373 lb): up 3.6%, still inside the 4.2% median variation of a real max test. The signal you can trust is the set itself: 145 × 5 now moves like 140 × 5 used to. Then you finish with touch-and-go 120 kg (265 lb) × 15, and Epley calls it 180.0 kg (397 lb), an 11 kg (24 lb) jump in one session. It isn’t. Different rep range, different setup, different fatigue: that number belongs on another chart.
What an estimated 1RM can’t tell you
- It only compares to itself. Same lift, same setup (depth, pause, dead-stop or touch-and-go, belt or no belt), similar reps, similar fatigue. An e1RM from a set of 12 at the end of a session and one from a triple at the start are measuring different things.
- It doesn’t transfer between lifts. In LeSuer’s data the equations missed by different amounts on each lift.
- You don’t need one to start. For novices the ACSM recommends loads in the 8–12 RM range: a weight you can lift 8 to 12 times, not a max.
- It won’t tell you why you stalled. If the trend drops for several weeks under the same conditions, the question becomes fatigue or programming, and sometimes the answer is a deload week.
Testing a true 1RM with shaky technique or without a spotter is where people get hurt. Persistent pain, an injury or a medical condition: see a healthcare professional before changing anything.
How EXE uses estimated 1RM
EXE computes your estimated 1RM with the Epley formula, the same way every time, so your trend line stays comparable to itself. PRs are detected and marked on the weight chart, and plateaus surface as insight cards showing the exercise, the weeks stagnating and your last PR. For next week, auto-progression is rule-based: it tracks consecutive successful sessions (at least 90% completion, RPE under 8.5) and suggests +1.25 or +2.5 kg (about 2.5 or 5 lb), holding, or a deload. The free tier has no time limit: see how EXE works.
FAQ
Which 1RM formula is the most accurate?
None of them, across the board. In LeSuer’s study most equations were significantly off on average, and all of them underestimated the deadlift. Pick one and never switch: consistency matters more than the choice.
How many reps should I use to estimate my 1RM?
A short, hard set: 5 reps or fewer is a sound rule of thumb. Past 10 reps, Epley and Brzycki drift further apart with every rep, and Brzycki is useless on a set of 20.
Do I need to test my true 1RM?
Not to make progress. A real test is reliable but still moves around, with a median coefficient of variation of 4.2% in Grgic’s review. If you need one, say for a meet, test it with solid technique and a spotter or safeties.
Why did my estimated 1RM drop this week?
Probably nothing to worry about. A 2–3% dip from a single session sits inside the median variation of an actual max test. Check the conditions first (reps, fatigue, setup), then judge the trend over several weeks.
Sources
- The Accuracy of Prediction Equations for Estimating 1-RM Performance in the Bench Press, Squat, and Deadlift — LeSuer et al., 1997: 7 equations, 67 untrained subjects, all underestimated the deadlift.
- Test–Retest Reliability of the One-Repetition Maximum (1RM) Strength Assessment: a Systematic Review — Grgic et al., 2020: 32 studies, median coefficient of variation 4.2%.
- Progression Models in Resistance Training for Healthy Adults — ACSM, 2009: 8–12 RM loads for novices, 2–10% load progression.
- Application of the Repetitions in Reserve-Based Rating of Perceived Exertion Scale for Resistance Training — Helms et al., 2016: RPE-to-RIR mapping, maximal strength work at 80–100% of 1RM.
