One-Rep Max, Estimated From Reps
After reading this you can turn a hard set of a few reps into a fair estimate of your one-rep max, understand why seven formulas disagree, and build a load table for any rep target you train.
What a 1RM estimate is
Your one-rep max (1RM) is the heaviest weight you can lift once with good form for a given exercise. You rarely need to test it directly. Instead you take a set close to failure at a lighter weight and let a formula predict the single you probably could have done.
Here is the hook. Suppose you squat 100 kg for 5 hard reps. The Epley formula predicts a 1RM of 116.7 kg. That same set feeds a table telling you that a 5-rep working weight sits near 85 percent of your max, a triple near 90 percent, and a light day of 10 reps near 74 percent. One honest set gives you a whole training menu.
When to use it and when not to
Rep-max formulas are calibrated on submaximal sets. They are most accurate from 2 to about 8 reps. Below that you are close to a true single already. Above roughly 12 reps, fatigue, breathing, and grip start to limit you before the target muscle does, so the estimate reads high.
An estimate is not permission to load a bar to that number and try it. If Epley says 116.7 kg, treat it as a ceiling to approach over weeks, not a target for today. Build up to heavy singles gradually and use a spotter.
Use the estimate to set working loads, to compare progress week to week, and to program percentages. Do not use it to decide a max attempt on a whim, and do not compare across exercises: a deadlift and an overhead press do not share the same rep-to-1RM curve, even though the formulas pretend they do.
The formula and the intuition behind it
The most common formula is Epley's. It says the extra weight you sacrifice per rep is a fixed fraction of your max:
Here w is the weight you lifted and r is the number of reps completed. The term r/30 is the boost: each rep you managed implies you could have added about 1/30 (3.33 percent) of the lifted weight if you had done one fewer rep. At r = 1 the formula returns w exactly, as it should.
Brzycki takes a different shape. Instead of a linear boost, it divides by a term that shrinks as reps rise:
The two agree closely at low reps and drift apart as reps climb. At r = 5 and w = 100, Epley gives 116.7 and Brzycki gives 112.5, a gap of about 4 kg. At r = 10 the gap widens to roughly 10 kg. That spread is the honest error bar on the whole method.
A worked example you can reproduce
100 kg for 5 reps
These are the tool's demo values: weight = 100, unit = kg, reps = 5.
- Epley: 100 \times (1 + 5/30) = 100 \times 1.1667 = 116.7 kg.
- Brzycki: 100 \times 36/(37 - 5) = 100 \times 1.125 = 112.5 kg.
- Lombardi: 100 \times 5^{0.10} = 100 \times 1.1746 = 117.5 kg.
- Average the seven published formulas and you land near
115kg.
So a fair single estimate is about 115 kg, with a plausible range of roughly 112 to 118 kg depending on which formula you trust. Report the range, not a single decimal.
The chart below shows how the seven-formula spread grows with reps for the same lifted weight of 100 kg.
Reading the percentage-of-1RM table
Once you have a 1RM, the same curve runs backward: for any rep target it tells you what fraction of your max you should be able to lift. Using an Epley-consistent table on a 115 kg max:
| Reps | % of 1RM | Load (kg) |
|---|---|---|
| 1 | 100% | 115.0 |
| 3 | 93.5% | 107.5 |
| 5 | 85.7% | 98.6 |
| 8 | 78.9% | 90.8 |
| 10 | 75.0% | 86.3 |
| 12 | 70.6% | 81.2 |
Read it as a starting guess, then adjust by feel. If a prescribed 5-rep load of 98.6 kg leaves you three reps short of failure, your real 1RM is higher than 115 kg and you should nudge the input weight up next session.
Common mistakes
Four errors account for most bad estimates.
- Counting a soft set
- If you stop with 3 reps in the tank, the formula reads low. Use a set taken close to genuine failure, ideally within 1 to 2 reps of it.
- Using too many reps
- A 20-rep set of 60 kg might "predict" a 100 kg squat, but that number is fiction. Stay at or below 12 reps.
- Mixing units mid-plan
- Pick kg or lb and keep it. A 100 lb max is
45.4 kg, not 100 kg. The formulas are unit-agnostic, but your loading is not. - Trusting one decimal
- An estimate good to about 5 percent should be rounded to the nearest 2.5 kg plate step, so
116.7becomes a planned115.
Related tools
Strength is one slice of fitness. To round out your training numbers, the Heart Rate Zone Calculator sets five effort zones for conditioning work, and the VO₂max Estimator gauges aerobic fitness. If you also run, the Running Pace Calculator converts pace to race times, and the Calories Burned Calculator estimates energy cost by activity.
Frequently asked questions
Which formula is most accurate?
No single formula wins for everyone. Epley and Brzycki are the most cited and agree within about 4 kg at 5 reps on a 100 kg lift. Averaging several formulas, as this tool does, smooths out the choice. Treat the average as your point estimate and the spread as the uncertainty.
How many reps should I use?
Two to eight is the sweet spot. At 5 reps the formulas are still tight and the set is hard enough to be honest but light enough to be safe. Above 12 the estimate drifts high because non-muscular fatigue ends the set early.
Why does my estimate change between exercises?
Because the true rep-to-1RM curve differs by lift. A back squat sustains more reps at a given percentage than an overhead press. The formulas use one fixed curve, so expect the estimate to fit big compound lifts better than small isolation moves.
Should I actually attempt the estimated single?
Not on the strength of one calculation. Approach heavy singles over several weeks, add weight in small steps, and use a spotter or safeties. This is an educational estimate, not medical advice; consult a professional for decisions about your health.
Does bodyweight or sex change the formula?
These formulas ignore both. They only see weight lifted and reps. That is a limitation, not a feature: two people who both press 60 kg for 5 reps get the same estimate even if their true maxes differ.
Related tools
Calories Burned Calculator
Energy burned by activity using MET values.
Running Pace Calculator
Pace ↔ speed ↔ race time, plus a race predictor.
Heart Rate Zone Calculator
Five training zones from age or measured max HR.
VO₂max Estimator
Aerobic fitness from a Cooper, heart-rate or run test.
Antibiotic Resistance Evolution Simulator
Watch bacteria evolve resistance — and see why finishing the course matters.
BMI Calculator
Body mass index, WHO category and healthy weight range.