1 Rep Max Calculator
Enter a weight and the reps you hit. This runs all seven published 1RM formulas and shows how far apart they land — because most calculators pick one silently, and past six reps they disagree by more than 10%.
Consensus of 7 formulas
What each formula says
Same set, seven equations. The wider the spread, the less you should trust any single number — that usually means your set ran too long to estimate from.
| Formula | Estimated 1RM | Equation | Behaviour |
|---|
Training percentages
Loads based on the consensus estimate. Strength work usually lives at 80–90%; hypertrophy around 65–80%.
| % of 1RM | Weight | Typical reps |
|---|
Why the formulas disagree
Every 1RM equation is a curve fitted to a particular group of lifters on a particular lift. None of them is measuring your strength — they are all extrapolating from one data point, and they extrapolate differently.
- Epley:
w × (1 + r/30)— The most widely used. Tends to read high at higher reps. - Brzycki:
w × 36 / (37 − r)— Reads low at high reps; breaks down past ~10. - Lombardi:
w × r^0.10— Gentle curve, stays usable at higher rep counts. - O'Conner:
w × (1 + r/40)— The most conservative of the common formulas. - Wathan:
100w / (48.8 + 53.8·e^(−0.075r))— Regression-derived; strong agreement in the 3–10 range. - Lander:
100w / (101.3 − 2.67123r)— Close to Brzycki through the low-rep range. - Mayhew:
100w / (52.2 + 41.9·e^(−0.055r))— Regression-derived from bench press data.
Here is how far apart they actually land, from a 100 kg set at each rep count:
| Reps | Consensus 1RM | Lowest – highest | Spread |
|---|---|---|---|
| 2 | 106.1 kg | 102.9 – 111.4 kg | 8.1% |
| 3 | 109.3 kg | 105.9 – 114.0 kg | 7.4% |
| 4 | 112.4 kg | 109.1 – 116.5 kg | 6.6% |
| 5 | 115.5 kg | 112.5 – 119.0 kg | 5.6% |
| 6 | 118.5 kg | 115.0 – 121.5 kg | 5.5% |
| 7 | 121.6 kg | 117.5 – 124.0 kg | 5.4% |
| 8 | 124.7 kg | 120.0 – 127.7 kg | 6.2% |
| 10 | 131.0 kg | 125.0 – 134.7 kg | 7.4% |
| 12 | 137.6 kg | 128.2 – 144.4 kg | 11.8% |
| 15 | 148.3 kg | 131.1 – 163.6 kg | 21.9% |
The formulas agree most at 5–7 reps, not at 1–3. That surprises people. Doubles and triples sit on the steepest part of every curve, so small differences in how each equation was fitted show up as a wider gap — about 8% at 2 reps. Agreement is tightest around 5–7 reps at roughly 5%, then falls apart fast: 12% at 12 reps, 22% at 15.
So the practical rule is not "go as heavy as possible". Estimate from a set of 5–7 clean reps. That is where the equations converge, and it is a far cheaper set to perform than a near-max double.
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Frequently asked questions
How accurate is a 1-rep-max calculator?
Estimates are sharpest in the 5–7 rep range, where the formulas agree most closely, usually landing within about 5–10% of a true max. Past 10 reps accuracy falls off, because endurance and technique start to dominate over raw strength. That is why this page shows you the spread between formulas rather than a single confident-looking number.
Why do the formulas disagree?
Each equation was fitted to a different population and lift. Epley was built to stay simple; Wathan and Mayhew come from regression on measured lifting data; Brzycki assumes a near-linear relationship that falls apart above roughly 10 reps. Agreement is tightest at 5-7 reps (about 5%), not at 1-3 reps where the curves are steepest (about 8% at 2 reps). By 12 reps the gap exceeds 11%, and by 15 it is over 20%.
Which number should I actually train from?
Use the consensus average, and treat the range as your error bar. If the spread is wide, your set was too long to estimate from — repeat it with heavier weight and fewer reps for a tighter answer.
How do I use the percentage table?
Once you have an estimated 1RM, the table shows what to load for a given percentage. Most strength programming lives in the 80–90% band; hypertrophy work usually sits around 65–80%.
Should I test a true 1-rep max instead?
You do not need to. A true max attempt is taxing and carries real injury risk without a spotter. Estimating from a clean set of 3–5 reps gives you everything you need to program around, at a fraction of the cost.
Can I use this calculator on my own site?
Yes. There is a copy-paste embed snippet further down the page. It is free to use, and the only thing asked in return is a visible link back to this page.
Programming around these numbers? See our workout plans and gear reviews.