Your one rep max (1RM) is the single most important number in strength training. It's the ceiling from which every percentage in your program is calculated — the reference point that turns "lift heavy" into a precise prescription.
But actually testing your 1RM by loading a bar to max and grinding out a single is both risky and taxing. That's why coaches and athletes use mathematical formulas to estimate their max from sub-maximal sets. Load the bar with a weight you can lift for 5 reps, plug the numbers into the right formula, and you get a reliable 1RM estimate without the stress of a true max effort.
This guide explains how those formulas work, when each one is most accurate, and how to use RPE to sharpen your estimate from non-maximal training sets.
■The Five Core 1RM Formulas
Decades of sports science research have produced dozens of formulas for estimating 1RM. Five have stood the test of time and are used in almost every reputable calculator.
| Formula | Equation | Best For |
|---|---|---|
| Epley | weight × (1 + reps/30) | General use, wide rep ranges |
| Brzycki | weight × (36 / (37 - reps)) | 1–6 rep ranges |
| Lombardi | weight × reps^0.10 | Higher rep ranges |
| O'Conner | weight × (1 + 0.025 × reps) | Conservative estimate |
| Mayhew | (100 × weight) / (52.2 + 41.9 × e^(-0.055 × reps)) | Moderate rep ranges |
Epley (1978)
The Epley formula is the most widely cited 1RM equation and appears in most commercial calculators. It's simple, linear, and remarkably accurate across a broad rep range (1–12):
1RM = weight × (1 + reps / 30)
If you bench 225 lbs for 5 reps:
1RM = 225 × (1 + 5/30) = 225 × 1.167 = 262.5 lbs
Epley tends to be slightly conservative at very low rep ranges (1–3) and becomes less accurate above 12 reps, where muscular endurance starts overriding raw strength.
Brzycki (1993)
The Brzycki formula uses a different mathematical model that produces slightly different results — and is often preferred by powerlifters working in the 1–6 rep range:
1RM = weight × (36 / (37 - reps))
For the same 225 lbs × 5 reps:
1RM = 225 × (36 / (37 - 5)) = 225 × (36/32) = 225 × 1.125 = 253 lbs
Brzycki gives a more conservative (lower) estimate than Epley for the same rep range, which is why many powerlifters prefer it — they'd rather underestimate their 1RM than overshoot their working weights.
Note: Brzycki's formula breaks down entirely above 16 reps (the denominator approaches zero). Only use it with sets of 12 reps or fewer.
Averaging Multiple Formulas
No single formula is universally most accurate. Individual differences in fiber type composition, muscle belly length, and training history all affect how closely any formula will match your actual max. The most robust approach is to average several formulas — which is exactly what the One Rep Max Calculator on this site does.
The averaged estimate tends to be less sensitive to the quirks of any single equation and performs better across a wider range of athletes and rep schemes.
■Using RPE to Sharpen Your Estimate
The formulas above assume you trained to failure — that you genuinely could not complete another rep. Most good programs don't ask you to train to failure. They prescribe sets at RPE 7, 8, or 9, meaning you stopped with 1, 2, or 3 reps still in reserve.
RPE (Rate of Perceived Exertion) is a 1–10 scale:
- ■RPE 10 = couldn't do another rep (true failure)
- ■RPE 9 = 1 rep in reserve
- ■RPE 8 = 2 reps in reserve
- ■RPE 7 = 3 reps in reserve
If you stopped a set with reps still in the tank, you need to account for those missed reps before plugging into the formula. The adjustment is simple:
Effective reps = actual reps + (10 - RPE)
So if you did 5 reps at RPE 8 (2 reps left in reserve):
Effective reps = 5 + (10 - 8) = 7
You then use 7 as your rep count in the formula, producing a more accurate 1RM estimate from a non-maximal set. This is essential for autoregulated programs where you never actually train to failure.
■How Often Should You Recalculate?
Your 1RM changes as you get stronger — or weaker during a deload. Here's a practical recalculation schedule:
- ■Beginners (< 1 year training): Every 2–4 weeks. Strength gains come fast early on.
- ■Intermediate (1–3 years): Every 4–8 weeks, or at the end of each training block.
- ■Advanced (3+ years): Every 8–12 weeks, or once per competitive cycle.
Avoid recalculating during a deload week. Your performance will be intentionally suppressed, and you'll underestimate your actual max.
■Turning Your 1RM Into a Training Program
Once you have your estimated 1RM, you can use training percentages to set precise working weights:
| % of 1RM | Rep Range | Use Case |
|---|---|---|
| 90–100% | 1–3 reps | Peaking, strength testing |
| 80–90% | 3–5 reps | Strength development |
| 70–80% | 5–8 reps | Hypertrophy/strength |
| 60–70% | 8–12 reps | Hypertrophy |
| 50–60% | 12–20 reps | Endurance, technique |
Most percentage-based programs (5/3/1, Texas Method, Cube Method) use 85–90% of your 1RM as a "training max" rather than your true estimated max. The built-in 10–15% buffer prevents the program from overwhelming you on bad days and leaves room for the weights to feel heavy when the program calls for it.
If you're running 5/3/1, use the 5/3/1 Training Max Calculator to set your training maxes correctly for each lift.
■Key Takeaways
- ■Use sets of 3–6 reps near failure for the most accurate formula input
- ■Average multiple formulas rather than relying on any single equation
- ■Apply RPE adjustment if you didn't train to failure
- ■Retest every 4–8 weeks to keep your programming current
- ■Use 90% of your estimated 1RM as your training max for most percentage-based programs
Ready to calculate? Use the One Rep Max Calculator to get your estimate, then check the Strength Standard Calculator to see how you rank.