What is a one rep max?
Your one rep max (1RM) is the heaviest weight you can lift for a single repetition of an exercise with proper form. It is the standard measure of maximal strength in the squat, bench press, deadlift and other barbell lifts, and it is the number most strength programs are built around: when a program tells you to lift "5 sets of 5 at 80%", that percentage refers to your 1RM.
Testing a true 1RM in the gym is demanding. It requires a full warm-up protocol, spotters or safety pins, and it produces a lot of fatigue that can disrupt the rest of your training week. That is why coaches and lifters usually estimate the 1RM from a submaximal set instead — a heavy set of 3, for example — using well-established prediction formulas. This calculator does exactly that.
How the calculator works
IronEval applies three of the most widely used prediction equations and shows you each result plus their average:
| Formula | Equation |
|---|---|
| Epley | 1RM = w × (1 + r / 30) |
| Brzycki | 1RM = w × 36 / (37 − r) |
| Lombardi | 1RM = w × r0.10 |
Here w is the weight lifted and r is the number of reps completed. Epley and Brzycki produce nearly identical numbers in the 2–10 rep range and are the two formulas you will see most often in strength research and training apps. Lombardi uses a power curve and tends to run slightly higher at high rep counts. Because each equation has small biases, the average of the three is usually the most dependable single estimate.
The plate diagram under your result shows how that weight would be loaded on a standard Olympic barbell (20 kg / 45 lb bar), rounded to the nearest loadable weight with standard plates — a quick sanity check before you walk up to the bar.
How to get an accurate estimate
The formulas are most reliable when your input set meets three conditions. First, use a low rep count: sets of 2–6 reps predict far better than sets of 10 or more, because fatigue mechanisms change at higher reps. Second, the set should be taken close to failure — if you stopped with three reps in reserve, the calculator will underestimate your max. Third, use a recent set: strength changes week to week, and a set from two months ago no longer reflects your current ability.
Rep-max relationships also vary between exercises and individuals. Lifters with more fast-twitch dominance often get fewer reps at a given percentage, while high-rep specialists get more. Treat the estimate as a smart starting point, then adjust based on how your working sets actually feel.
Using your 1RM in training
Once you know your estimated max, the percentage table above becomes your programming reference. Broadly: 90–100% builds maximal strength and is used sparingly; 80–90% is the classic strength zone for sets of 2–5; 70–80% suits sets of 5–8 and builds both strength and muscle; 60–70% works well for volume work and technique practice; and 50–60% is typically used for warm-ups, speed work and deloads.
Recalculate every training block — roughly every 4–8 weeks — or whenever you hit a clear rep PR. Keeping your percentages tied to your current strength is what makes percentage-based programs work.