What a 1RM actually is
Your one-rep max is the heaviest load you could lift for a single clean rep. You rarely test it directly because it is risky and fatiguing. Instead you estimate it from a set you already did.
Free strength tool
One-rep max calculator.
Enter a weight and the reps you hit. We estimate your one-rep max the same way the Ironstead app does it for your logged sets, then turn it into training percentages, warm-up sets and plate math, and show how every established formula compares.
How it works
From one hard set to a number you can program with.
Which formula we use
Ironstead estimates your 1RM with one transparent method: Epley for higher-rep sets and Brzycki near your max, the pairing that sports-science validation studies put in the most accurate group. We still show Lander, Lombardi, Mayhew, O'Conner and Wathan so you can see how the published formulas compare, but we do not average them: they correlate almost perfectly and differ only in their built-in bias.
Where estimates are sharpest
The published formulas agree most closely between 2 and 10 reps. Below that you are basically testing; above roughly 12 reps, endurance and technique start to dominate and every estimate drifts high. Keep test sets crisp.
Turning it into training
Once you have a 1RM, percentages do the rest: 5s around 80%, 3s around 87%, speed work at 70%. Ironstead tracks your estimated 1RM automatically from every logged set, so the number stays current without a max-out day.
Common questions
About 1RM estimates.
- How accurate is an estimated 1RM?
- For sets of 2 to 10 reps taken close to failure, estimates land within a few percent of a true tested max. Accuracy drops as reps climb, because high-rep performance depends as much on conditioning as on maximal strength.
- Which formula does this use?
- One transparent method: Epley for higher-rep sets and Brzycki near your max, which validation studies place in the most accurate group. The other formulas are shown only for comparison. We deliberately do not average them, because they correlate almost perfectly and averaging just blends their biases.
- Should I enter reps to failure?
- Yes. The math assumes the set was taken at or very near failure. If you left several reps in the tank, your real 1RM is higher than the estimate.
- kg or lb?
- Either. The formulas are unit-agnostic; loads, plate math and the bar weight all follow the unit you pick.
- Does Ironstead calculate this for me?
- Yes. The app derives an estimated 1RM from every working set you log, charts it over time, and flags new personal records automatically, so there is no separate test day.
Stop recalculating. Start logging.
Ironstead tracks your estimated 1RM from every set, builds your training percentages, and shows plate math live in the gym. Free for athletes.
Estimates are guidance, not medical advice. Warm up properly and use a spotter for heavy attempts.