Formcast Fitness Free tools

Free tool

One-rep max calculator

Enter a set you actually did — weight and reps — and get an estimated one-rep max using the Epley formula, plus what that predicts for every rep count from 1 to 12.

Units

Estimated one-rep max—lb

Estimated weight for each rep count, derived from the same Epley equation
Reps% of 1RMWeight (lb)

Rows are the same Epley equation solved the other way round: weight = e1RM ÷ (1 + reps ÷ 30).

It spots your PRs so you don't have to

Formcast Fitness runs this same estimate on every set you log, compares it against your history for that exercise, and tells you when you have beaten a record.

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iPhone only · free trial · subscription after the trial

How this calculator works

Formcast Fitness estimates one-rep maxes with the Epley formula, in src/services/prDetection.ts. This page runs that same function:

  • e1RM = weight × (1 + reps ÷ 30)

So 225 lb for 5 reps estimates 225 × (1 + 5/30) = 262.5 lb. The app rounds the result to one decimal place, and so does this page.

Two deliberate edge cases, copied from the app

  • One rep or fewer returns the weight itself. A single is already a one-rep max; there is nothing to extrapolate.
  • Reps are capped at 100 and the first number in the field is the one used — the app reads reps out of free text like “8–12”, where it takes the 8. Both behaviours exist so a typo can never fabricate a personal record.

The app uses this estimate for automatic PR detection: after a set, it compares the set's e1RM against the best e1RM in your prior history for that exercise and tells you if you have beaten it. It waits until you have at least three prior recorded sets before announcing anything, and it never compares a kilogram set against a pound one.

The rep table

The table is the same Epley equation solved the other way round — weight = e1RM ÷ (1 + reps ÷ 30) — so a set predicted from it will estimate back to the same one-rep max. It is a derivation of the formula above, not a second formula.

What this assumes, and where it is wrong

  • It is an estimate, not a max. The only way to know your one-rep max is to lift one. Epley is a straight line fitted to averaged data; real strength curves are not straight.
  • It is fitted to low-rep sets. Between about 1 and 10 reps it is reasonable. Past 10 it drifts high, sometimes badly — a 20-rep set will produce a number most people cannot lift.
  • It assumes the set was close to failure. Five reps with three left in the tank and five reps to grinding failure give the same input and the same output, and only one of them is honest.
  • It ignores the lift. Rep performance at a given percentage differs between a deadlift, a bench press and a squat, and between trained and untrained lifters. One equation for all of them is a simplification.
  • Do not use it as a prescription to attempt. Never walk up to a bar loaded to a calculated number and try a single without building to it, without a spotter, and without safeties.

Estimate only. This number is produced by a formula from one set, not by testing you. Warm up properly, use safeties or a spotter for heavy singles, and stop if something hurts.

More free calculators

All four use the same code the Formcast Fitness app uses, so the numbers match what you would see in the app.