What is a one rep max — and why estimate it?
Your one rep max (1RM) is the heaviest weight you can lift once, with proper form, on a given exercise: one squat, one bench press, one deadlift. It is the reference number almost all serious training is built on, because programs prescribe work as percentages of 1RM — 5 sets of 5 at 80%, speed work at 60%, a heavy single at 92%. Knowing the number lets you load the bar correctly instead of guessing.
The catch is that testing a true 1RM directly is genuinely costly: it demands a full warm-up, spotters or safety arms, peak freshness, and it carries the highest injury risk of anything you do in a gym, all to measure a number that fatigue alone can move by several percent from day to day. So most lifters estimate it from a hard submaximal set instead — lift a weight you can manage for a handful of reps, push that set close to failure, and let a formula project what a single would be. That is safer, repeatable in an ordinary training session, and accurate enough for programming, which is exactly what this calculator does with the two most widely used formulas.
The two formulas
The Epley formula assumes each extra rep you can perform represents about a 30th more strength in reserve: ten reps adds a third to the weight. The Brzycki formula, from strength coach Matt Brzycki, models the same relationship as a fraction — at 10 reps you are working at 27/36 of your max. The two agree closely in the low rep ranges, which is reassuring, and this calculator shows both plus their average, a sensible single number to program from. Brzycki breaks mathematically at 37 reps, where its denominator reaches zero; if you enter 37 or more reps, the Brzycki column simply falls back to the Epley value and says so. A set of one rep needs no formula at all: the weight you lifted once is your 1RM, and both formulas return it unchanged.
Accuracy: the honest warning
These formulas estimate; they do not measure. Their shared weakness is that a set's rep count reflects endurance and pain tolerance as well as strength, and the further the set is from a single, the more those other qualities pollute the projection. Both formulas are at their best with sets of 2 to 10 reps taken close to failure. Above about 10 reps the estimates drift noticeably — often optimistically — and by 15 or 20 reps individual differences in muscular endurance dominate, so two lifters with the same 20-rep set can have genuinely different true maxes. Formulas also assume the set was truly maximal: stopping three reps short of failure understates the result just as surely. Use the number to program training, not as a fact about your body, and re-test with a fresh hard set every few weeks as your training progresses.
Using the percentage table
The table below the calculator converts your estimated max into working weights. The pattern is consistent across lifters: around 85–95% builds maximal strength in sets of 2–6, 70–85% is the classic strength-and-size zone, and 50–65% suits volume work, technique practice and speed work. The rep ranges in the table are typical, not guarantees — your own reps at a given percentage will sit a little above or below them depending on the exercise (upper-body lifts usually allow fewer reps at a percentage than squats or deadlifts) and on your training history. Round table weights to what your plates actually allow.
Worked example
You lift 100 kg for 10 reps. Epley: 100 × (1 + 10 ÷ 30) = 100 × 1.3333 = 133.3 kg. Brzycki: 100 × 36 ÷ (37 − 10) = 3600 ÷ 27 = 133.3 kg. The two formulas agree exactly at 10 reps here, so the average estimated 1RM is also 133.3 kg — and 80% training work would be about 106.7 kg.
How to use this calculator
- Choose a recent, hard set on the lift you want a max for — ideally 2 to 10 reps taken close to failure.
- Enter the weight you lifted and the reps you completed.
- Choose kilograms or pounds to match the weight you entered.
- Read the average estimated 1RM in the headline panel, with both formulas shown in the breakdown.
- Use the percentage table to pick training weights, and note the accuracy warning if your set had more than 10 reps.
Safety, stated plainly
An estimated max exists precisely so you do not have to test a real one often — so do not turn around and test it casually. Never attempt a true one-rep max alone, and never without a proper setup: a rack with safety arms set at the right height, competent spotters who know the plan, collars on the bar, and a clear bail-out route you have practised with lighter weights. Warm up progressively, stop any attempt where form breaks down rather than grinding through it, and leave genuine max attempts for days when you are rested, fed and focused. Most progress comes from the percentage-based work in the table, not from maxing out — competitive lifters, who max for a living, still only peak a handful of times a year.
One more calibration note: an estimated 1RM is specific to the lift, the equipment and the conditions of the set you entered. A Smith machine, a different bar, a pause versus touch-and-go bench press, or lifting belted versus beltless can each shift the true number. Keep the conditions consistent when you re-estimate, and the trend over time — which is what actually tells you whether your training is working — will be trustworthy even if the absolute number is a few kilos off.
Important note
For information only — not medical advice. This calculator uses standard published formulas to estimate values for general education and planning. It does not diagnose, treat, or replace the advice of a doctor or other qualified health professional. If you have a health concern, are pregnant, or are making decisions about treatment, medication, or a major diet or exercise change, please talk to a healthcare professional first.