How to calculate one rep max (1RM) and program a client's loads from it

A 1RM is specific to one lift. A 100 kg squat 1RM means 100 kg goes up once and 102.5 kg does not. It is not a general strength score: every exercise has its own, and it changes as the person trains.

The short definition lives in the glossary. This guide is for the coach who has to use that number with clients: when to test it and when to estimate it, which formula to pick, how to turn a 1RM into the load for each set, and how often to update it.

The 1RM does two jobs. The first is tracking strength: if a client's estimated bench press 1RM goes from 70 to 77.5 kg in three months, they got stronger on that lift, even if every set they did was a set of 8.

The second is prescribing. A program that says "4 sets of 6 at 80%" needs a 1RM to become pounds or kilos. With a 100 kg 1RM, that is 80 kg; with a 60 kg 1RM, it is 48 kg. The same program fits different clients because the load is written relative to each of them.

That is why the number has to be trustworthy. An inflated 1RM makes every set of the block heavier than planned, and one that is too low leaves the client training far from where they should be.

The direct test is repeatable. A systematic review by Grgic and colleagues, published in Sports Medicine - Open in 2020, pooled 32 studies with 1,595 participants and found very high test-retest reliability: a median intraclass correlation of 0.97 and a median variation of 4.2% between one test and the next.

The usual protocol ramps the load up: a few low-rep warm-up sets, then single-rep attempts with 3 to 5 minutes of rest between them, aiming to reach the max within 3 to 5 attempts so fatigue does not muddy the result. An attempt counts only if the technique matches training: same squat depth, same pause on the chest in the bench press.

Testing makes sense for clients who already own the technique, and for the main lifts such as the barbell back squat, the bench press or the deadlift. For a beginner, for isolation work, or on machines where the last kilo changes no decision, an estimate gives the same information with less risk and less fatigue.

The alternative is to estimate it from a multi-rep set taken close to failure. The two most used formulas are Epley, 1RM = weight × (1 + reps / 30), and Brzycki, published in 1993, 1RM = weight × 36 / (37 − reps). Both work the same in pounds or kilos: the result comes out in the unit that went in.

Accuracy depends on how many reps the set has. Reynolds, Gordon and Robergs tested 70 adults aged 18 to 69 at 1, 5, 10 and 20 rep maxes on the bench press and leg press: the 5-rep set predicted the 1RM best, and the authors advised against using sets of more than 10 reps in linear equations. On the leg press, even from the 5-rep set, the typical error was about 16 kg.

The coaching rule follows from that: a set of 3 to 6 reps near failure gives a solid estimate, a set of 10 to 12 gives a reference, and a set of 15 or more says little about the 1RM. The one rep max calculator runs both formulas and shows where they part ways.

The two agree closely around 10 reps and drift apart from there. With 80 kg for 8 reps to failure, Epley estimates 101.3 kg and Brzycki 99.3 kg: a 2 kg gap, smaller than the swing in a person's performance between a good day and a bad one.

So the choice of formula matters less than two decisions that do move the number. The first is to always use the same formula for the same client, so the trend is comparable. The second is to count the reps left in reserve. If the client did 6 reps with 2 left, the set is worth 8 to failure: at 80 kg, Epley gives 101.3 kg. Count only the 6 and it gives 96 kg, a 1RM 5 kg below the real one.

RIR is a subjective estimate that improves with practice, so with new clients it pays to estimate from sets with little margin (RIR 0 to 2), where people judge it best.

Inverting the formula gives the chart used for programming. With Epley, the percentage of 1RM a person can move for a given number of reps to failure is 1 / (1 + reps / 30). Rounded:

A concrete case. An intermediate client, training for hypertrophy with some strength work. In her last session she logged the barbell back squat at 80 kg for 6 reps with RIR 2. That is 8 equivalent reps, and Epley gives an estimated 1RM of 101.3 kg. For programming, round it down to 100 kg: a conservative 1RM costs little, and an inflated one wrecks the block.

To write the load, add the set's reps to the target RIR and read that total off the chart. The RIR to percentage of 1RM calculator does the same math for any combination:

A 1RM from three months ago no longer describes the person. The American College of Sports Medicine's 2009 position stand on progression recommends raising the load by 2% to 10% once the person can do one or two reps more than prescribed at the current load. Each of those raises is, in practice, a 1RM that moved.

With a direct test, the sensible cadence is at the end of a multi-week block, not every week: it is tiring and it disrupts training on those days. With the estimate, the number can be recalculated from every logged set, and it is worth smoothing: one bad session after a short night should not pull down the loads of the whole block. Averaging the latest estimates handles that.

The same position stand puts beginners' work in the 8 to 12 rep max range, and more experienced lifters in a 1 to 12 range with growing emphasis on heavy loads of 1 to 6. For a beginner, then, the 1RM is mainly a tool for writing loads; testing their true max adds little until the technique is stable.

With one client, the math fits in a spreadsheet. With twenty, each with six or seven main lifts, that is more than a hundred 1RMs changing every week, each of them moving the loads of several sessions.

In Kaizer, a set's load can be written as a percentage of 1RM and it converts to the weight matching the client's latest 1RM. If the client has not logged that exercise yet, the coach enters the 1RM by hand on the spot and the weight lands on the set. Each exercise's progress shows the estimated 1RM next to strength relative to body weight.

Automatic progression works out the next load from the average of the last three estimated 1RMs, so a single session does not move the whole plan. The coach keeps the decisions: any week edited by hand leaves the automatic calculation, and automatic progression switches on or off for everyone or per client. The math is laid out in how load adaptation works, and the logic of progressing with RIR in the piece on progressive overload with RIR.

The 1RM is the heaviest single rep in a given exercise, and in a program it is the reference every load is written from.

The direct test is highly repeatable and makes sense with stable technique on the main lifts. For everything else, a set of 3 to 6 reps near failure, with RIR counted, gives a good enough estimate.

The formula matters less than consistency: same formula, same technique standard, and a number that updates with what the client logs.