How to calculate a one-rep max estimate

A one-rep max, often written 1RM, is the most weight a person can lift once for a named exercise under stated conditions. A calculator does not observe that maximum. It predicts a value from a load you lifted for several repetitions. To make the calculation, record the total load in kilograms or pounds and the number of completed repetitions in one set. Use the load on the bar or machine, including the bar when its weight is part of the lift. Keep the exercise name beside the result. A bench-press estimate is not a squat estimate, even when both sets use the same load.

The repetitions need a clear meaning. The equations studied by LeSuer and colleagues used repetitions to fatigue with consistent technique, not an arbitrary set stopped while many repetitions remained. In a normal training record, you may not know how many more repetitions were possible. Write down whether the set was close to your usual limit and whether technique changed. That note helps explain why the same load and repetition count might not represent the same effort on another day. If you stopped well before fatigue, the calculator still performs the formula arithmetic, but that set cannot support an estimate of your current maximum. Do not push a set to failure simply to make an online result look more exact.

Use the same exercise and a similar range of movement when you compare later estimates. A pause on the bench, a different squat depth, or a new machine can change what counts as a completed repetition. In the 1997 study, the researchers specified technique for bench press, squat, and deadlift before comparing predictions with actual one-repetition tests. That controlled setting is one reason to read the study as evidence about those lifts and participants, rather than a guarantee for every exercise or person. The result is an estimate of performance under a set of assumptions.

Suppose a hypothetical lifter completes five bench-press repetitions with a total load of 100 kg, close to the lifter’s usual limit and with consistent technique. Enter 100 kg and 5 repetitions. The Epley equation is load × (1 + repetitions ÷ 30). Five divided by 30 is about 0.1667, so 100 kg × 1.1667 gives about 116.7 kg. The Brzycki form used by Body Numeral is load × 36 ÷ (37 − repetitions). It gives 100 kg × 36 ÷ 32 = 112.5 kg. Both answers have the same unit as the input load. They differ because the equations model the relation between repetitions and maximum load differently.

The calculator also uses the Wathen equation. For 100 kg and five repetitions, its unrounded result is about 116.6 kg. Body Numeral averages Epley, Brzycki, and Wathen without rounding their intermediate results. The average is about 115.25 kg, displayed as 115.2 kg to one decimal place. The three estimates do not become a direct measurement when averaged. The cited studies tested individual equations and repetition tests; they did not validate Body Numeral’s three-equation average as a combined predictor. In the table, e is the mathematical constant about 2.71828. Use the exp function on a scientific calculator to evaluate e raised to a power in Wathen’s equation. The summary tells you which calculation this particular tool chose to display first. Another site may show one formula or average all seven, so save the method name with the number.

Hypothetical 100 kg × 5 bench-press set. Equation results are rounded to one decimal place after calculation; the summary uses unrounded results.

The table is a worked calculation, not a lifting plan. It lets you check the inputs, formulas, unit, and rounding behind the headline result. If you enter pounds instead of kilograms, keep pounds through the formulas; do not mix units within a calculation. The site also displays percentage loads. For this example, 80% means 115.2497 kg × 0.80, or about 92.2 kg before the table rounds to a whole kilogram. This multiplication is useful for understanding the display. It does not tell you that 92 kg is a safe or suitable load for a workout.

Body Numeral lists seven named equations as well as its three-equation summary. The wider list makes model disagreement visible. Each equation turns the same pair of inputs into a different prediction, and the gap can change with the repetition count. A table of seven values is not seven independent measurements of a person's strength. All seven depend on the same completed set. The site's variation percentage describes the spread among formula outputs divided by their mean. It is a measure of disagreement between models, not a tested error range for you.

LeSuer and colleagues compared seven equations with actual one-repetition results for bench press, squat, and deadlift. They found high correlations between predicted and achieved lifts, yet many equations had systematic average differences. All seven significantly underestimated deadlift results in that group, despite the high correlations. For bench press, two equations had average differences that were not statistically different from zero under the study's test. Those are group findings from untrained college students. They do not identify the closest equation for an individual set, and they do not justify correcting every deadlift estimate by one fixed amount.

A second study by Reynolds and colleagues tested 70 adults aged 18 to 69 on bench press and leg press, using one-, five-, ten-, and twenty-repetition tests. In its analyses, five-repetition data gave the strongest prediction among the tested multiple-repetition conditions for those two exercises. That result supports treating very long sets with more caution, but it does not establish a universal five-repetition rule for all lifts. The Body Numeral tool accepts one through thirty completed repetitions and warns when the count is high. A permitted input range means the calculation can run; it does not mean every point in that range has equal evidence.

Open the one-rep max calculator and enter the total weight lifted and whole number of completed repetitions. Choose metric kilograms or US pounds in the measurement-unit control. The tool converts an entered pound load to kilograms for the calculation, then shows the result in the selected unit. Keep the original unit with your record. Check the exercise and technique in your own note because the calculator does not ask for them. It shows a three-equation estimate, seven individual estimates, model variation, and a percentage table. It does not measure your current maximum or decide which exercise-specific equation fits you best.

Read the displayed decimal as a calculated output, not a promise that you could lift that exact load. A difference of 0.1 kg between two displays can come from rounding. A larger difference can come from the chosen formula, the repetition count, or whether a set ended near fatigue. If you want to see how sensitive the result is, hold the load fixed and change the repetition count by one in the calculator. That is an arithmetic comparison of scenarios. It does not say a repetition happened or that performance changed. Record the real completed set separately from any scenario you explored.

The percentage table multiplies the unrounded three-equation summary by values from 100% down to 55% and rounds each listed load to a whole kilogram or pound. It does not account for warm-up, fatigue, exercise technique, equipment, pain, or training history. Treat the table as an explanation of percentage arithmetic. A coach or qualified professional can help with a training decision when technique, injury, or health concerns matter. You do not need to attempt a maximum lift to use this article or the calculator. The cited studies used defined repetition-test protocols. A web form cannot check whether a set matches those procedures.

When you repeat a calculation later, keep the exercise, load unit, completed repetitions, formula or summary, and date together. Also note meaningful changes in equipment and technique. If the newer set uses a different repetition count, the two estimates include both a performance difference and a change in the equation input. You can still compare them, but first look at the actual sets. A 100 kg × 5 set and a 100 kg × 6 set are different observations; the calculator only translates each observation with its chosen formulas. A single estimated maximum is less informative than the recorded work that produced it.

Do not average bench, squat, and deadlift predictions into one strength score. The 1997 comparison showed that formula behavior differed across those lifts. Likewise, do not copy a percentage load from one exercise into another just because both are labeled 80%. Keep machine and free-weight versions separate if their loading or movement differs. The guide on why calculators disagree can help when two sites produce different numbers from the same set. The guide on comparing measurements explains how to keep method changes visible across dates. Those links support the record; they do not validate a personal maximum.

A useful next step is to save one complete entry: exercise, 100 kg total load, five completed repetitions, technique note, date, and the Body Numeral three-equation estimate of about 115.2 kg. Label this example hypothetical if you copy it as a template; replace every value with your own observed set before drawing any conclusion. If a later result differs, check the raw set and method before assigning a cause. The estimate can help describe a set in a consistent way. It cannot show what you should lift next or remove the need for judgment about a real session.