One Rep Max (1RM) Calculator

Formula	Est. 1RM ({{ unit }})	Copy
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% of 1RM	Load ({{ unit }})	Copy
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One repetition maximum is the highest load you can lift once with sound form. Use it as a one rep max calculator for planning strength work and setting realistic targets. Knowing this anchor helps you scale training, compare sessions, and track gradual progress without constant max attempts.

You provide the heaviest weight used in a clean set and the number of strict reps, and the estimate combines several established formulas to return a single average alongside each individual result. The display favors clarity, so you see one figure to guide decisions and a spread that shows method differences.

You may round results to match available plates and then build a percent table that maps the estimate to target intensities from a chosen start to an end in steady steps. This keeps work sets consistent from week to week and makes quick adjustments simple when energy or time is tight.

A practical example is a set of five with 100 kilograms returning an estimate near the mid one teens, and your table listing training loads at 60 to 90 in small jumps. Keep technique consistent and avoid grinders since fatigue and loose form can inflate numbers and mislead programming.

Expect small differences between formulas and treat the cluster as a range rather than a promise. This tool provides informational estimates and does not substitute professional advice.

Technical Details:

The quantity of interest is the one repetition maximum, an estimate of the peak load a lifter can move for a single controlled repetition. Inputs are the working weight from a submaximal set and the completed repetitions in that set. The calculation is a snapshot for that performance, not a long‑term capacity record.

The estimate is computed per formula and then averaged to yield a single guide value. Individual formulas transform the same inputs differently, which is helpful because the relationship between repetitions and maximum strength varies by lift and person. The percent table then scales the estimate to target intensities for programming.

Results have no fixed risk bands; instead, they form a small spread whose center is the average. Values near the edges of the spread are not “wrong” but reflect different assumptions about how repetitions carry over to a single. Treat the average as the planning anchor and the spread as reasonable uncertainty.

Estimates assume strict reps and are most reliable for about 1 to 12 repetitions. Outside that range, fatigue, pacing, and technique drift can introduce noise, so treat outputs cautiously.

\begin{array}{lcl} M & = & w (1 + \frac{r}{30}) (Epley) \\ M & = & \frac{36 w}{37 - r} (Brzycki) \\ M & = & w \cdot r^{0.10} (Lombardi) \\ M & = & \frac{100 \cdot w}{52.2 + 41.9 \cdot e^{- 0.055 \cdot r}} (Mayhew) \\ M & = & w (1 + 0.025 \cdot r) (O’Conner) \\ M & = & \frac{100 \cdot w}{48.8 + 53.8 \cdot e^{- 0.075 \cdot r}} (Wathan) \\ M & = & \frac{100 \cdot w}{101.3 - 2.67123 \cdot r} (Lander) \end{array}

Symbol and unit definitions
Symbol	Meaning	Unit/Datatype	Source
M	Estimated one repetition maximum	kg or lb	Derived
w	Working weight in the set	kg or lb	Input
r	Completed strict repetitions	integer	Input
p	Target percent of M	percent	Input
inc	Rounding increment	kg or lb	Input

Worked example. For w = 100 kg and r = 5:

\begin{array}{lcl} Epley: & = & 100 (1 + \frac{5}{30}) = 116.67 kg \\ Brzycki: & = & \frac{36 \cdot 100}{37 - 5} = 112.50 kg \\ Lombardi: & = & 100 \cdot 5^{0.10} \approx 117.44 kg \\ Mayhew: & = & \frac{10000}{52.2 + 41.9 \cdot e^{- 0.055 \cdot 5}} \approx 119.04 kg \\ O’Conner: & = & 100 (1 + 0.025 \cdot 5) = 112.50 kg \\ Wathan: & = & \frac{10000}{48.8 + 53.8 \cdot e^{- 0.075 \cdot 5}} \approx 116.57 kg \\ Lander: & = & \frac{10000}{101.3 - 2.67123 \cdot 5} \approx 113.71 kg \\ Average: & \approx & 115.49 kg \end{array}

Rounding is optional and applied after each formula or table step to a chosen increment using nearest, down, or up.

Compute all enabled formulas using w and r.
Average the unrounded results to get a planning anchor.
Optionally round each figure to increment inc by selected mode.
Build a percent table from start to end in chosen steps.

Validation rules and bounds
Field	Type	Min	Max	Step/Pattern	Error Text	Notes
Weight lifted	number	0	—	0.1	Enter a positive weight.	Unit selectable as kg or lb.
Reps performed	integer	1	—	1	Reps must be at least 1.	Estimates beyond about 12 reps are less reliable.
Round to increment	number	0	—	0.1	—	Mode: nearest, down, or up.
Percent start	integer	1	100	1	—	Used for table construction.
Percent end	integer	1	100	1	—	Must be ≥ start.
Percent step	integer	1	50	1	—	Determines table row spacing.
Included formulas	checkboxes	—	—	Epley \| Brzycki \| Lombardi \| Mayhew \| O’Conner \| Wathan \| Lander	—	Average uses enabled formulas only.

Units, precision & rounding

Display shows two decimals; internal calculations keep four decimals before rounding to increment.
Nearest uses standard half-up rounding; down and up use floor and ceil after scaling by the increment.
Percent table multiplies the unrounded average by each percent and then applies rounding.

I/O formats & encoding

Input and output formats
Input	Accepted Families	Output	Encoding/Precision	Rounding
Weight, reps, options	Numbers; unit selection	Metrics table; percent table; JSON payload; CSV export	Display 2 decimals; JSON numeric values	Optional increment; nearest/down/up

Networking & storage

All calculations occur locally; no server calls are required for computation.
Copy and download actions create data on your device when requested.

Diagnostics & determinism

Identical inputs produce identical outputs for each formula and the average.
Warnings surface for zero or negative weight, reps under one, and very high reps.

Security & privacy

No data is transmitted or stored server‑side. Avoid entering personal health information; numbers alone are sufficient for planning.

Assumptions & limitations

Heads‑up Assumes strict technique and consistent tempo.
Assumes adequate warm‑up without fatigue carryover.
Most reliable for 1 to 12 repetitions.
Different lifts may favor different formulas.
Rounding can bias loads up or down at small increments.
Unit choice does not convert plates; it labels outputs.
Average treats formulas equally without weighting.
Denominator guards prevent division by zero at extreme reps.

Edge cases & error sources

Very low weight values round to zero when increment is large.
Reps above 12 inflate estimates due to model limits.
Start percent greater than end produces an empty table.
Percent step of zero is invalid and yields no rows.
Rounding up near 100 can overshoot practical top singles.
Rounding down at low percents can collapse to the same value.
Copy actions can fail if clipboard permission is denied.
Downloads may be blocked by restrictive browser settings.
Changing units does not convert prior numbers; re‑enter weight if needed.
Extreme increments can hide variation among formulas.

How to Use:

Estimate a one repetition maximum and build a matching percent table for training.

Enter Weight lifted and select kg or lb.
Enter Reps performed for that set.
(Optional) Set Round to increment and choose the rounding mode.
(Optional) Choose Percent start, end, and step.
Enable or disable formulas to suit the lift and compare the spread.
Review the average, copy tables, or download the payload for notes.

Example. 100 kg for 5 reps gives an average near 115.5 kg; at 80%, plan about 92.4 kg, rounded per your increment.

Use plate‑sized increments to keep loads practical.
Favor reps under ten on technique‑demanding lifts.

FAQ:

Is my data stored or sent?

No. Calculations happen locally and copy or download occurs only when you request it.

How accurate is the estimate?

It is a practical guide, not a test. The average of multiple formulas reduces bias, but fatigue, exercise choice, and technique still affect results.

Which units can I use?

Kilograms or pounds. Select the one that matches your plates so rounding aligns with your equipment.

Can I use reps higher than twelve?

You can, but reliability drops as reps climb. Prefer sets of three to eight for steadier planning numbers.

Why do formulas disagree?

Each formula models the rep‑to‑max curve differently. Use the average as your anchor and treat the spread as normal variation.

How do I estimate from 5 reps?

Enter the working weight and 5 as reps. Review the per‑formula values and the average, then round to your plate increment.

Does this work offline?

Once loaded, calculations and viewing work without a connection. Copy and download features depend on your device permissions.

Is there a cost or license?

No account is required and the estimator is provided for general educational use.

Valid ranges and warnings follow the on‑page inputs and messages.

Glossary:

One repetition maximum: Highest load liftable once with good form.
Working weight: Load used for the recorded set.
Repetition: One complete controlled movement of the lift.
Rounding increment: Step size that matches available plates.
Percent of maximum: Target intensity as a fraction of the estimate.
Formula spread: Range of values across different methods.