Estimate a 1RM
Weight and reps in. A cited formula out. No account.
1RM = w × (1 + r / 30) · Epley, 1985. Default.
How the estimate is made
A one-rep max is the heaviest load you could lift for a single repetition. Most training never tests that single. This fitness calculator estimates it from a set you already did: the weight on the bar, the repetitions you completed, and an optional RPE or reps in reserve. The default formula is Epley. Brzycki, Lombardi, Mayhew, and Wathan are available when you want a second reading of the same set. Each card cites the year, the equation, and the source, so the number can be checked instead of trusted on appearance.
One number, three surfaces
The browser, GET /api/v0, and the MCP server at /mcp run the same function. There is no account and no API key. Send Accept: text/markdown, or open the .md twin, when you want the page as Markdown. llms.txt names the jobs this site is for and the jobs it is not. The OpenAPI document is the contract for the HTTP API. Errors come back as problem details with a code and a resolution, not as an HTML page.
From a 1RM to a session
Once you have a one-rep max, the other tools go the other direction. A target rep count becomes a working load. A bar plus plates becomes a loading plan. Warm-up sets walk up from the empty bar. Meet attempts, work sets, and a rep-max table use the same estimate. Points scores cover IPF GL, DOTS, Wilks, and Sinclair. Field tests cover the Cooper run, the Rockport walk, and the Léger shuttle. YMCA bench press scoring sits with the protocols. Units default to kilograms.
What the number is not
These are estimates from published formulas or common gym practice. They are not a training plan, a coaching service, or a lab measurement. RPE on a 1RM estimate means leftover reps: 100 for 5 at RPE 8 is treated as 100 for 7 at RPE 10, then the formula runs. That rule is not the Helms percent chart. The chart is a separate lookup. Read the formula card before you treat a result as a tested max.
A worked call
One hundred kilograms for five repetitions is the example on this page. Epley reads that set as about 116.7 kg. The same call on the API is GET /api/v0/one-rep-maxes?weight=100&reps=5. Add formula or unit when the default is wrong. Add rpe or rir when the set was not taken to failure, and do not send both. A load in the other direction is GET /api/v0/loads with one_rep_max and reps. Agents that want the whole contract should open /api/v0/openapi.json, and agents that want a tool call should use /mcp. The about, contact, and privacy pages name who publishes the site and how to reach them.