One Rep Max Calculator
Estimate your 1RM from a weight and reps, with a percentage training table.
Results update as you type.
About this calculator
A one-rep-max (1RM) calculator estimates the most weight you could lift for a single repetition from a set you actually performed. It uses the Epley formula (1RM = w × (1 + reps/30)) by default, with Brzycki, Lombardi and O’Conner as alternatives, and prints a table of training loads at common percentages of your max.
Say you squat 100 kg for 5 clean reps. Epley estimates 100 × (1 + 5 ÷ 30) = 100 × 1.167 ≈ 116.7 kg, while Brzycki gives 100 × 36 ÷ (37 − 5) = 112.5 kg. The two agree closely for sets of about 2–10 reps and drift apart on very high-rep sets, which is why the tool shows several formulas side by side.
From the estimated max it builds a percentage table so you can program training loads — for example roughly 85% for a hard triple, 70% for hypertrophy work. Estimating a 1RM from a moderate set is safer than testing a true single, especially without a spotter, and lets you set weights without maxing out every session.
Frequently asked questions
How is one-rep max estimated?
From a set you completed. The Epley formula is 1RM = weight × (1 + reps ÷ 30); Brzycki is weight × 36 ÷ (37 − reps). Both are accurate for about 2–10 reps and diverge for very high-rep sets.
Why show percentages of my 1RM?
Strength programs prescribe loads as a percentage of your max — for example 5 reps near 85% or hypertrophy work around 70%. The table converts your estimated 1RM into those working weights.
Is estimating safer than testing a true max?
Usually yes. A calculated 1RM from a moderate set avoids the injury risk of grinding out a genuine single, especially without a spotter or experience.
How much should I lift for 5 reps?
Roughly 85–87% of your 1RM. The training-load table converts your estimated max into a weight for each percentage and estimates the reps each load allows using the Epley relationship.
Which 1RM formula is most accurate?
All are estimates and none is exact. Epley and Brzycki agree closely from 2–10 reps and diverge at higher reps; the tool shows Epley, Brzycki, Lombardi and O’Conner so you can compare them for your set.
How often should I retest my max?
Every 4–8 weeks is typical, or simply recalculate from a hard set whenever your working weights climb. Estimating from a rep set avoids the fatigue and injury risk of frequent true-max attempts.
Does this work for every lift?
It works best for barbell lifts like the squat, bench press and deadlift where you can perform a controlled set of 2–10 reps. It is less reliable for isolation exercises and very high-rep endurance sets.
API — use this calculator from code
Call this calculator as a free JSON endpoint — no key required. Send the field values below as query parameters or JSON. Read the full API docs →
Endpoint
GET https://calculator.free/api/v1/one-rep-max/curl
curl "https://calculator.free/api/v1/one-rep-max/?weight=100&reps=5&unit=kg"JavaScript fetch()
const r = await fetch(
"https://calculator.free/api/v1/one-rep-max/?" + new URLSearchParams({
"weight": "100",
"reps": "5",
"unit": "kg"
}));
const data = await r.json();
console.log(data.results);Results are estimates for general guidance only, not financial, medical or tax advice.