One-Rep Max Calculator — Estimate 1RM from Sub-Maximal Lifts
Estimate one-rep max (1RM) from any rep × weight set using Epley, Brzycki, Lombardi, O'Conner, Wathan formulas. Free.
About One Rep Max Calculator
A one-rep max (1RM) calculator estimates the maximum weight you could lift for a single repetition based on a sub-maximal set (e.g., 100 kg × 5 reps) using formulas like Epley, Brzycki, Lombardi, O'Conner, and Wathan — useful for programming training percentages without actually testing 1RM (which is fatiguing, time-consuming, and somewhat injury-prone). The ZTools 1RM Calculator runs all five formulas at once for comparison, supports kg and lb, generates a full percentage table (50% through 100% of estimated 1RM in 5% steps), and notes that all formulas lose accuracy beyond ~10 reps — pick a 3–8 rep set for the most reliable estimate.
Use cases
- Program design without max-testing. Lifters set training percentages (5×5 at 80%, 3×3 at 90%) without doing a true 1RM test. Calculator gives the working numbers.
- Progress tracking. After a 4–8 week training block, recompute estimated 1RM from a heavy AMRAP set. Direct measure of strength change.
- Coaching novice lifters. Coaches use estimates rather than testing because novice 1RMs change weekly and testing wastes a session.
- Comparing formulas. Different formulas suit different rep ranges. Side-by-side view shows the spread (~3–5 kg between formulas typically).
How it works
- Enter weight × reps. A real set you completed near failure. Best for 3–8 reps; 1–2 reps gives accuracy ≈ true 1RM, 10+ reps overestimates.
- Compute via 5 formulas. Epley: w × (1 + 0.0333 × r). Brzycki: w / (1.0278 − 0.0278 × r). Lombardi, O'Conner, Wathan have similar shapes.
- Average or pick. Default returns the average of all five. Pick a single formula if you have a preference.
- Generate percentage table. Estimated 1RM × 50% through 100% in 5% steps for programming.
- Refresh after each block. Recompute from a heavy AMRAP every 4–8 weeks; track progression.
Examples
Input: 100 kg × 5 reps
Output: Epley 116.7 · Brzycki 112.5 · Lombardi 114.9 · average ≈ 114 kg estimated 1RM
Input: 80 kg × 8 reps
Output: Estimated 1RM ≈ 100 kg (5 formulas average)
Input: 60 kg × 12 reps
Output: Estimated 1RM ≈ 84 kg — note: 12 reps is at the edge of formula accuracy
Frequently asked questions
How accurate are these estimates?
Within 5% for most lifters at 3–8 rep sets. 1–2 rep sets give near-true 1RM. Beyond 10 reps, all formulas overestimate (training factor matters more than maximal strength at high reps).
Which formula should I use?
Epley is the most common; Brzycki is more conservative for high reps. The average across formulas hedges against single-formula bias.
Should I retest 1RM?
For competitive lifters, occasionally yes. For most, estimates suffice and avoid risk. If you do test, follow proper warmup, ramp-up, and have a spotter.
Do estimates work for all lifts?
Best for compound lifts (squat, bench, deadlift, overhead press). Isolation lifts (curls, tricep extensions) have different fatigue profiles; estimates run high.
Why does my 1RM seem to fluctuate?
Sleep, food, hydration, stress all affect daily strength by ±5%. Single-day estimates are snapshots, not absolutes.
How often should I recompute?
After each training block (4–8 weeks). More frequently for novices; less for advanced lifters.
Pro tips
- Use 3–8 rep sets for the most reliable estimates.
- Rest 3–5 minutes before the test set — fatigue inflates rep count and overestimates 1RM.
- Same lift, same equipment for tracking — comparing barbell squat to leg-press 1RM is meaningless.
- Recompute after every block; use the new number to set the next block's percentages.
- Conservative beats heroic — programming 5% under estimated 1RM rarely costs progress; 5% over costs missed reps.
Reviewed by Ahsan Mahmood · Last updated 2026-05-05 · Part of ZTools.
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