How the One Rep Max Calculator Works
Give the calculator one hard set, the weight and how many reps you got, and it estimates the heaviest single you could lift today. That estimate is your E1RM, or estimated one rep max. It lets you track your strength every week without grinding out a true max every week.
Say you squat 225 lb for 5 reps. The Brzycki formula, the default here, divides the weight by 1.0278 − 0.0278 × reps: 225 ÷ (1.0278 − 0.139) = 253 lb. So a solid set of five at 225 says you are good for about 253 lb on a single, today. Put in your own set above and the calculator runs the same sum through all seven formulas at once.
The 7 One Rep Max Formulas Compared
Every formula is someone's best fit to how reps fall away as the bar gets heavier. Here is each one run on the same three sets, where w is the weight and r is the reps.
| Formula | Equation | 225 lb × 5 | 185 lb × 10 | 135 lb × 15 |
|---|---|---|---|---|
| Brzycki | 253 lb | 247 lb | 221 lb | |
| Epley | 263 lb | 247 lb | 203 lb | |
| McGlothin | 256 lb | 248 lb | 220 lb | |
| Lombardi | 264 lb | 233 lb | 177 lb | |
| Mayhew | 268 lb | 242 lb | 191 lb | |
| O'Conner | 253 lb | 231 lb | 186 lb | |
| Wathan | 262 lb | 249 lb | 204 lb |
At 5 reps the seven formulas land within 6% of each other (253–268 lb). At 10 reps the gap is 8% (231–249 lb). At 15 reps it opens to 25% (177–221 lb), and the formulas stop agreeing on what your set means. That is the real lesson of the table: the formula matters far less than the rep count you feed it.
Which Formula Should You Use?
Pick one and stick with it. For tracking progress, a formula you use every time beats a slightly more accurate one you swap between, because the trend is what tells you whether you are getting stronger. Brzycki is the default here because it behaves well in the 1–6 rep range where most strength work lives. Epley is one of the most widely used and runs a little higher at 5 reps.
If you want the full story on where each equation came from, read how to calculate your E1RM.
Using Your Estimate in Training
Most programs are written in percentages of your max, which is what the percentage table above is for. A 5-rep max usually sits around 85–90% of a one rep max (89% in the example above), so working sets of 5 that leave a rep or two in the tank tend to live at 75–85%. Volume work sits lower, around 65–75%.
The rep max table works the other way. If your program says "a heavy triple", read off the 3RM row and start a little under it. Before any of that, the warm-up sets calculator builds your ramp from the estimate so you are not guessing plates between sets.
When to Trust the Estimate, and When to Test a True Max
Estimates are most reliable from a hard set of about 3–10 reps. A 2006 Journal of Strength and Conditioning Research study found that 5-rep sets gave the most accurate predictions. Above 10 reps, the formulas themselves become unpredictable, which is why the table above scatters at 15.
The formulas also describe an average lifter. If you have always been good at high-rep sets, they will flatter you; if you are a natural at heavy singles, they will sell you short. Watch how your estimates line up with the singles you actually hit, and trust the number accordingly.
Test a true max when it earns its place: before a meet, at the end of a training block, or when you want a real number to set your percentages from. The rest of the time the estimate is the safer tool. How often should you test your one rep max?
