How the 1RM estimate works
The calculator runs six formulas side by side: Epley, Brzycki, Lombardi, O'Conner, Mayhew, and Wathan. Each formula was built from a different data set and a different mathematical fit, which is why the answers can differ by a few pounds for the same set. e1RM shows the spread instead of hiding it behind one default number.
That spread is useful. If all six formulas cluster tightly, the set was probably a good input for estimating strength. If the formulas disagree by 5-8%, the set was probably too high-rep, too far from failure, or affected by technique fatigue. Treat the range as an error bar, not as six competing claims.
The six formulas
Epley (1985) uses 1RM = w x (1 + r / 30). It is the most common default in strength training apps and works well for hard sets of roughly two to eight reps. It can read high when a lifter is unusually good at rep work.
Brzycki (1993) uses 1RM = w x 36 / (37 - r). It is usually a little more conservative than Epley. That makes it useful when returning from a layoff, choosing a training max for a high-volume block, or estimating from a set that had a rep in reserve.
Lombardi (1989) uses 1RM = w x r^0.10. It is a power-curve fit rather than a simple linear adjustment. It often stays close to the middle of the group for low-rep barbell work.
O'Conner (1989) uses 1RM = w x (1 + 0.025 x r). It is a simple linear approximation and often lands near Brzycki for common working sets.
Mayhew (1992) uses 1RM = (100 x w) / (52.2 + 41.9 x e^(-0.055 x r)). It came from bench press research with athletic populations, so it is useful as an upper-body cross-check.
Wathan (1994) uses 1RM = (100 x w) / (48.8 + 53.8 x e^(-0.075 x r)). It is another curved model designed to handle common strength-test rep ranges without assuming every extra rep adds the same amount.
In every formula, w means weight lifted and r means reps completed. The calculator keeps the math visible because lifters should be able to cite the formula, check the number, and understand why two calculators may disagree.
Which formula should I trust?
For most barbell lifts on sets of two to eight reps, the formulas usually land within 3-5% of each other. When they do, use the middle of the range or your preferred default. Epley is a practical everyday choice; Brzycki is a useful conservative choice when you want a training max that will survive several weeks of volume.
If one formula is an outlier, do not chase it. A 225 x 5 bench press may give one estimate around 253 lb and another around 268 lb. That does not mean you are guaranteed to press 268 today. It means the model spread is telling you how much uncertainty exists. For programming, the lower number usually produces better training than the biggest number.
Why low-rep sets give better estimates
A clean triple at roughly 90% of true max is mechanically similar to a heavy single. Bar path, bracing, joint angles, and intent are close to max-strength work. A 12-rep set at 65% is different: breathing, pacing, local muscular endurance, and pain tolerance all become limiting factors. That is why high-rep estimates often inflate a lifter's usable max.
The sweet spot is usually two to six reps. Sets of seven to ten can still work, but the error bar widens. Anything above ten reps should be treated as a rough estimate until you confirm it with a heavier set.
From estimated 1RM to training loads
The percentage table converts the selected formula into practical barbell loads. Use 95-100% for singles and max-strength exposure, 85-92.5% for heavy strength work, 75-82.5% for strength and hypertrophy overlap, 65-72.5% for volume and technique, and 50-60% for speed work, warm-ups, deloads, or AMRAP-style variations.
A common mistake is plugging an inflated estimate into 85% work and grinding through bad reps for three weeks. If your first set at 85% moves like an RPE 9.5 max effort, the estimate was too high. Drop the training max by 5%, retest with a cleaner set, and rebuild the percentage table.
When to retest
Intermediates can retest every four to eight weeks without taking a true max. Use a top set of three or five reps, keep the standard consistent, and compare the new estimate to the old one. A cleaner set with the same weight is progress even when the formula output barely moves.
Estimated maxes are best used as planning tools. They are not meet attempts, they are not proof of a gym PR, and they are not a substitute for judgment. The reason to use six formulas is simple: you get a number, a range, and a warning when the input is not reliable enough to over-trust.
How to choose the input set
Use a set that matches the way you want to train the lift. For bench press, decide whether the reps were paused or touch-and-go. For squat, keep depth honest. For deadlift, note whether straps were used. For overhead press, keep strict press and push press separate. The calculator can only estimate from the information you give it, so a consistent movement standard matters more than chasing the hardest-looking set in your logbook.
Do not enter warm-up sets, backoff sets taken far from failure, or sets where technique changed dramatically across reps. A clean 225 x 5 at RPE 9 is a better input than 225 x 8 with three questionable reps. If you are unsure, calculate both the aggressive and conservative set, then program from the lower training max until performance confirms the higher one.
Sources
- Epley, B. (1985). Poundage chart. Boyd Epley Workout.
- Brzycki, M. (1993). Strength testing: Predicting a one-rep max from reps-to-fatigue. JOPERD, 64(1), 88-90.
- Lombardi, V.P. (1989). Beginning weight training: The safe and effective way. Wm. C. Brown Publishers.
- O'Conner, B., Simmons, J., & O'Shea, P. (1989). Weight Training Today. West Publishing.
- Mayhew, J.L., et al. (1992). Relative muscular endurance performance as a predictor of bench press strength. Journal of Applied Sport Science Research, 6(4), 200-206.
- Wathan, D. (1994). Load assignment. In Essentials of Strength Training and Conditioning.
- LeSuer, D.A., et al. (1997). The accuracy of prediction equations for estimating 1-RM performance in the bench press, squat, and deadlift. JSCR, 11(4), 211-213.