Your one-rep max is the foundation of every percentage-based strength program. When a program prescribes sets at 80% of your 1RM, that number is worthless if the underlying 1RM is a guess. Working at 80% 1RM tends to place you in the hypertrophy range — roughly 8–10 quality reps — while 85–90% shifts you toward maximal-strength adaptation, and 90%+ is reserved for peaking and competition preparation. Without an accurate 1RM, every prescribed load in your program is off.
The problem is that actually testing your 1RM is risky and impractical for most training contexts. A true max attempt demands a competent spotter, full CNS recovery (typically 48–72 hours of reduced training volume beforehand), and technical mastery under near-maximal load — conditions that are rarely met in a regular training week. More importantly, the cumulative CNS fatigue from frequent max testing undermines the very training it is supposed to calibrate. A better approach is to estimate your 1RM from a recent sub-maximal set — a weight you moved for 3–8 reps with solid form and a couple of reps left in reserve. That is exactly what the calculator below does, using four validated formulas so you can see how the estimates compare.
Enter any recent working set and get your estimated 1RM from four validated formulas — Epley, Brzycki, Lombardi, and O'Conner — plus instant training percentages for every intensity zone.
iPhone · iOS 17 +
Calculator
Accuracy is highest for sets of 2–6 reps. Estimates diverge at 10+ reps — see formula comparison below.
Formula Guide
The four formulas included here are the most widely validated equations in the strength science literature, but they were developed from different populations and datasets — which means they do not always agree, particularly at higher rep ranges.
The Epley formula (weight × (1 + reps / 30)) is the most commonly cited and tends to produce conservative estimates that hold up well across a broad range of rep counts. It was popularized in the early 1990s and remains a default in most strength testing software. The Brzycki formula (weight × 36 / (37 − reps)) is mathematically similar to Epley at low rep counts but grows more aggressively as reps increase — producing notably higher 1RM estimates when you input 10 or more reps. It is well-suited to intermediate lifters performing sets in the 4–8 rep range.
The Lombardi formula (weight × reps^0.1) uses a power function rather than a linear multiplier. It tends to underestimate 1RM slightly compared to Epley and Brzycki at very low rep counts (1–3 reps) but holds accuracy more consistently across moderate rep ranges. The O'Conner formula (weight × (1 + 0.025 × reps)) is the most conservative of the four. Because it applies a smaller multiplier per rep, it is the formula least likely to overestimate your 1RM — which can make it the safer choice when your set was not taken close to failure.
The key takeaway from Mayhew et al. (1995) and subsequent validation studies is that all four formulas are reasonably accurate for sets of 2–6 reps but begin to diverge meaningfully at 10 or more reps. At 10 reps on a 135-lb set, for example, Brzycki will estimate a higher 1RM than O'Conner by roughly 10–15 lbs. If you regularly train in higher rep ranges, the "Average of all 4" option provides a hedge against any single formula's systematic bias.
No estimation formula replaces actual 1RM testing when precision matters — such as peaking for a meet — but for everyday training load management, a well-performed 3–6 rep set provides all the data you need.
Example 1 — Epley: 225 lbs × 5 reps
Estimated 1RM: 248 lbs. At 85% (strength zone): 211 lbs. At 75% (hypertrophy zone): 186 lbs. A 225-lb squat for 5 clean reps with 2 reps in reserve is a reliable input — the Epley formula handles this rep range well and produces an estimate close to what most intermediate lifters would hit on a true max day.
Example 2 — Brzycki: 315 lbs × 3 reps
Estimated 1RM: 335 lbs. At 85% (strength zone): 285 lbs. At 75% (hypertrophy zone): 251 lbs. At 3 reps, Epley and Brzycki produce nearly identical outputs (within 2–3 lbs), so either formula is a good choice for heavy triples. This is the sweet spot where all four formulas agree most closely.
Example 3 — Average of all 4: 135 lbs × 10 reps
Estimated 1RM: ~178 lbs. This example illustrates formula divergence at higher rep ranges. Brzycki estimates roughly 184 lbs; O'Conner produces about 169 lbs — a 15-lb spread from a single input. Using the average of all four (approximately 178 lbs) reduces the impact of any single formula's bias. For sets of 8 or more reps, averaging is recommended unless you have reason to prefer one formula based on your training history.
Why it matters
You estimate 1RM manually from memory
Zenith calculates 1RM from every logged set automatically
Percentages calculated by hand before each session
Training loads auto-set from your current 1RM estimate
No alert when your 1RM estimate goes stale
1RM updates with each new personal best or max-effort set
Separate spreadsheet for tracking strength progress
Strength curves visualized in-app across all lifts
Miss a PR because you didn't notice the trend
PRs flagged in real-time during your session
Static program regardless of strength changes
Program weights auto-adjust as your strength improves
Train smarter
Zenith uses your estimated 1RM to auto-set RPE-based training percentages across every lift in your program — no spreadsheets, no manual math.
Download Zenith FreeMarcus Chen
NSCA-CPT, MS Exercise Science · Reviewed May 2026