Testing a true one-rep max, the heaviest weight you can lift for exactly one repetition, carries real injury risk and requires significant warm-up and recovery, which is why most lifters never actually attempt it directly. Instead, 1RM formulas let you lift a lighter, safer weight for several reps and back-calculate an estimated max, a genuinely useful shortcut, but one built on statistical relationships rather than a direct measurement of your specific body.
How the formulas actually work
The two most common formulas are Epley and Brzycki. The Epley formula estimates 1RM as weight × (1 + reps ÷ 30), so lifting 100 kg for 5 reps predicts a 1RM of about 116.7 kg. The Brzycki formula uses weight × 36 ÷ (37 − reps), which for the same 100 kg × 5 reps gives roughly 112.5 kg, a noticeably different number from Epley despite using identical inputs. Both were derived by fitting equations to real lifting data, but from different studies and lifter populations, which is exactly why they diverge. The One Rep Max Calculator typically uses one of these formulas (often Epley) to convert a submaximal set into an estimated max instantly.
Why the estimate gets less reliable at higher rep counts
All 1RM formulas become progressively less accurate as rep count increases, they're built for and validated on rep ranges roughly between 1 and 10, and a formula applied to a set of 20 reps extrapolates well beyond the range it was designed for, often overestimating true 1RM significantly. This is because the relationship between weight and reps isn't perfectly linear across all rep ranges, muscular endurance and fatigue resistance start to matter more than raw strength at higher reps, which the simple formulas don't fully capture.
The formulas also don't account for exercise-specific factors like leverage, technique efficiency, or how a particular movement pattern fatigues, which is why an estimated 1RM for a squat and a bench press built from the same formula carry different amounts of real-world accuracy depending on the lifter's technique in each movement.
Using the estimate safely
Because these are estimates, it's worth treating the output as a training guide (for calculating percentage-based training loads, for example) rather than a number to chase directly with a maximal attempt. If you do want to test a true 1RM, doing so with proper warm-up, spotters or safety equipment, and ideally professional supervision reduces the injury risk that comes with attempting a genuine single-rep maximal lift, and pairing strength training with adequate protein intake supports the recovery that underlies real strength gains over time.