Calculate your leg press one-rep max from a set’s weight and reps with the Epley or Brzycki formula, or find the working weight for a target rep count.
Seated Leg Press One-Rep Max Formula
The calculator estimates machine-load one-rep max (1RM) from machine resistance and completed repetitions, or algebraically finds a load from a target estimated 1RM and target repetitions. Choose Epley or Brzycki as the primary formula; both estimates, their range and their spread are shown. Optional body weight affects only the 1RM/body-weight ratio and is never added to machine resistance. Results are estimates, not guaranteed maxima or safe-load prescriptions.
Epley convention: M = L × (1 + r/30). Brzycki convention: M = L × 36/(37 − r). These forms apply for r ≥ 2 here; at r = 1 the calculator returns M = L for both.
- M (1RM) = estimated machine-load one-repetition maximum
- L = machine resistance used, in kg or lb; do not add body weight
- BW = optional body weight, used only for the ratio M/BW after unit conversion
- r = whole repetitions from 1 to 36; values above 10 are high-rep extrapolation, not a reliable prediction guarantee
Machine resistance alone supplies L in both formulas. The selected estimate divided by optional body weight gives a physical weight ratio, with units converted when kg and lb differ. Blank or zero body weight omits the ratio; negative body weight is invalid. Machine design, seat position, friction and technique affect comparability, so use the same machine and setup consistently.
How to Use the Calculator
- Choose Estimate 1RM from a set or Load from target estimated 1RM, then choose Epley or Brzycki.
- For an estimate, enter the machine resistance in kg or lb and the completed repetitions. For inverse mode, enter the target estimated 1RM and target repetitions.
- Use a comparable set of full, controlled repetitions near its repetition limit. The accepted arithmetic range is 1–36; more than 10 reps is explicitly marked as high-rep extrapolation.
- Optionally enter body weight for the ratio only. Read both formula estimates, their range/spread and the limitations; percentage rows are arithmetic examples, not personal training prescriptions.
For comparison, keep depth, tempo, seat position and effort consistent. Enter displayed machine resistance as the Machine load used; do not combine it with body weight. Per-field kg/lb selectors convert entered values, and the main result follows the active machine-load or target-1RM unit. This model does not correct for sled angle, friction or a particular machine’s mechanical advantage.
Rearranged Forms of the Formula
Load from target estimated 1RM mode inverts the selected declared formula. This calculator does not solve for body weight or repetition count:
Inverse Epley: L = M ÷ (1 + r/30).
Inverse Brzycki: L = M × (37 − r)/36.
At exactly one repetition, both inverse modes return L = M. Both formulas use the same machine-load basis. At 10 reps their factors both equal 4/3, so the formula spread is zero; that agreement is not a confidence interval or proof of accuracy.
Example
For 200 lb of displayed machine load and 10 reps, both declared formulas give the same estimated 1RM. Optional body weight of 180 lb is used only for the ratio:
M = 200 × (1 + 10/30) = 200 × 36/(37 − 10) ≈ 266.6667 lb.
The estimated machine-load 1RM is 266.6667 lb. With 180 lb body weight, the ratio is about 1.4815×; changing body weight does not change the 1RM estimate.
Arithmetic Percentages of Estimated 1RM
Multiplying the selected estimated 1RM by a fraction gives an arithmetic machine-load value. The calculator shows 60%, 70%, 80% and 90%; these do not determine an appropriate program, set count, effort or safe load for an individual.
Arithmetic machine load = selected estimated 1RM × fraction.
| Fraction of estimated 1RM | Multiplier |
|---|---|
| 60% | 0.60 |
| 70% | 0.70 |
| 80% | 0.80 |
| 90% | 0.90 |
Example: 80% of the 266.6667 lb estimate is about 213.3333 lb by arithmetic. This does not establish a safe or suitable training load.
Arithmetic 80% load = (800 ÷ 3) × 0.80 ≈ 213.3333 lb.
What Affects Accuracy?
| Factor | Why It Matters |
|---|---|
| Machine design | Different seated leg press machines can feel heavier or lighter even at the same displayed load. |
| Range of motion | Quarter reps and full-depth reps can produce very different estimates. |
| Foot placement | Foot height and stance width can change leverage and muscle emphasis. |
| Rep quality | Bouncing, shortening the rep, or using momentum can inflate the estimate. |
| Rep count | Very high-rep sets usually make any 1RM estimate less precise than moderate-rep sets. |
Best Practices for Consistent Results
- Use the same seated leg press machine when comparing results over time.
- Keep foot placement, seat position, and depth as consistent as possible.
- Count only repetitions completed with control and full intended range.
- If you use the optional ratio, record body weight in the correct unit near the session date; it does not enter the 1RM formula.
- Use the estimate as a programming tool, not as a guarantee of what you can safely max for one rep.
Common Input Mistakes
- Adding body weight into the machine load field: do not add it. The optional body-weight input is used only for the dimensionless ratio.
- Mixing completed and target reps: estimate mode uses the reps actually completed; inverse mode uses the intended target count.
- Comparing different machines directly: a new machine can change the estimate even if your strength has not changed.
- Ignoring technique changes: shorter range of motion can make progress look larger than it really is.
FAQ
Is this the same as a squat one-rep max?
No. A seated leg press and a squat use different mechanics, support, balance demands, and loading patterns, so the numbers should not be treated as interchangeable.
Should I test a true 1RM on the seated leg press?
The calculator does not decide whether maximal testing is appropriate or safe. Discuss the testing method, technique and safeguards with an appropriately qualified professional rather than treating an estimate as a permitted attempt.
Why does the calculator include body weight?
Body weight is used only to show estimated 1RM divided by body weight after converting units; it is never added to machine resistance. Blank or zero omits this optional ratio. No strength classification or safe-load recommendation is inferred from it.
What is the best way to track progress?
Use the same machine, setup, range of motion and comparable effort. Treat trends as estimates rather than direct measurements. Reynolds and colleagues’ 2006 plate-loaded leg-press and chest-press study found better prediction from 5RM than higher-repetition tests and advised limiting linear estimates to 10 repetitions; it does not validate every seated machine or user. This calculator retains its inherited exact 1/30 and 36/(37 − r) conventions, while that paper’s comparison table uses rounded coefficients. Above 10 reps, the accepted 1–36 arithmetic domain is marked as extrapolation.
