Leg Extension 1RM Calculator

Read your leg-extension target weight

Use the target on the same two-leg leg-extension machine and recorded stack or plate convention. Choose the equipment-rounded value only when the machine cannot match the calculation.

Keep the leg-extension setup consistent

Use both legs on the same leg-extension machine and record the indicated stack weight or recorded plate weight for a consistent hard set. Keep the machine, seat, lever settings, range of motion, unit, and selected weight convention the same when you compare.

The number represents the machine weight you recorded; it is not a measurement of force at the pad. This calculator uses both legs together and does not double a single-leg result or infer knee torque.

Weight unit

Choose the unit shown on your weights.

Weight lifted

Enter the weight used for your completed set, following the weight convention below.

Repetitions completed

Enter 1–10 full repetitions from a hard set with consistent technique.

Repetitions to estimate

Choose how many repetitions you want to estimate a weight for, from 1 to 10.

Percentage of your estimated max

Enter the percentage weight you want to calculate.

Available weight increase

Leave blank for the exact estimate, or enter the smallest total weight increase your equipment allows.

Legs used

This estimate uses both legs together.

How are you counting the weight?

Choose the description that matches the number you entered. Keep this choice the same when comparing results.

Machine name

Optionally name the exact machine so you can compare like-for-like next time.

Setup notes

Record the seat, grip or stance you want to repeat next time.

Compare leg-extension targets across reps

The curve compares rep targets for the same two-leg machine setup. It does not show a strength standard or the force acting at the knee.

The example below shows how to read the graph. Your graph updates after you calculate, and the complete values remain available in the table beneath it.

Follow a leg-extension rep-max example

The result uses only the values you enter and the method shown below. Rounded workout values stay separate from the underlying calculation.

A checked leg extension with both legs example

This checked example uses pounds. It comes directly from the same calculator that produces your result.

Select “Use example” to load these values. If you choose kilograms in the calculator, the chart and personalized tables recalculate in kilograms.

Use the estimate within the same task

Use this as a repeatable estimate, not proof of your true one-rep max. The available study used to check the Brzycki equation tested free-weight bench press in 103 college women. That study checks the equation in that setting; It did not test leg extension with both legs. Leg-extension machines can differ in lever arms, stack calibration and seat settings, so compare results only on the same machine and setup. Use a hard set of 1–10 full repetitions. Stopping early can understate the estimate, and this calculator does not accept higher-repetition sets.

• Strength Testing—Predicting a One-Rep Max from Reps-to-Fatigue — Matt Brzycki (1993).
• Accuracy of Prediction Equations for Determining One Repetition Maximum Bench Press in Women Before and After Resistance Training — Mayhew et al. (2008).
• Guide for the Use of the International System of Units (SI) — NIST (2008).

Explore the matching leg-extension task

• Leg Extension Live Endura

  Explore leg-extension standards as a separate comparison task.

• RPE Calculator

  Record how close the completed set felt to its repetition limit.

• Warm-Up Set Calculator

  Calculate chosen warm-up percentages on the same machine.

• Leg Press 1RM Calculator

  Track a leg-press set separately from leg-extension performance.

• Hack Squat 1RM Calculator

  Keep a hack-squat estimate with its own machine and setup.