Your One Rep Max (1RM) is the maximum amount of weight you can lift for a single repetition of a given exercise with proper form. It is the gold standard for measuring muscular strength and is used to determine training intensities and percentages for strength and hypertrophy programs.

You can estimate your 1RM by lifting a sub-maximal weight for as many reps as possible and using a mathematical formula. The most common is the Epley formula: 1RM = Weight × (1 + Reps / 30). For example, if you bench press 100 kg for 5 reps, your estimated 1RM is 100 × (1 + 5/30) = 116.67 kg. This is much safer than testing a true 1RM.

The Epley formula is one of the most widely used equations to estimate a 1RM. It is expressed as: 1RM = w × (1 + r / 30), where w is the weight lifted and r is the number of repetitions performed. It is highly accurate for rep ranges between 1 and 10, but may slightly overestimate 1RM if you perform more than 10-12 reps.

The Brzycki formula is another popular 1RM estimation equation: 1RM = w × (36 / (37 – r)), where w is weight and r is reps. It is very accurate for low rep ranges (1-10 reps). However, it becomes mathematically invalid if you perform 37 or more reps, and it tends to underestimate 1RM for higher rep ranges compared to Epley.

1RM calculators are generally very accurate (within 1-5%) if you use a weight that allows you to perform between 3 and 10 repetitions to failure. If you perform more than 12-15 reps, the formulas tend to overestimate your true 1RM because muscular endurance plays a larger role than pure maximal strength. For the best accuracy, use a heavy weight where you fail between 4 and 8 reps.

Testing a true 1RM is highly taxing on the nervous system and carries a higher risk of injury, especially for inexperienced lifters. It is generally recommended to use sub-maximal 1RM calculators for most training purposes. Only advanced lifters preparing for powerlifting competitions should regularly test their actual 1RM, and they should do so with proper spotting and after a thorough warm-up.

For maximal strength, train at 85-100% of your 1RM for 1-5 reps. For hypertrophy (muscle growth), train at 65-85% of your 1RM for 6-12 reps. For muscular endurance, train below 65% of your 1RM for 13+ reps. This calculator provides a full percentage chart so you can easily find your training zones.

You should recalculate your 1RM every 4-6 weeks, or whenever you significantly increase the weight you can lift for a given number of reps. As you get stronger, your training percentages need to be updated to ensure you are continuing to progress without overtraining.