One Rep Max Calculator

TL;DR: To calculate your one rep max (1RM), use the Epley formula: 1RM = Weight × (1 + Repetitions / 30), so for 80 kg lifted for 5 reps, your estimated one rep max is 93.3 kg, with training percentages of 84 kg at 90%, 74.7 kg at 80%, and 65.3 kg at 70%.

What Is the One Rep Max Calculator?

The One Rep Max Calculator is a free online tool that estimates the maximum weight you can lift for a single repetition (your 1RM) based on a submaximal set. Instead of risking injury by attempting a true one-rep max, you enter a weight you can comfortably lift for multiple reps, and the calculator uses a proven formula to project your theoretical 1RM. This is a cornerstone of strength training, powerlifting, and bodybuilding programs, as it allows you to prescribe training loads as percentages of your true max.

The calculator is essential for athletes, coaches, and recreational lifters who want to train with precision. By knowing your estimated 1RM, you can structure your workouts to target specific adaptations—like pure strength (85-100% 1RM), hypertrophy (70-80% 1RM), or muscular endurance (50-60% 1RM). For example, if your goal is to build muscle, you can program sets at 75% of your calculated 1RM, ensuring you are lifting heavy enough to stimulate growth but light enough to maintain proper form and volume. This calculator removes the guesswork and provides concrete numbers you can plug directly into your training log.

Beyond gym enthusiasts, this tool is valuable for anyone following structured programs like 5/3/1, Starting Strength, or periodized routines where intensity is calculated as a percentage of a daily, weekly, or monthly max. The calculator provides immediate results for your primary output (the 1RM) and three secondary outputs (90%, 80%, and 70% of that max). These percentages are the most common training intensities used in periodization, giving you a ready-made roadmap for your next mesocycle without needing to do extra math.

How to Use the Calculator

Using the One Rep Max Calculator is straightforward and takes less than ten seconds. Follow these steps to get your estimated maximum and training percentages:

1. Enter the weight lifted (peso_kg): In the input field labeled 'peso_kg', type the total weight you lifted, in kilograms. For this example, enter 80. This value must be a positive number greater than zero. This is the actual weight on the bar, not including the bar's weight if you are using a standard Olympic bar (though you should include the bar weight in the total). For instance, if you have one 20 kg plate per side, the total is 20 (plate) + 20 (plate) + 20 (bar) = 60 kg.
2. Enter the number of repetitions (repeticiones): In the input field labeled 'repeticiones', type the exact number of reps you performed. For this example, enter 5. This should be the number of reps you completed with good form before reaching technical failure. If you can do more than 12 reps, the formula's accuracy drops, so aim for a rep range between 1 and 10 for best results.
3. Initiate the calculation: Click the 'Calculate' button. The calculator will process your two inputs using the Epley formula.
4. Review your results: The tool will display four key outputs:
   • one_rep_max: Your estimated maximum lift for a single rep (e.g., 93.3 kg).
   • at_90pct: 90% of your 1RM (e.g., 84.0 kg). This is typically used for heavy working sets or top-end strength work.
   • at_80pct: 80% of your 1RM (e.g., 74.7 kg). This is commonly used for volume training and hypertrophy.
   • at_70pct: 70% of your 1RM (e.g., 65.3 kg). This is ideal for speed work, technique practice, or deload weeks.

Formula and Calculation Method

The calculator uses the Epley formula, one of the most widely validated and accurate equations for estimating one rep max. The formula is simple and requires only your lifted weight and the number of reps performed. It is expressed as:

1RM = Weight × (1 + Repetitions / 30)

In plain language, this formula says: your one rep max equals the weight you lifted, multiplied by one plus the number of reps you did divided by thirty. The logic is that for every 30 additional reps you can perform, your estimated max increases by the entire weight of the load. So, if you can lift a weight for 10 reps, your 1RM is roughly one-third higher than the weight on the bar.

Let’s walk through the concrete example from the calculator inputs. Suppose you lifted 80 kg for 5 repetitions. Plugging these numbers into the formula:

1RM = 80 × (1 + 5/30)
1RM = 80 × (1 + 0.1667)
1RM = 80 × 1.1667
1RM = 93.3 kg

So, your estimated one rep max is 93.3 kg. Once this is established, the calculator derives the secondary outputs using simple percentages:

• 90% of 1RM: 93.3 × 0.90 = 84.0 kg
• 80% of 1RM: 93.3 × 0.80 = 74.7 kg
• 70% of 1RM: 93.3 × 0.70 = 65.3 kg

These derived numbers are directly actionable. If you want to train at 80% intensity, you would load the bar with approximately 75 kg (rounded to the nearest 2.5 kg plate increment), which is a manageable weight for higher-rep sets. This method ensures you are always training at the right intensity relative to your true strength level.

Practical Examples

To illustrate the utility of the calculator, here are three realistic scenarios showing how different inputs yield different training prescriptions. Each scenario uses the same formula but provides distinct outputs tailored to different lifting profiles.

In the powerlifter scenario, lifting 140 kg for 3 reps projects a 1RM of 154 kg. This lifter would use the 123.2 kg (80%) for volume deadlifts (e.g., 5 sets of 5) and 138.6 kg (90%) for heavy triples or singles. In the hypertrophy scenario, the lifter lifting 60 kg for 10 reps has a projected max of 80 kg. Their 80% (64 kg) is perfect for 8-12 rep sets to maximize muscle growth. The beginner scenario shows a lifter working with 40 kg for 8 reps, projecting a 50.7 kg max. Using the 70% value of 35.5 kg helps this athlete focus on technique and bar speed without excessive load, reducing injury risk while building a solid foundation.

Tips for Accurate Results

To get the most reliable estimate from this calculator, you must pay attention to how you input your data. The most common mistake is confusing unit options—this calculator requires weight in kilograms (kg). If you usually train in pounds, divide your pound weight by 2.20462 to convert to kg before entering it. For example, a 175 lb lift converts to approximately 79.4 kg. Entering pounds directly will skew your results significantly (e.g., entering 175 as kg would suggest a 1RM of over 204 kg, which is wildly inaccurate for a 175 lb lifter).

Another critical pitfall is entering zero or negative values. The calculator requires positive numbers for both peso_kg and repeticiones. If you enter 0 or a negative number, the formula will produce a nonsensical result (like a negative 1RM or a 1RM equal to 0), which is not useful for programming. Always double-check that you have entered a real weight you actually lifted, not a symbolic placeholder.

Finally, be mindful of the recommended rep range for the formula's accuracy. The Epley equation is most accurate for rep ranges between 1 and 10. If you enter a rep count higher than 12, the formula tends to overestimate your true 1RM. This is because, at higher reps, muscular endurance and fatigue play a larger role than maximal strength. If you can perform 20 reps with a weight, your actual 1RM will be lower than what the formula projects. For accuracy, always test your 1RM or use a weight you can lift for 6-8 reps with near-maximal effort. Also, ensure you perform your tested set to near-failure (about 2 reps in reserve) for the estimate to be valid; if you stop at 5 easy reps, the projection will be low.

Frequently Asked Questions

How accurate is the Epley formula compared to an actual 1RM test?

Research on the Epley formula shows it is typically within 2-5% of a lifter's actual tested one rep max for trained individuals. For most lifters, the formula's estimate is accurate enough to guide training programming without requiring a risky maximal lift. However, its accuracy decreases for very high rep sets (above 12 reps) and for lifters who are highly trained in endurance versus pure strength. If you are a powerlifter needing exact numbers, you should periodically test your true 1RM directly, but for daily training, this calculator provides an excellent and safe proxy. The at_90pct and at_80pct outputs are particularly reliable for setting up weight, as small discrepancies in 1RM translate to only 1-2 kg differences in your working sets.

Can I use this calculator for any lift (squat, bench, deadlift, etc.)?

Yes, the One Rep Max Calculator is exercise-agnostic. You can use it to estimate your 1RM for the squat, bench press, deadlift, overhead press, barbell rows, or any other compound or isolation lift. The Epley formula is based on the general relationship between load and repetitions, which holds true across different muscle groups. However, keep in mind that your 1RM will differ for each lift. For example, you might have a 1RM of 140 kg on deadlift but only 100 kg on bench press. Always input the specific weight and reps for the lift you are programming, rather than using a generic number. For best results, perform the estimation set with the exact exercise you intend to train, as neural adaptations and leverage changes can affect the relationship.

What if I fail to complete a rep on my testing set? Should I use the partial rep count?

If you fail to complete a rep, you should count only the successfully completed repetitions, not the attempted one. For example, if you attempted 6 reps with 80 kg but failed on the 6th rep, you should enter 5 as the rep count. This is because the completed reps represent your true capacity, and adding a failed rep will inflate your estimated 1RM. In this case, the formula would calculate 80 × (1 + 5/30) = 93.3 kg. If you mistakenly entered 6, the formula would give you 1RM = 96.0 kg, an overestimate that could lead you to select a working weight that is too heavy. Always err on the side of conservative estimates when in doubt, as under-perceiving your max is safer for joint health and long-term progress than overestimating.