1RM stands for one rep maximum - the maximum weight that can be lifted once. It is perhaps the best way to measure true strength. The most accurate way to find out your one rep max (1RM) is to test it under the barbell. This however can mess your program, affect recovery time and possibly increase the chance of injury. 1RM can also be estimated using different methods. The formulas defined by Brzycki, Epley and Landers are quite reliable and popular. More advanced lifters may want to look at exercise specific estimates from NSCA (National Strength and Conditioning Association).
Some programs suggest training with a certain percentage of your 1RM. In order to follow the program you need to know your 1RM. Sometimes you may also want to monitor progress when you are not performing one rep lifts and may even train with periodization schemes where you may wish to compare work done using different reps/sets (e.g. have I increased my bench if I did 90kg x 10 reps 2 weeks ago vs 105kg x 6 reps today). Maybe you are preparing for a meet and want to lift a certain amount. Without testing you can calculate how much you should lift with more reps when conditioning. Another great reason to calculate your 1RM is to actually compare it to your tested 1RM. If you calculate a 1RM and your tested 1RM is way lower then it is a good indicator that there's something wrong with either your lifting technique or a lagging muscle group which prevents you from reaching true 1RM. However note that technically difficult olympic exercises (snatch etc) do not work well with calculations due to huge variance in technique which may be the limiting factor instead of strength.
The exact formula for each calculation used in this site can be found in the references listed below. The formulas are based on a regression analysis of large sample of lifters performing a certain amount of reps on a given weight and testing their true 1RM. A formula is defined to match the regression pattern and tested on a new group of lifters. Each of the formulas used on this site have been used for a long time and are fairly accurate for typical exercises. They can never be fully accurate due to differences between lifters (e.g. difference in fast/slow muscle fiber ratio).
1RM may be calculated using a formula such as Brzycki (A), Epley (B) or Lander (C) or it can be calculated using coefficients such as the ones defined by NSCA (D).
Still not sure how to use the calculations? Let's say you lifted X weight (unit is irrelevant kg/lbs/sheep) for Y reps. Put the X in the weight input box and Y in the reps input box. You can see the tables update with estimated max weight you can lift once and weights that you can lift for 1-10 times. The first row (General A, estimate calculated with Brzycki formula) is a good estimate for most people for most exercises. If the weight and reps your entered are for bench press, squat or deadlift and you are an intermediate/advanced lifter then you may want to check the NSCA numbers for specific exercises.
This site was created to be the best and most intuitive way to calculate 1RM and see the amount of reps you can get with a given weight. I created the site because I was unhappy with existing calculators. This service is not affiliated or sponsored by anyone.