The most accurate way to know your 1RM is to test it under a barbell but oftentimes that can be detrimental to your current programming or maybe you're not confident with doing a max effort lift due to previous injuries or lack of experience.
This 1RM calculator can be used to help you with finding your starting point for more advanced strength and conditioning programs that require the use of your 1RM percentages.
While there are a multitude of formulas out there that help to calculate 1RM, the most proven accurate is the 1RM formula created by Matt Brzycki, expertly named as the Brzychi Formula:
weight performed / ( 1.0278 – ( 0.0278 × reps performed ))
Use The Embedded Calculator Below To Get Your 1RM - For most accurate results, it's recommended that you use a multi-rep max of no more than five repetitions.