How To Calculate 1RM (rep max)

1RM Calculator - Link to Google Sheet

WHAT IS 1RM AND WHO NEEDS IT?
One rep max is measurement of muscular strength, not endurance.  Athletes who dominantly use the anaerobic system would get the most out of a one rep maximum strength test.

WORD OF CAUTION
If a spotter isn't present or if the participant has a medical condition that could be aggravated by maximal intensity weight lifting, use the equation.  Individuals with hypertension should especially avoid 1RM testing.  Because heavy weights cannot be lifted quickly in a one rep max test, the primary movers and the core muscles will contract almost isometrically and increase blood pressure quickly.  This can lead to serious problems or even death.  Consult an experienced or competent fitness professional to spot the lift.  

SAFE EXERCISE FOR 1RM TESTING
Generally, power exercises should be avoided due to its explosive nature.  Power exercise also cannot be spotted, so the risk is even greater.  If it has to be done, it's wise to estimate one rep max through multiple repetitions.  For those competing in weightlifting, make sure you have above normal mobility so that you have multiple options to safely exit the lift.

For general fitness purposes, one rep max is best for larger muscle groups and exercises that may be performed with control:

• Bench press - Pectoralis major

• Squat - Gluteus maximus

• Split squat - Gluteus maximus, hamstrings

• Romanian dead lift - Gluteus maximus

• Step up - Gluteus maximus

• Shoulder press - Trapezius, triceps

• Knee extension - Quadriceps

• Knee flexion - Hamstrings

It's not appropriate or safe to 1RM test stabilizing muscles like the core, rotator cuff, neck, and deep hip rotators.  Asking a stabilizing muscle to function as a primary mover is a recipe for disaster.  Here are some exercises to avoid:

• External rotation - Rotator cuff muscles

• Hip abduction - Gluteus minimus/ medius

• Hip adduction - Adductor magnus

• Spine extension - Erector spinae

• Crunch - Rectus abdominis

• Oblique crunch - External/ internal oblique

EQUATIONS TO ESTIMATE 1RM:
You'll get a more accurate estimation with less repetitions than with more repetitions.  However, between these two equations, you'll get a reasonable upper and lower theoretical 1RM value.

Equation 1

• 1RM = [(reps/30) + 1] * weight (lbs)

Brzycki Formula (2)

• 1RM = w / (1.0278 - (0.0278 * repetitions))

1RM Calculator - Link to Google Sheet

References:

1. Bloomfield, John, Timothy Robert Ackland, and Bruce C. Elliott. Applied anatomy and biomechanics in sport. New York: Blackwell Scientific Publications, 1994. Print.
2. Brzycki, Matt (1998). A Practical Approach To Strength Training. McGraw-Hill. ISBN 1-570-28018-5.
3. Plowman, Sharon A., and Denise L. Smith. Exercise physiology for health, fitness, and performance. 3rd ed. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins, 2011. Print.