Enter "Weight Lifted" and "Reps Performed". The repetitions must be between 1 and 10. Press "Calculate" for the weight you may be able to perform for a single repetition.

Brzycki, M. Strength testing: Predicting a one-rep max from a reps-to-fatigue
Journal of Physical Education, Recreation and Dance 64 (1): 88-90, 1993.