http://www.bodybuilding.com/fun/1rm.htm

Some calculator I found at bb.com

However, it's impossible to guess your 1rm without taking into consideration other stresses that may effect your 1rm.

Do what andrew.cook said and "simply get your butt under a bar".

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Deadlift - 405X1
Squat - 325X1
Bench - 225X1

Golf Tee - Anyone who can bench more than they can squat and/or deadlift.