Door Closers are rated by opening/closing force "Size". The Size ratings are from #1 to #6, with Size #1 the weakest strength and Size #6 the strongest.
Size 1 - 2 lbs. minimum closing force
Size 2 - 3 lbs. minimum closing force
Size 3 - 5 lbs. minimum closing force
Size 4 - 8 lbs. minimum closing force
Size 5 - 11 lbs. minimum closing force
Size 6 - 14 lbs. minimum closing force