I have building with 16 showers , 6 toilets, 8 sinks and 6 urinals what size of water pipe should be for the supply?
Usually a fixture will receive a 1/2 inch supply line; one for hot and one for cold.
Based on "16 showers, 6 toilets, 8 sinks, and 6 urinals" we would want 2 lines to each shower and sink:
(16 * 2) + 6 + (8 * 2) + 6 = 60
pie * radius squared formula, we know that a half inch diameter circle has an area of 0.2 sq. inches (0.19635 if you want to be exact).
Now multiply by the number of half inch lines you need to run
0.2 * 60 = 12
Now that we know our supply pipe needs to have an area of 12 sq. inches we can work backwards:
( ( square root of ( 12 divided by pie ) ) and double the result ) to get a diameter of 4 inches.
If you think that only half of the fixtures will be used at any given time then we cut our square inches (12) in half to 6 and work backwards to give us a 2.75 inch pipe.