The following chart is a working spreadsheet that will allow you to determine the pressure drop through an orifice, both in metric and in US Customary units.
Simply select the Metric or US Customary chart. Then select the item in the left chart column that you want to analyze. Then move across that row and change any of the colored cells to enter your number. The chart will calculate the results in the non-colored cells. The non-colored cells contain formulas and so they are locked to prevent accidental changes.