Inches of water to pounds per square inch conversion calculator shown above calculates how many pounds per square inch are in 'X' inches of water (where 'X' is the number of inches of water to convert to pounds per square inch). In order to convert a value from inches of water to pounds per square inch (from inH2O to psi) just type the number of inH2O to be converted to psi and then click on the 'convert' button.