This function converts any quantity in many different volume measurements into US gallons. You can use this as a template for any conversion function just substitute the names and conversion factor.
ToUSLiquidGallon (quantity;unit)
Case (
unit = "Quart" ; quantity * .25;
unit = "Pint" ; quantity * .125;
unit = "UKGallon" ; quantity * 1.200949925;
unit = "UKPint" ; quantity * .150118741;
unit = "UKQuart" ; quantity * .300237481;
unit = "Litre" ; quantity * .264172052;
unit = "BarrelOil" ; quantity * 42;
unit = "Bushel" ; quantity * 9.309177482;
unit = "CubicFeet" ; quantity * 7.480519478;
unit = "CubicMeter" ; quantity * 264.17205;
unit = "USDry" ; quantity * 1.163647185;
unit = "Gill" ; quantity * .03125;
unit = "Firkin" ; quantity * 9;
quantity
)