Calculating Geometry Measurements in Python
All below from Esri:
Python expressions can use the geometry Area and Length properties with an areal or linear unit to convert the value to a different unit of measure.
!shape.area@acres!Areal unit of measure keywords: Unit of measure keywords: ACRES | ARES | HECTARES | SQUARECENTIMETERS | SQUAREDECIMETERS | SQUAREINCHES | SQUAREFEET | SQUAREKILOMETERS | SQUAREMETERS | SQUAREMILES | SQUAREMILLIMETERS | SQUAREYARDS | SQUAREMAPUNITSLinear unit of measure keywords: CENTIMETERS | DECIMALDEGREES | DECIMETERS | FEET | INCHES | KILOMETERS | METERS | MILES | MILLIMETERS | NAUTICALMILES | POINTS | YARDS