Force readings are converted to pressure readings by dividing the force reading by the specimen cross sectional area. For example if your peak force reading is 100 pounds and the measured cross section of the specimen is 0.50 inches, peak pressure is 100 / 0.05 or 200 pounds per square inch(PSI). If you are using other force or deflection units, the formula still applies but the result units will vary accordingly. Kg/mm. Nt/meter ect.