This improved method of calibrating wood testing machines includes improved test bars, shim and software to direct the operator through the steps of calibration, storing interim results and calculating new calibration factors based on values read when the test bars are placed in the wood testing machine. This method avoids a number of problems in previous methods and apparatus. It properly corrects for changes in deflection in the measurement apparatus and changes in straightness of the test bars. The result is a more stable and reliable calibration that is referenced to precise measurement of the EI product for the test bar.