Measurement of a stress intensity factor using a compact sandwich specimen involves other material properties such as elastic moduli and Poisson’s ratios of interlayer and holder materials, and therefore it affects uncertainty. No attempt was made in the past to quantify the uncertainty due to the systematic effects involved in the measurement a of stress intensity factor (K) using a compact sandwich specimen and energy release rate (G) by the elastic compliance technique. Analysis of Type-B uncertainty gave comparative estimates of uncertainties in (1) Mode I stress intensity factor obtained from CT test, CS test, and CS test of negligible interlayer thickness (h→0), (2) Mode I energy release rate using elastic compliance (EC) technique, and (3) Indirect stress intensity factor obtained using the relation with energy release rate. Using a set of nominal values of cadaver bone tests of a specimen-size bovine femur and a set of instrument/sensor errors and specimen dimension tolerances, without the consideration of a series of experimental/statistical data, gave estimates of type B uncertainty. The analysis additionally gave percentage contributions of (1) relevant sources of uncertainty namely, caliper, tolerance of specimen dimensions, load cell and displacement sensor, and (2) individual parameters like load, specimen dimensions, and material constants included in the formulae, towards the uncertainty in fracture toughness measures. This study provided an understanding of the role of sensor, instrument, and specimen fabricating machine errors on the tests considered. Proportional variation in uncertainty of different measures of fracture toughness was verified for different sizes of specimens. This study showed that the selection of instruments/sensors of viable error levels would scale down the uncertainty of fracture toughness measures obtained by the CS test, and brought it very close to that of the CT test of the same specimen size, with a difference of less than 0.05 %.