The optimised limit for polarimetric calibration of fully polarised synthetic aperture radar systems is derived by establishing an error model as a function of cross-talk, channel imbalance and system noise. Compared with noise equivalent sigma zero, the polarimetric error below the optimised limit is too small to affect the signal of cross-polarised channel. Thus, polarimetric calibration could be relaxed or even ignored in this case. With the backscatter model, optimised limits for cross-talk and channel imbalance at X, C and L-bands are presented. Moreover, when ignoring channel imbalance, the limit for cross-talk is given in a quantitative way. These results are very useful in practice, allowing significant reduction in calibration cost.