A length smaller than the crown height is correlated with higher failure rates. Empirically, the post should be 2/3 of the root length leaving at least 4-5 mm of gutta purcha. However, removal of gutta purcha beyond 5-7 mm has been shown to significantly increase susceptibility to leakage. Furthermore, removal of gutta-percha to a level of 6 mm has been shown to lead to an unpredictable and significantly inferior seal compared with an intact root canal filling.