A new vector analytical method has been developed in order to obtain the true isotopic composition of the (42)Ca-(48)Ca double spike. This is achieved by using two different sample-spike mixtures combined with the double spike and natural Ca data. Because the natural sample (two mixtures) and the spike should all lie on a single mixing line, we are able to constrain the true isotopic composition of our double spike using this new approach. Once the isotopic composition of the Ca double spike is established, we are able to obtain the true Ca isotopic composition of the NIST Ca standard SRM915a, (40)Ca/(44)Ca = 46.537 +/- 2 (2 sigma(m), n = 55), (42)Ca/(44)Ca = 0.31031 +/- 1, (43)Ca/(44)Ca = 0.06474 +/- 1, and (48)Ca/(44)Ca = 0.08956 +/- 1. Despite an offset of 1.3% in (40)Ca/(44)Ca between our result and the previously reported value (Russell et al. 1978), Our data indicate all offset of 1.89 parts per thousand in (40)Ca/(44)Ca between SRM915a and seawater, entirely consistent with the published results.