Checksum
The checksum letter is calculated by converting the letters into numbers, i.e. where A=1 and Z=26, potentially giving 7 individual numbers from each registration plate. However, only two letters of the prefix are used in the checksum, for now, so the first letter is discarded for a three-letter prefix. SBA 1234 would therefore give 2, 1, 1, 2, 3 and 4 (note that "S" is discarded). For three-digit numbers, a "0" is appended to the front of the number. Each individual number is then multiplied by 6 fixed numbers (14, 2, 12, 2, 11, 1). These are added up, then divided by 19. The remainder corresponds to one of the 19 letters used (A, Y, U, S, P, L, J, G, D, B, Z, X, T, R, M, K, H, E, C), with "A" corresponding to a remainder of 0. In the case of SBA 1234, the final letter should be a G.
Read more about this topic: Vehicle Registration Plates Of Singapore