This method puts a decreasing weight on interest and an increasing weight on principal across the repayment periods. The number 78 comes from adding up the number of months of a 12-month-loan:
1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 + 11 + 12 = 78
The interest is allocated to the repayment period in a reverse portion, such that 12/78 (first interest portion) of the total interest will be applied to the first month, 11/78 (second interest portion) of the total interest will be applied to the second month, etc.
Illustration of showing the proportion of interest and principal in each monthly repayment using rule of 78:
A customer borrows HK$120,000 for 12 months, at a monthly flat rate of 0.14% and handling fee is waived. As such, the total loan principal is HK$120,000
Monthly interest = HK$120,000 x 0.14% = HK$168
Total interest for the full term = HK$168 x 12 months = HK$2,016
Monthly repayment = HK$(120,000 + 2,016) ÷ 12 = HK$10,168