信用卡與樓宇按揭利息及本利和的計算方法

if P is the loan at time 0,
if i is the effective interest rate per payment period,
if n is the total payment period no.
if payment starts at the end of the first payment period (but not at the beginning)

then,

monthly payment = P / {[1- (1+i)^(-n)]/i}
interest payment at that period is the outstanding loan x i
repayment of loan (本金) = monthly payment - interest payment

The outstanding load reduces as time passes. As a result, your interest payment reduces also.

e.g.

P = 1,000,000
i = 5% per year = (1+5%)^(1/12)-1 = 0.4074124% effectively per month
n = 1 year = 12 periods if 1 period = 1 month
monthly payment = 85,556.60
outstanding loan by the end of first month = 1,000,000
interest payment at first month= 1,000,00 x 0.4074124% = 4,074.12
repayment of loan at the end of first month = 85,556.60 - 4,074.12 = 81,482.48
so, outstanding load by the end of second month = 1,000,000 - 81,482.48 = 918,517.52
interest payment at second month = 3,742.15
repayment of loan = 85,556.60 - 3,742.15 = 81,814.45
and so on ... ...

You can produce the amortisation schedule with an excel very easily, as what bank also does.

因你個個月都有還本金