方程式如下,取自 http://www.vertex42.com/ExcelArticles/amortization-calculation.html
Amortization Calculation
Usually, whether you can afford a loan depends on whether you can afford the periodic payment (commonly a monthly payment period). So, the most important amortization formula is probably the calculation of the payment amount per period. Calculating the Payment Amount per PeriodThe formula for calculating the payment amount is shown below. 
where
- A = payment Amount per period
- P = initial Principal (loan amount)
- r = interest rate per period
- n = total number of payments or periods
Example: What would the monthly payment be on a 5-year, $20,000 car loan with a nominal 7.5% annual interest rate? We'll assume that the original price was $21,000 and that you've made a $1,000 down payment. You can use the amortization calculator below to determine that the Payment Amount (A) is $400.76 per month. P = $20,000
r = 7.5% per year / 12 months = 0.625% per period
n = 5 years * 12 months = 60 total periods
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