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This question is for those who know how to produce a amortization loan scehedule in code. If you know about validating code with excel that would be great, also.

I have three parameters: principal (original balance), interest rate, and terms (in months). I want to output the monthly payment. For that monthly payment, I can code it.

For example. P = 500, i = (8.25/100)/12, and n = 12 (months).

monthlypayment = P [(i(1 + i)^n / ((1 + i)^n) - 1]

43.55 = monthlypayment.

However, I want to produce x = 1...12, and y = remaining principal until it equals 0 and cumulative interest rate.

The values of x and ya and yb, should be something like.

Month BeginningBalance Payment Principal Interest CumulativePrincipal CumulativeInterest EndingBalance

1 $500.00 $43.55 $40.11 $3.44 $40.11 $3.44

2 $459.89 $43.55 $40.39 $3.16 $80.50 $6.60

3 $419.50 $43.55 $40.67 $2.88 $121.17 $9.48

4 $378.83 $43.55 $40.95 $2.60 $162.12 $12.08

5 $337.88 $43.55 $41.23 $2.32 $203.35 $14.40

6 $296.65 $43.55 $41.51 $2.04 $244.86 $16.44

7 $255.14 $43.55 $41.80 $1.75 $286.66 $18.19

8 $213.34 $43.55 $42.08 $1.47 $328.74 $19.66

9 $171.26 $43.55 $42.37 $1.18 $371.11 $20.84

10 $128.89 $43.55 $42.66 $0.89 $413.77 $21.73

11 $86.23 $43.55 $42.96 $0.59 $456.73 $22.32

12 $43.27 $43.55 $43.25 $0.30 $499.98 $22.62

I don't the other columns just the current balance and cumulative interest, where x = numberOfMonths, y(a) = remaining principal and y(b) = cumulative interest.

I can program in Java/C++ but a pseduo-code would be fine.

I have three parameters: principal (original balance), interest rate, and terms (in months). I want to output the monthly payment. For that monthly payment, I can code it.

For example. P = 500, i = (8.25/100)/12, and n = 12 (months).

monthlypayment = P [(i(1 + i)^n / ((1 + i)^n) - 1]

43.55 = monthlypayment.

However, I want to produce x = 1...12, and y = remaining principal until it equals 0 and cumulative interest rate.

The values of x and ya and yb, should be something like.

Month BeginningBalance Payment Principal Interest CumulativePrincipal CumulativeInterest EndingBalance

1 $500.00 $43.55 $40.11 $3.44 $40.11 $3.44

2 $459.89 $43.55 $40.39 $3.16 $80.50 $6.60

3 $419.50 $43.55 $40.67 $2.88 $121.17 $9.48

4 $378.83 $43.55 $40.95 $2.60 $162.12 $12.08

5 $337.88 $43.55 $41.23 $2.32 $203.35 $14.40

6 $296.65 $43.55 $41.51 $2.04 $244.86 $16.44

7 $255.14 $43.55 $41.80 $1.75 $286.66 $18.19

8 $213.34 $43.55 $42.08 $1.47 $328.74 $19.66

9 $171.26 $43.55 $42.37 $1.18 $371.11 $20.84

10 $128.89 $43.55 $42.66 $0.89 $413.77 $21.73

11 $86.23 $43.55 $42.96 $0.59 $456.73 $22.32

12 $43.27 $43.55 $43.25 $0.30 $499.98 $22.62

I don't the other columns just the current balance and cumulative interest, where x = numberOfMonths, y(a) = remaining principal and y(b) = cumulative interest.

I can program in Java/C++ but a pseduo-code would be fine.

2 Solutions

Formula for periodic payment calculations:

PeriodInterestRate

Payment = Principal * --------------------------

( 1 )

(1 - --------------------------

( (1 + PeriodInterestRate)^Period

If you are looking at getting balances from each payment, do the following

Multiply the principle times the annual interest rate divided by 12 (assuming monthly payments) , - this gives you the amount going to interest for that payment. Subract that amount from the payment (calculated above). This gives you the amount of principal reduced in that payment.

do this interatively, for each month, reduce the principal by the amount reduced in the previous month, recalculate the interest for that month (should be less than the month before) and then cacluate the remainder as principal. This loop will iterate until the total number of periods has been calculated. If the calculation is correct, the final principal will be 0.

Hope this helps!

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Cheers

Dave