(SR-52) Easy Mortgage Math

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An excerpt from Electronics Today international, Vol. 2 No. 9, September 1978, pgs, 64-65, 67

"  Anyone who has attempted to use a regular annuities program to calculate house mortgages, may well have been disappointed to discover errors in the results. An error of even a few cents in the monthly payment can make a large difference over the time period of a normal mortgage. The normal annuity program assumes that the compounding period and payment period are the same, whereas a mortgage is usually compounded semi-annually, but paid monthly. Another difference that can occur arises because the "professional" computer print-out rounds all dollar and cent figures to two decimal places. The program outlined here, takes account of both factors and produces correctly the total interest, monthly payment and outstanding debt after n payment's. It does not, of course, include property tax and insurance which are often included in the monthly payment. After using such a program you may well decide to renegotiate your mortgage. I did! Tables 1 and 2 show the status for the first four months of a $23,000 Mortgage at 10.5% per annum. By just over doubling the monthly payment, the time is reduced to one quarter and there is a saving of $25,000 in interest.
  The program was written for a Texas Instrument SR-52 … "

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