[GNC] Mortgage & Loan Repayment Assistant woes

Derek Atkins derek at ihtfp.com
Thu Sep 16 16:54:39 EDT 2021


On Thu, September 16, 2021 4:22 pm, Chris Mitchell wrote:
> On Thu, 16 Sep 2021 10:39:10 -0700
> Jean Laroche <ripngo at gmail.com> wrote:
>> The fact that the interest amount is twice as much seems to indicate
>> to me that GC does not do the proper calculation given your
>> every-two-week frequency.
>> That would happen I think if GC thought you're paying your mortgage
>> every month instead of every two-weeks.
>> Could you try doing the same with a monthly schedule, instead of
>> two-week schedule and see if what you get seems right?
>> That would help debug the issue.
>> Jean
> My Gnucash version is 4.4 (more specifically 4.4+(2020-12-28) running
> on Debian); thanks for reminding me to mention that, Stan!
> Jean, I tried setting the payment frequency to monthly, leaving
> everything else exactly as it was. All of the payment amounts remained
> exactly the same; only the dates changed.
> The formula showing for "Amount" is exactly the same as before.
> Assuming that my hypothetical monthly total payment, interest portion,
> and principal portion should all be 26/12 × my actual biweekly amounts:
> * The total payment amount is roughly 1.75× what I would expect;
> * The interest portion is about what I would expect;
> * The principal amount is thus way too high;
> Let me know if there's any more info I can provide.

The inputs into the pmt() formula are:
starting balance
ending balance
interest rate *
number of periods

Normally, for a monthly, 30-year mortgage with a fixed rate of 5%, you
would enter an interest rate of .05/12 and 360 periods.  If you're doing
semi-monthly payments this would be 0.05/24 for interest and 720 periods. 
If you're doing bi-weekly, it gets a little more complicated.  I would
START by saying 0.05/26 and 780 periods.

The druid is kinda dumb in that it doesn't equate number of periods to
dates.  So just changing the dates without changing the PMT() entries will
keep the same amounts but just change... you know.. the dates!

Hope this helps get you pointed in the right direction.

> Cheers!
>  -Chris

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       Derek Atkins                 617-623-3745
       derek at ihtfp.com             www.ihtfp.com
       Computer and Internet Security Consultant

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