Since I couldn't easily get my unpaid principal balance, I calculated my mortgage balance using a handful of online calculators. Today I took a look at the actual balance, and was rather surprised to see it's off by $1,000 from what online calculators showed.

I've been poking around online to see why the calculators may be inaccurate, or why my balance my be inaccurate. There's some interesting mentions of JavaScript calculators using floating point numbers, and a few mentions of various rounding issues, but I'm having a hard time chalking up an off-by-1000 error to rounding (especially since the balance started under $90,000, six years ago).

Before I go yell at my bank, I'd like to know if there's a surefire way to know the proper balance. Aside from the original balance, the interest rate, what I've been paying, and how long I've been paying it, is there any other information I need to accurately calculate my mortgage? Is there a reliable way I can calculate my balance without relying on questionable online calculators?

  • 4
    Ask your bank to print out a mortage payment schedule since you started with them. It should detail how much of each payment went towards interest vs. principal. Ask them to explain the calculations. Mar 19, 2014 at 3:01
  • Please edit the last sentence of your question and remove the words in <strike>strike-out</strike> style. Mar 19, 2014 at 3:03
  • @DilipSarwate Feel free to
    – rtf
    Mar 19, 2014 at 3:12
  • @DilipSarwate "This question does not show any research effort; it is unclear or not useful". If something offends you, or you find it inappropriate, feel free to flag or edit; you have the required reputation.
    – rtf
    Mar 19, 2014 at 3:30
  • I have removed my down-vote since your question now reads less like a rant and more like something that might be useful. Mar 19, 2014 at 10:55

1 Answer 1


Among possible sources of error:

What is the nominal interest rate and compounding period?

What is the effective interest rate per payment period?

Does the lender take into account the different number of days in the months, in amortizing a monthly payment mortgage?

Has the lender added any fees or costs to the principal amount? Some lenders insist on paying property taxes (since the city gets first crack at the property ahead of any lender), or fire insurance premiums (since it's their collateral that could burn down)

  • The OP already marked this as accepted though it looks like they were trying to get a formula or the method to get to the mortgage from the APR/Interest rate. Is there no standard formula that banks use? May 21, 2019 at 22:30

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