# Program Bugs

Rather than a feature, if you notice some behavior of the calculator that isn’t what you expect or values that you believe to be incorrect, make a note of such things by adding comments to this page. You’ll need to create an account for yourself (if you haven’t already).

## 27 Replies to “Program Bugs”

1. Hi, Earl. The calculator is behaving as it has been designed: the calculator looks for blank (or zero) fields to figure out what to calculate. So when you change a value in one of the fields, you need to blank out (or set to 0) the field that you wish to be re-calculated using the new values. By leaving only the Balloon Payment field blank, the calculator assumed that this is the value you wanted to calculate.

Bret

2. earlreb says:

Today (July 1, 2009) I used the calculator to calculate a monthly payment. Then I changed the amount to see what the payment would be on the different amount–but the calculator did not change the payment amount. It threw a huge unknown number into “balloon payment” where I had nothing before. I had to close the browser and reopen the calculator to enter the new amount. You should be able to change the amount and recalculate a new payment amount.
Earl

3. Andy says:

Sorry Bret, I made my second post before I saw yours. Thanks for your help

4. Andy says:

In further analysing this, I think I have answered my own question. The monthly payment is not sufficient to even fully cover the Interest amount and therefore \$0 can go to the principal reduction and so the principal increases each month by the shortfall.

5. With a 9.5% interest rate, the monthly interest payment is already over \$1329. If you don’t pay at least that amount, you’ll owe even more the following month. If you want to figure out how much you need to pay monthly to liquidate the debt after 15 years, leave the “Payment Amount” field blank, and let the calculator determine that for you.

6. Andy says:

I have entered the following numbers in the calculator

Principal: 167895.00
Payments per year: 12
Annual Interest rate: 9.5
Number of payments: 180
Payment Amount: 1200.00
and when I calculate it shows a Balloon payment of 220773.25

Is there something wrong or am I doing something wrong

7. Hi, Rob. More years than what? That extra payment per year that you’ve calculated has knocked off nearly 4.5 years from a 30-year mortgage. But note that this method may not accurately reflect the real payoff rate.

The calculator assumes that the payments are equally distributed throughout the year. If instead you make an additional payment at the end of the year, the 13-payments/year method will underestimate the amount of interest you will actually pay. To demonstrate, run the normal amortization assuming a 12-payment/year schedule, and look at the principal balance at the end of the first year. Now subtract your extra \$965 from that balance (the one extra payment at the end of the year). Now run the 13-payment/year schedule as you had done and look at the principal balance: it’s nearly \$22 less.

If the confusion is over the total number of payments, remember that 13 payments/year means there’s a total of 390 payments over 30 years in the schedule, not 360. So your 13-payments/year schedule eliminated 59 payments, not a mere 29.

I hope I didn’t confuse the issue for you. Let me know if you have any further questions.

8. robkluck says:

Hi Bret,
Great program. I punched in \$185,000, 4.75% interest, \$965.05 payment, 13 pmts/yr, and it said 331 payments (25.46 years). Seems like more years. Thanks. Rob

9. tzerrilla says:

Brett, I was trying to use the calculator for a \$ 30 million project, and had some trouble after defining the loan parameters.
Could you determine whether the calculator has an upper threshhold ? It appeared that I had trouble after posing a loan amount over