Simple, Free Way To Calculate Interest To My Friend

Factual Questions
1

Okay.
I borrowed $6000 from a friend for a business investment, and have been paying him back for a few years, normally 75 per week. I agreed to pay him 12% interest. Now that I'm getting to the point where I've paid off the initial loan but will still owe him the interest. I tried using Wolframalpha to calculate the interest based on my payments and the balances, but I am getting wildly inconsistent output... the balance will go down, and the interest amount will go up.
Is there something simple and free to calculate how much I owe this guy?

2

I don’t imagine there’s much online if you varied your payments month to month. Making a spreadsheet should only take a couple minutes.

3

How about this?

Using a fixed payment of $300/month, it comes to $6727.72 over 22.43 months.

4

It sounds like you may be using some unconventional ways to calculate interest versus principal as well but I am not sure. You normally calculate this using an amortization schedule in which interest gets compounded on some schedule and any payments are subtracted from the principal balance plus the accrued interest. Here is a sample if your interest compounded weekly and you made a weekly payment of $75. I built that in Excel in about a minute and you can set it up to handle payments of different amounts or at different intervals if you want.

Balance Weekly Interest Payment
$6,000.00 $13.85 $75.00
$5,938.85 $13.71 $75.00
$5,877.55 $13.56 $75.00
$5,816.11 $13.42 $75.00
$5,754.54 $13.28 $75.00
$5,692.82 $13.14 $75.00
$5,630.95 $12.99 $75.00
$5,568.95 $12.85 $75.00
$5,506.80 $12.71 $75.00
$5,444.51 $12.56 $75.00
$5,382.07 $12.42 $75.00
$5,319.49 $12.28 $75.00
$5,256.77 $12.13 $75.00
$5,193.90 $11.99 $75.00
$5,130.88 $11.84 $75.00
$5,067.73 $11.69 $75.00
$5,004.42 $11.55 $75.00

5

Here’s the one I use:

[Here’s the one I use - http://www1.aoc.state.nc.us/jpc/calculator.do](Here’s the one I use - http://www1.aoc.state.nc.us/jpc/calculator.do)

6

It depends on how often the interest compounds.

Assuming that the 12% represents the interest over one year and that you’re compounding monthly, your effective monthly rate of interest would be

0.9489%

Increasing the balance of your debt by that amount each month would add up to a 12% increase over a year.

To find out what you owe now, set up a spreadsheet. Each month, multiply the previous balance by 1.9489 and subtract your payments ($300 if you paid every week).

If you made every payment on time you should still owe him about $675 after paying off the principle.

7

If he paid $75.00 per week, then assuming $300 per month would not be accurate. There are, on average, 4.333 weeks per month (52 weeks / 12 months). The more accurate monthly principle reduction @ $75 per week would be $325.

8

Thanks guys!
Both Sateryn and Shagnasty gave me great solutions!