It's not all that hard. But to make it easier on op, I've created a sheet:

http://www.filedropper.com/insterest...ularpayments_1 <-- wrong, this is better
http://www.filedropper.com/insterest...mentscorrected

The basics are calculating interest based on
(previous balance) * 1.1 ^ ((days passed since last change)/365) - (previous balance)
for each interest accrual during that period.

Add up those and you're there.

Just check out the sheet.

Yeah, it's not *that* hard--I gave him another way to calculate it as well above.

And I made a nice mistake in there, too.
So much for not being *that* hard.

Here's a corrected version:
http://www.filedropper.com/insterest...mentscorrected

Yea I know that. What I meant was for calculating the effect of additional prepayments on that kind of loan. It's very simple. I learned this in my first finance class, so I'm just surprised something like that isn't around. But I assume that kazana just showed how easy it is anyway.

If there was EVER a time to set up a reasonable payment plan this is it. Ask the guy how much he can afford to pay you a month. Then take that figure and work out when he'll have paid you back in full. Make sure that you're not making his life difficult by asking for more than he can easily afford to give you.

You really need to be careful here for obvious reasons.

EDIT: If you can get him into a mindset of you being just another bill he has to pay every month you stand a much greater chance of getting your money back than if you pressure him at all to get your money back.

Amount you should expect by July 30, 2012 is
$9000*1.10*0

thx to all for the advice & especially Kazana for the calculator.

Again, I'll be satisfied if I just receive the full $15,000 back. Per everyone's advice I'll leave the part about the lawyer/collection agency out, and am considering adding language to the effect of "if you pay me the outstanding balance in 30 days I'll forgive interest owed.

I'm sure this guy has some cash-flow problems, but I "think" that if push came to shove &/or if he really wanted to, he could come up with the money to pay me. I think he hasn't so far simply because I've made it too easy for him, i.e., all I've really done is send him the occasional text message or called him every 4-6 weeks asking when he was going to drop off another payment.

Again, if I end up having to chalk this one up to a $9,000 lesson, oh well, perhaps it will have served to protect me from a larger, more painful (i.e., expensive) mistake in the future. From now on, when people want to borrow money, I will do so only if they give me collateral worth more than the loan. Otherwise my pre-prepared stock answer will be "I'm sorry, but I'm not in a position to loan money at the moment."

gl, OP.

Would not add in any language to the letter that does not need to be there. A simple letter with as little as possible is best in this situation. Just something along the lines of "Per the promissory note executed on (date), please consider this letter notice of demand requiring payment in full of outstanding principal and interest no later than (date)". Have it notarized and preferably mailed by a third party (your attorney, if you have one) via a trackable mailing service, preferably one requiring signature. You can also have it delivered by a courier, requiring a signature for delivery.

The delivery method will get your message across without any additional carrots or sticks.