Month: June 2010

Over the years, a lot of people have asked for calculator features like occasional extra principal reduction payments, tracking fees or late charges, summary of interest over a fiscal year. My pat response is “this kind of thing needs a spreadsheet or special-purpose software.”

Well, I’ve taken a little time to work up a basic amortization spreadsheet. It doesn’t have all the calculation options that the web calculator provides (it only calculates the payment, for example), but the amortization schedule it produces will allow one to track extra payments and fees, to include payments for insurance and taxes, and it provides a nice fiscal year summary of interest paid.

It took a few days of playing around with Excel to get it working mostly right, and it’s free for you to download and try, if you’re into that kind of thing. Maybe it will give you some ideas about how to build your own custom spreadsheet. But of course, use at your own risk! Please don’t base any high-finance decisions on the spreadsheet’s results until you’ve verified that it’s doing everything correctly. Like the web calculator, I consider this a planning tool only!

If you want to give it a shot, here’s a link to the spreadsheet. Enjoy!

I’ve made a few more improvements to the calculator today. Since I added some comma separators to the output to improve the readability of larger numbers, I thought I should do something about the upper bound of the calculator. Before today, the calculator couldn’t amortize amounts larger than about 21.4million. This upper bound was imposed by the computer’s native word size and how I chose to round values to the nearest whole penny. By changing the rounding procedure, I have boosted that upper bound to under 10billion. Above this value, the calculator will silently start to lose resolution at the low end, but amortizing values in the 100s of millions should be possible now.

I just can’t help myself: when I start to tweak things, I start coming up with more ideas for making things a little better or easier. Nothing dramatic, just little things.

Today’s idea was to make it easy to identify that point in the amortization schedule when the principal component of a payment first exceeds the interest component of a payment. Let’s call that the cross-over point. If you run an amortization schedule now, the cross-over point is highlighted in green. There may be schedules without a cross-over point (usually shorter term loans), in which case there’s no special highlighting.

Who knows what might come next, now that I’ve started tweaking. 🙂

It’s been a while since I’ve made any improvements to the calculator, but today I finished a few cosmetic upgrades. The Summary section has some nicer layout and formatting, and the addition of comma-separators should make larger numbers easier to read.