Spreadsheet to Calculate ROI on Principal Pre-Payment (monthly)

[Joel Trowbridge]

Hello everyone. This is my first post on here! Forgive me if I sound misinformed throughout because I likely am. I know there are plenty of free spreadsheet templates out there for mortgage calculators. I have several, but what I'm trying to find is a template that can calculate your monthly ROI on any extra cash you put towards the principal on the loan.

I've seen several that will show your total interest saved after summing all extra principal payments together, but I'd like it broken down by month. The ROI percentage isn't even that big of a deal, I just want to see the dollar amount saved each month.

Has anyone come across (or created) something like this? I've only seen one out there (from Vertex42) that includes interest saved but if I'm honest, I don't understand how they come up with the calculations for the interest savings. I tried plugging in an extra $100 payment per month in it, but the interest savings start out at like only a few dollars each month.

Shouldn't the interest saved on extra principal payments be drastically higher in the beginning due to the front-loaded nature of amortized loans? Anyway, I'm hoping someone can point me in the right direction, or let me know if this is just something I'll need to calculate in a more manual way.