Show HN: Mortgage Calculator: 30-year vs. 5/1 ARM (Monte Carlo, Brownian Motion)

Since the payments on a 5/1 ARM vary with interest rates, I used Monte Carlo simulation (with Brownian Motion) to generate possible payment paths. Then I took the 75th-percentile of 100 possible ARM paths (yep, this is a small sample size!), diffed it with the 30-year fixed payments and discounted everything back to the present day to generate a (for entertainment purposes only) recommendation.

The state of ARM calculators is bad. Check out the products offered by Zillow, Bank Rate or Nerd Wallet and you'll see 1) they don't really exist and 2) they might have some weird assumptions like interest rates staying fixed or only increasing. This isn't reality.

Moreover, comparing two mortgages (e.g. 30-year fixed vs. 5/1 ARM or 30-year fixed from one lender vs. 30-year fixed from another lender) is sorcery for most people. This stuff lives in Excel sheets on your banker's desktop, but with some effort, I could make make the "which type of mortgage and which lender?" question more transparent and actionable.

In any case, this calculator is a simple Google sheet. It doesn't really work on mobile: sorry. Also, you can break it with little effort. Please don't, it's just a headache for anyone else playing with it. Also, yes, this sheet is multi-tenant. Play with it directly or download a copy. Be sure to hover over the cells to see notes about relevant stuff.

I'm curious who's interested: brntsllvn@gmail.com or linkedin.com/in/brent-sullivan-350230209

Some background on Monte Carlo and Brownian Motion: I'm convinced Monte Carlo is the right approach, but we might debate Brownian Motion as the right model. The reason I chose Brownian Motion is that the daily difference between interest rates is mostly Gaussian. Tweak the params and see what happens.