This tool compares the long-term financial outcomes of buying a home versus renting. It projects your net worth over 30 years in both scenarios by considering factors like home appreciation, rent increases, and investments.
The calculator invests any difference in the monthly payments between renting and buying in to the stock market. So, if your rent is $1500 per month, and your total mortgage/insurance/taxes are $2000 per month, the rental scenario invests $500 per month into stocks. If you increase your rent to $2500 per month, that extra $500 now is invested in the buying scenario instead (the idea being, you indicated that you are able to afford a total $2500 per month outlay).
This one is better in some ways and worse in others. The main feature here is the ability to add refinancing and moves to a more expensive apartment, instead of a single point-in-time comparison. However, this calculator is missing some other (perhaps minor) components, such as taking into the mortgage interest tax deduction that the New York Times calculator does.
This calculator uses current dollars for everything - no assumptions are made about inflation. So the returns from the stock market and home price/rental price growth should use pre-inflation numbers. For instance, you might typically assume that the stock market averages a real return of 7%, with average inflation 3% and 10% average unadjusted returns. For this calculator, you should use the 10% returns as your assumption, and then interpret the outputs in current dollars.
Awesome! Please head to the GitHub repo and open a pull request.