OK Here we go. I used this mortgage calculator
Mortgage Calculator. As a reference and put 0% for taxes and 0% for PMI because we are only concerned in the difference of interest paid vs stock market gains.
Since I've been mortgage shopping I used my own mortgage as the example:
218,000 loan amount, no PMI (never get a mortgage with PMI, it's throwing money away for no good reason)
The following makes some assumptions:
1) You're in your "forever" house, meaning you don't plan on moving 5 years after you buy the place (that's an entirely different discussion)
2) You can actually afford the house, therefore you could afford to choose between a 30 year fixed and a 10 year fixed
4% was the lowest interest rate I was quoted in recent weeks for a 30 year fixed at my loan to mortgage value which is at roughly 75%.
3.25% is what I actually locked in at for the 10 year fixed I will be switching to at that loan to mortgage value
30 year fixed at 4% gives a monthly payment of $1040.77. This is just principle and interest.
10 year fixed at 3.25% gives a monthly payment of 2130.27. Again, just principle and interest.
That's a difference of 1089.50 a month or for simplicity sake $13074 a year.
Investing that difference of $13074 (instead of paying down your mortgage in 10 years) and getting 7% gains over the course of that 10 years gets you $62540.62 in investment profit. ($13074 the first year gains $915.18, add that together and add another $13074 for the next year and you gain 1894.42 in year 2, etc). Now that's at a VERY good 7% return rate year in and year out for 10 years. That is much better than your average investor.
Source:
Why The Average Investor's Investment Return Is So Low - Forbes
Meanwhile, you've spent $78640.86 in interest over that same 10 year period on your 30 year loan. So you've spent $16100.24 more in interest than you've gained in investments had you invested that extra monthly income rather than paying down your mortgage. Not only that but you'll have to pay taxes on your investment gains (though if you're investing in index funds you'll be earning some dividends on those investments as well). And that's if you beat average rate of return on investments by almost 5% per year.