First off, I apologize for adding fuel to the mortgage payoff debate fire. I also apologize for reposting from the "stop worrying..." thread, but I added a table of results that I'm very proud of.
I'm not going to try to answer the question whether you should pay off your mortgage or invest, I'm just going to try to answer the question of how much you need invested to retire with a mortgage vs without a mortgage, and the relative risk of the two options. Early Retirement Now wrote an
essay on the subject but I don't agree with this methodology of assuming fixed 2% inflation through all historical cohorts.
Thought experiment: you have a $1M, 30 year mortgage and can live off $40k per year. What size portfolio will allow you to retire?
Simple answer: $2M. On your first day of retirement, pay off the mortgage (ignoring taxes). Now you have $1M left, and you decide to use a 4% withdrawal rate. You withdraw $40k per annum and live happily ever after.
More complicated: slowly pay down my mortgage with half the portfolio. Let's model how much is needed to cover the mortgage. The mortgage payment is obviously not CPI adjusted. Taxes and insurance have to be paid separately whether the loan is paid off or not, so ignored in this analysis.
Let's pick a 70/30 portfolio with annual fees of 0.04%. The "4% Rule" fails 5/125 cycles, or 4% of the time, so that's our baseline for "safe". A hypothetical $1M mortgage at 4% costs $57k annually in principle and interest, giving a 5.7% withdrawal rate. Plug a 5.7% withdrawal rate into cFireSIM and select "not CPI Adjusted". Failure rate: 3/125. So a 4% mortgage funded by a portfolio is safer than the "4% Rule".
A 3% mortgage is a 5.5% withdrawal rate. This fails 2/125 cycles. In fact, to match the failure rate of the "4% Rule" only $875k invested is needed to cover a $1M mortgage!
A 5% mortgage is a 6.4% withdrawal, which fails 15/125 cycles. This is much riskier than commonly accepted.
In tabular form:
Mortgage Rate | | Withdrawal Rate | | Failure Rate | |
2% | 4.40% | 0.00% |
3% | 5.06% | 1.60% |
4% | 5.70% | 2.40% |
5% | 6.40% | 15.20% |
The "Mortgage Rate" is a 30 year fixed mortgage rate. The "Withdrawal Rate" is the annual mortgage payment, excluding taxes and insurance, divided by the size of the portfolio. The "Failure Rate" is how often, historically speaking, the portfolio would have been depleted before the loan was paid off.
Of course, the more conservative you are with your withdrawal rate, the less compelling the mortgage becomes. Luckily it's easy to repeat this analysis with shorter mortgage terms, different withdrawal rates, etc.