Ask most homeowners what their down payment cost them and they will say "$80,000, twelve years ago." Ask them what that $80,000 would be worth today if they had never bought the house, and the answer gets quiet.
That silence is the whole point of this article. The down payment is not a check you already wrote; it is capital that could have been earning a return for you the entire time you have been paying property tax. Comparing rent to buy without accounting for it is like comparing two jobs by looking only at commute times.
What "opportunity cost" actually means
Money invested in a house is not invested somewhere else. That is the entire idea. In the rent scenario, your $80,000 sits in an index fund and grows. In the buy scenario, it sits in the walls of your kitchen and does not.
Your house may appreciate. That is not the same as growing — your down payment might grow at 3 percent per year in a house that appreciates at 3 percent per year, but so would the whole house, and the whole house is not yours until you sell (and pay 6 percent to the agents to do it). The invested version has no such friction.
The honest way to model this is to give the down payment two possible futures — one in a house, one in a diversified portfolio — and let them race.
What number should the invested version grow at
This is the input that most calculators cheat on. Some use 1 percent, which is a rounding error on a savings account and makes buying always win. Some use 10 percent, which is the nominal U.S. large-cap return and makes renting always win. Neither is the honest number.
The honest number is a real (inflation-adjusted) expected return on a diversified global portfolio. Historically that is somewhere around 4 to 6 percent per year, depending on your allocation between stocks and bonds. A useful default is 5 percent real — matching the number you would see in any credible retirement planner.
You can move it up if you would hold 100 percent equities. You can move it down if you would hold cash. But you cannot move it to zero without conceding you would set the money on fire.
The calculator lets you set this input directly. It is the second most sensitive input in the whole model, after "how long you stay."
The compounding, in dollars
Here is what $80,000 becomes at 5 percent real, over the horizons that matter for a rent-vs-buy decision:
- 5 years: $102,000 — the invested version is $22,000 ahead.
- 7 years: $113,000 — the invested version is $33,000 ahead.
- 10 years: $130,000 — the invested version is $50,000 ahead.
- 15 years: $166,000 — the invested version is $86,000 ahead.
- 20 years: $212,000 — the invested version is $132,000 ahead.
That $132,000 at year 20 is the number the house has to beat, on top of covering all the other costs of owning it, before buying is the winning move.
It is also the number that turns "the mortgage is basically the same as rent" into "the mortgage is basically the same as rent plus a portfolio I quietly gave up."
But the mortgage is forced savings
This is the honest counterargument, and it deserves an honest answer. Yes, a mortgage principal payment forces you to save every month. Yes, most people who rent do not automatically invest the difference. Yes, "the math works out if you invest the difference" is doing a lot of work in the rent-side argument.
Two responses.
First: the calculator does not assume you invest the difference. It only asks about the down payment. You can invest the down payment once and never think about it again — that requires no discipline. The forced-savings argument is about the monthly cash flow, which is a smaller pool of money.
Second: if the counterargument is "I need my house to force me to save," it is worth asking whether that is an argument for the house, or an argument for a savings automation. Setting up an automatic transfer to a brokerage account solves the discipline problem for free, and does not lock you into a specific city.
When opportunity cost swings the answer
The opportunity cost of the down payment gets bigger as three things get bigger: the size of the down payment, the expected return on the alternative, and the horizon. So the buy case is weakest — and the rent case is strongest — when you would be putting a large amount down, expecting decent market returns, and planning to hold for a long time.
That last part is counterintuitive. Most people think "I will stay a long time" makes the buy case stronger. It does help amortize the closing costs and it does give appreciation more time to work. But it also gives the invested down payment more time to compound. The two effects fight each other, and which one wins depends on the specific numbers.
The calculator's answer to this: run it. Try different horizons. Watch which side pulls ahead and by how much. That is what "defensible break-even year" actually means.
Recommended reading
Two books that shaped the way we think about the opportunity-cost side of this calculator. As an Amazon Associate we earn from qualifying purchases — at no extra cost to you.
- A Random Walk Down Wall Street — Burton G. Malkiel. The best single book on why "invest the down payment in a diversified index fund and let it compound" is a defensible baseline, not a heroic assumption.
- The Millionaire Next Door — Thomas J. Stanley & William D. Danko. Long-term wealth is built by people who take opportunity cost seriously — the book that made "the invisible cost of the house you bought" a mainstream idea.