Every rent-vs-buy calculator prints a break-even year. Almost none of them tell you it's underdetermined.
I ran into this building the model behind a rent-vs-buy tool, and it's a good example of a modelling
decision that's invisible in the output but changes the answer by years.
The setup
Standard inputs: $450,000 home, 20% down ($90,000), 6.66% mortgage rate, $2,400/month rent, $9,000
closing costs, 5%/year assumed investment return. Nothing exotic. The question the calculator is
supposed to answer is: at what year does owning become cheaper than renting?
Here's the part that doesn't show up in the UI. Renting doesn't require $90,000 up front. That money
either goes into a down payment, or it stays invested and keeps compounding. Whether you charge the
buying scenario for that forgone return is a modelling choice, not a fact about the mortgage.
Same inputs, same math library, two defensible break-evens:
// pseudocode, not the actual engine — illustrates the branch, not the implementation
function breakEvenYear(inputs, chargeOpportunityCost) {
let ownerCost = inputs.downPayment + inputs.closingCosts;
let renterCost = 0;
for (year of horizon) {
ownerCost += mortgagePayment + propertyTax + maintenance - homeEquityGained;
renterCost += annualRent;
if (chargeOpportunityCost) {
// the buyer sank this cash into the house; the renter's copy of it stays
// in an index fund compounding at investmentReturn, so the forgone return
// is a cost carried by the BUYER
ownerCost += (downPayment + closingCosts) * ((1 + investmentReturn) ** year - 1);
}
if (ownerCost < renterCost) return year;
}
}
With the opportunity cost charged, break-even lands at year 8. With it switched off — treating the
$90,000 as gone the moment it's spent, on both sides — break-even is 4 years 3 months. Same house,
same rate, same rent. The gap between the two answers is just what $99,000 would have earned at 5%
a year, compounded, and that's a big enough gap that "when does buying win" depends entirely on which
branch the tool took silently.
Where this actually bites
It's not just an academic gap between the two break-even numbers. At a 7-year horizon in this same
scenario, the buyer's projected net worth comes out to $193,878 and the renter's to $200,559 — the
renter is still ahead. But the break-even card (computed with opportunity cost charged) says the
crossover happens in year 8. Someone reading only the headline break-even number at year 7 would
conclude they're one year from the finish line, when the net-worth comparison for their actual horizon
already has them on the wrong side of it. The two numbers are answering different questions — "when do
the running totals cross" versus "who's ahead at my horizon" — and they can legitimately disagree by a
full year around the crossing point.
A second underdetermined branch: the deduction
Most of these calculators also assume the buyer itemizes and gets the full value of mortgage-interest
and property-tax deductions. In this scenario, year-one deductible interest is $23,858 and property tax
is $4,116 — $27,974 itemizable. The 2026 standard deduction for married filing jointly is $32,200 (IRS
inflation-adjusted figures for tax year 2026). $27,974 loses. A model that doesn't check this gives the
buyer a tax break they wouldn't actually get.
I checked how a few well-known tools handle both of these branches:
- Calculator.net's default case prints a single line — "Buying is cheaper if you stay for 4.9 years or longer" — no visible branch.
- Zillow publishes one breakeven horizon, no second scenario.
- Redfin skips a break-even year entirely and compares four cost categories instead.
- NerdWallet's stated methodology assumes itemizing: "We assume buyers can save on taxes by itemizing federal tax deductions for both property tax and mortgage interest," with no gate against the standard deduction.
- michaelbluejay.com's calculator does gate the deduction against the standard deduction — credit where due — though its defaults are 2020-era ($12,400 standard deduction, $1,000,000 mortgage-interest cap, the latter superseded by the $750,000 cap for loans taken after 2017). The fields are editable, so that's a stale default rather than a missing branch.
None of this makes any of these tools wrong, exactly. It makes the break-even year a much softer
number than its single-line presentation suggests.
What I don't model either
Being honest about the limits: every rate in this model is held constant for the whole horizon, so
there's no sequence-of-returns risk — a market crash in year 2 versus year 9 produces the identical
output, which isn't how markets work. It's US federal income tax only, no state tax, no AMT, no phase-
down of the SALT cap (which the model holds at the 2026 $40,400 figure for the entire horizon, even
though the statute drops it back to $10,000 starting in 2030). No utilities, no moving costs, no
commute changes, no rent control. And it assumes you sell at the end of the horizon and eat the full
selling cost, which isn't everyone's plan.
The honest way to use a model like this isn't to trust either single number. It's to run it with the
opportunity-cost branch on and off and look at the spread — if your actual decision only makes sense
under one of the two assumptions, that's the thing worth knowing before year 8 arrives.
If you want to poke at the two branches yourself with your own numbers, the calculator this came out of
is at https://smart-calculators.net/en-US/tools/rent-vs-buy — it shows both break-even years side by
side by default rather than picking one for you.
