Question 1

Why does this calculator include opportunity cost on the down payment?

Accepted Answer

Most online rent-vs-buy calculators ignore that the buyer's down payment + closing costs could have been invested elsewhere — a $120K initial outlay foregoing a 7% market return for 10 years is ~$236K in lost compounding. The buy path needs to be charged for that opportunity cost in any honest comparison. Operator-grade rent-vs-buy includes it; carrier-side and lender-side calculators skip it because including it tilts the math toward renting in many scenarios.

Question 2

What's the right alternative-investment return assumption?

Accepted Answer

Damodaran NYU Stern long-run real return on diversified equity is ~7% inflation-adjusted (S&P 500 1928-2025). Conservative scenarios use 5-6%; aggressive use 8-9%. The default here is 7%. The key sensitivity: 1% lower return for 10 years cuts the renter's portfolio by 10-12% — meaningful but rarely enough to flip the recommendation when the home price + closing-costs are a meaningful share of net worth.

Question 3

How does the mortgage interest deduction affect the math?

Accepted Answer

Per IRC §163(h), home mortgage interest is deductible up to $750K loan balance for loans originated post-TCJA (2017). The deduction reduces taxable income, so the buyer's effective tax savings = mortgage interest paid × marginal tax rate. Property tax is also deductible per IRC §164 — but capped at $10K total state-and-local-tax (SALT) per year. This calculator applies the SALT cap on the property-tax portion. Standard-deduction households (most middle-income filers post-TCJA) get no benefit from these deductions; check the 'itemizes deductions' box only if you itemize.

Question 4

How long does it take buying to break even vs renting?

Accepted Answer

Highly dependent on inputs, but typical: 5-8 years in most U.S. metros at 2026 mortgage rates (6.5-7.5%) with 3-5% appreciation. Below 5 years, transaction costs (closing both ends + agent commissions at sale) usually dominate; above 7 years, mortgage paydown + appreciation + tax benefits usually compound enough to overtake rent. The break-even point is highly sensitive to closing costs (low closing → faster break-even) and home-price appreciation (high appreciation → faster break-even).

Question 5

What's the IRC §121 home sale exclusion?

Accepted Answer

Per IRC §121, taxpayers selling their primary residence can exclude up to $250K of capital gains from federal income tax ($500K married filing jointly), provided they've owned + used the home as primary residence for 2 of the last 5 years. This is a significant tax advantage of buying that doesn't appear in the basic calculator math — exit gains under the threshold are entirely tax-free, while the renter's invested-capital portfolio gains get taxed at long-term capital gains rates. For high-gain markets, §121 alone can shift the comparison toward buying meaningfully.

Question 6

Should I include PMI (private mortgage insurance) in the calculation?

Accepted Answer

Yes if your down payment is below 20% (PMI is typically required by conforming lenders below 20% LTV until you reach 78% LTV automatically or 80% LTV by request). PMI runs 0.3-1.5% of loan balance annually depending on credit. The calculator doesn't have an explicit PMI field; bundle it into the 'annual insurance' input if applicable. Note: PMI is NOT tax-deductible after 2021 (the §163(h) PMI deduction expired and hasn't been reinstated as of 2025).

Question 7

Does this calculator handle ARMs (adjustable-rate mortgages)?

Accepted Answer

No — the calculator assumes a fixed-rate mortgage for the full amortization period. ARMs add rate-reset risk that's hard to model deterministically. If you're considering an ARM, run the calculator at the initial rate for the fixed period, then re-run with a projected rate after reset (typically index + margin, capped at the lifetime cap). The break-even and recommendation should be valid at both scenarios for the deal to be defensible against ARM reset.

Question 8

Why ignore lifestyle factors in the math?

Accepted Answer

Because lifestyle factors are hard to quantify and vary enormously by household. The calculator surfaces the financial decision cleanly; lifestyle considerations are listed in the 'close-call' commentary (flexibility, maintenance burden, household stability, school-district lock-in, IRC §121 exit-gain exclusion). When the financial difference is material (>10% over the hold period), the math should drive the decision. When it's a close call, lifestyle factors usually dominate.

Yr	Cum. rent	Cum. buy	Diff	Renter portfolio
1	$30,300	$147,823	+$117,523	$125,398
2	$61,509	$183,146	+$121,637	$138,290
3	$93,654	$218,469	+$124,815	$151,148
4	$126,764	$253,793	+$127,029	$163,942
5	$160,867	$289,116	+$128,249	$176,638
6	$195,993	$324,439	+$128,446	$189,200
7	$232,173	$359,762	+$127,589	$202,444
8	$269,438	$395,085	+$125,647	$216,615
9	$307,821	$430,408	+$122,587	$231,778
10	$347,356	$465,731	+$118,376	$248,003

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