What each actually costs you
Owning is not the mortgage payment. Tax, insurance, maintenance and any HOA are the part that gets left out of the comparison people run in their head — and maintenance is the one nobody budgets.
Net wealth, both paths, if you sold that year
How far the break-even moves
Against the mortgage rate, holding everything else fixed. This is why a single "you should stay N years" answer does not exist — the rate alone moves it by years.
Get the spreadsheet
The full model — every month of amortisation, every cost line, the itemising test — plus the next model when it's published.
Method, assumptions and what this does not do
The fairness rule. The renter starts by investing the money the buyer just spent — the down payment plus closing costs. After that, whichever side has the lower monthly outflow invests the difference, and both portfolios are taxed on their gains when the comparison is struck. Without that, every rent-versus-buy comparison flatters buying, because it silently lets the buyer's down payment vanish.
What "break-even" means here. The year in which selling the house — after agent and transfer costs, after paying off the mortgage, after capital gains tax on anything above the §121 exclusion — leaves you with more than the renter's after-tax portfolio. Before that year, renting won.
The tax deduction. Mortgage interest on acquisition debt up to $750,000, plus property tax capped by the 2026 SALT limit of $40,400, are compared against your standard deduction. Only the excess is worth anything, and only at your marginal rate. State income tax also counts toward the SALT cap and is not included here, so this understates itemising for people in income-tax states.
Validation. In a degenerate world — no appreciation, no costs, no growth, a 0% mortgage and 100% down — buying and renting must leave identical wealth. The engine returns a difference of $0.000000. It is also cross-checked against an independent Python implementation across 12 input combinations and 48 assertions, agreeing to floating-point precision.
What it does not do. It ignores rent control, the risk that you cannot sell when you want to, mortgage points, refinancing, recasting, PMI removal by re-appraisal rather than amortisation, and any of the reasons people buy that are not financial. It assumes you keep the house until you sell it and that you would genuinely invest the difference — which, as the Roth model found, is where most of these comparisons actually break.
It is a model, not advice. Its assumptions are sliders. Move them.