Model integrity
These models assert their own accounting and refuse to report a result when one of the checks fails. Those checks run every time this site is built, and this page is their output — not a list somebody typed.
All 12 checks passed in this build. Last run September 14, 2026.
Each one re-derives its quantity independently rather than reading a number the engine already computed and comparing it against itself. That distinction matters: a model checking its own arithmetic against its own arithmetic will always agree with itself.
Rent vs. Buy
PASSPrincipal is conserved
Every dollar borrowed is either repaid or still owed. Repaid principal plus the remaining balance must equal the original loan, at every point in the schedule.
residual $0.00
PASSBoth households spend the same
The rule the whole comparison rests on. If either side were handed money the other did not get, every figure downstream would be meaningless.
difference over 30 years $0.00
PASSMortgage cash reconciles
Total cash paid on the loan equals cumulative interest plus cumulative principal.
residual $0.00
PASSThe portfolio adds up
The renter's final balance is rebuilt from the starting cash and every monthly contribution, compounded independently of the engine's own running total.
rebuilt $1,159,898.66 vs reported $1,159,898.66
PASSThe loan retires on schedule
A fixed-rate loan left alone must amortise to exactly zero on its final scheduled payment.
balance at month 360 $0.00
Car: Lease vs. Buy
PASSEvery dollar is dated correctly
Each option's cost is rebuilt by compounding every outflow from the month it is paid to the horizon. A dollar paid today and a dollar paid in five years must not be treated as equal.
worst deviation across five options $0.00
PASSThe chart and the table agree
The final point of the committed-cost chart is the same number as the net cost in the summary table.
worst deviation $0.00
PASSCash and financing converge
A loan of nothing at zero percent must cost exactly what paying cash costs. If the two ever diverge, the financing arithmetic is adding or losing money.
difference $0.00
PASSLease obligations are carried
A lease cannot be walked away from. Any contract still running at the horizon is charged as the present value of what is still owed, rather than vanishing.
outstanding at horizon $0.00
Rental Property vs. Investing
PASSBoth sides invest the same
The landlord and the investor start with identical capital, and every month the property needs feeding the investor contributes the same amount. Lifetime out-of-pocket must match.
difference $0.00
PASSPrincipal is conserved
Repaid principal plus the outstanding balance equals the original loan.
residual $0.00
PASSHigher appreciation never hurts the landlord
The sensitivity grid is checked for monotonicity in both directions. Linking operating costs to market value can break this, and when it does the grid says so rather than presenting a misleading picture.
grid is monotone in both directions
What this does not tell you
That the arithmetic is sound is not the same as the model being right for your decision. These checks cannot tell you whether 3% appreciation is a sensible assumption, whether the tax treatment left out of a rental model matters in your bracket, or whether the thing you are really deciding is financial at all. Each calculator’s methodology page sets out what it deliberately does not model, and those omissions are usually a bigger deal than any rounding.
Beyond these, the project runs a test suite of several hundred cases covering the same invariants across edge scenarios — zero-rate loans, hundred-percent deposits, one-month horizons, price crashes — plus an independent reference implementation of the rent-vs-buy model, written from its published economics rather than its code, which reproduces every headline figure to the cent.