General principle
Every calculator uses the French amortization system (fixed monthly payment) — the same formula any commercial bank in the EU or Latin America uses. All math runs in your browser with IEEE 754 double precision, without intermediate rounding.
Monthly payment — French system
The formula is:
M = P · [r·(1+r)^n] / [(1+r)^n − 1]
P= principal (loan amount after the down payment).r= monthly rate = annual rate / 12, as a decimal.n= total number of monthly payments.
Total interest is M · n − P. The amortization schedule uses the classic recurrence: monthly interest = outstanding balance × r; principal paid = M − interest; new outstanding balance = outstanding balance − principal paid.
APR — Newton-Raphson
The APR is computed by solving the present-value equation with Newton-Raphson, not a linear approximation. It converges in fewer than 50 iterations to 10⁻¹⁰ tolerance. It includes origination fee, mandatory insurance, and any formalization costs entered by the user.
Note: the naming differs by country (TAE in Spain, CAT in Mexico, CFT in Argentina, APR in the US). The math is the same; only the list of items included as "cost" changes per local regulation.
Data sources
- Euribor 12M (Spain and EU): European Central Bank SDMX API. Daily updates.
- TIIE 28-day (Mexico): Banxico SIE API. Daily updates.
- UVA and CER (Argentina): BCRA API v4.0. Daily updates.
- UF (Chile): CMF Chile via mindicador.cl. Daily updates.
- DTF and IBR (Colombia): datos.gov.co Socrata API. Banco de la República reference.
- US mortgage rates: FRED (Federal Reserve Bank of St. Louis) and Freddie Mac PMMS. Weekly.
- Spanish bank offers: editorial comparators (Kelisto + HelpMyCash) plus manual verification against the issuing bank. Weekly review.
- Local taxes (US state property tax, ISAI, ITP by CCAA, stamps): BOE, AEAT, SAT, state departments of revenue. Updated when legislation changes.
Affordability (DTI) by country
- Spain: 35% rule over net income (Banco de España).
- Mexico: 30% recommended by CNBV.
- Argentina: 25% per BCRA guidance.
- Colombia: 40% under Decreto 583.
- Chile: 25% as CMF guidance for dividend-over-income.
- United States: 43% under the CFPB's Qualified Mortgage Rule; 28% front-end as prudence.
The calculator flags a warning when effort crosses the country threshold and a red warning when it exceeds the threshold × 1.4.
Known limits
- Home and life insurance are not automatically included; they are computed only when you mark them as bundled products (Spain) or explicitly enter them (US PITI).
- Payroll bonifications are modeled as a flat discount on the nominal rate. They do not reflect amount-minimum requirements.
- APR includes what you enter. Undisclosed fees from your lender will push your real APR higher.
- City-level home prices refresh quarterly and may lag the market by a quarter.
- Argentina UVA projections are scenarios, not forecasts. Use them to understand ranges, not to commit.
- Chile's DS1 / DS49 subsidies are applied only if you meet the bracketed requirements; the tool displays the theoretical saving but the real grant depends on Minvu.
Open-source math
All calculator code is publicly auditable in the project repository. The pure calculation functions (monthly payment, amortization, APR, affordability, costs, early repayment) carry unit tests that run on every deploy.
Final note
The math may be correct, but the outputs are for information only. Always verify with your lender before signing anything. See the Terms of Use for the full limitation of liability.