Methodology

Monthly payment: French amortisation

Pirfila calculates the monthly payment using the French amortisation method (constant instalment), the most common method for mortgages in Spain. Every payment is equal throughout the life of the loan. Each payment is split into two parts: interest on the outstanding balance and principal repayment.

P = outstanding principal · r = monthly rate (annual nominal rate ÷ 12) · n = number of remaining payments

Early repayment

Each early-repayment strategy is simulated independently. You can choose whether the additional payment reduces the term (same monthly payment, fewer months) or reduces the instalment (same term, lower monthly payment). Strategy 0 is always the no-prepayment baseline and is used as a reference for calculating interest saved and months saved.

Rounding

All monetary amounts are rounded to two decimal places. The final payment includes a correction that absorbs any accumulated rounding differences, ensuring the total capital paid matches the original loan amount exactly.

Limitations

The calculator assumes a fixed interest rate throughout the life of the loan. It does not model variable-rate mortgages, interest-only periods, arrangement fees, bundled insurance, or other costs that affect the true total cost of the mortgage.

Compound interest with monthly contributions

Pirfila projects investment growth using the compound interest formula with beginning-of-period contributions. This assumes each monthly contribution is invested on the first day of the month.

P = initial capital · A = monthly contribution · r = monthly rate (annual nominal rate ÷ 12) · n = number of months

Reference rates

The reference rate table includes historical returns for market indices, index funds, and bank deposits sourced from public data. Each rate shows its last-updated date. Selecting a reference rate auto-fills the return field; data is indicative and may not be available for all time horizons.

Limitations

The calculator does not account for inflation, the tax impact on gains, investment vehicle fees and expenses, or market volatility. Projections assume a constant rate of return, which is a simplification that does not reflect real market behaviour.