Mortgage Payment Calculator
Calculate the monthly payment of a mortgage based on principal, rate and duration.
Formulas
Standard mortgage amortization formula (French method). Computes the fixed monthly payment for a fully amortizing loan. Pure arithmetic — the formula is mathematically identical across all jurisdictions. Inputs are caller-supplied; no jurisdiction-specific constants are embedded.
monthly_payment = principal * (annual_rate / 12) * (1 + annual_rate / 12) ^ periods / ((1 + annual_rate / 12) ^ periods - 1)
Assumptions and limitations
Scope
Calculate the monthly payment with French amortization plan (constant payment, decreasing interest portion). Applicable to fixed-rate mortgages with constant monthly payments.
Assumptions
- Standard mortgage amortization formula (French method). Computes the fixed monthly payment for a fully amortizing loan. Pure arithmetic — the formula is mathematically identical across all jurisdictions. Inputs are caller-supplied; no jurisdiction-specific constants are embedded.
Limitations
- Non valido per mutui a tasso variabile (la rata cambia ad ogni revisione)
- Non include costi accessori: assicurazione obbligatoria, imposta sostitutiva, perizia, istruttoria
- Assume assenza di periodi di preammortamento
- Il tasso mensile è calcolato come TAN/12: per confronti precisi usare il TAEG
Validation
Approach: Validated by verifying the declared formula and plausibility checks (range, units, edge cases).
Examples
Average 25-year mortgage
- €200.000 al 3,5% per 25 anni → rata ≈ €1.001/mese. Interessi totali ≈ €100.400, pari al 50% del capitale. Accorciando a 20 anni la rata sale a ≈ €1.160 ma si risparmiano ≈ €22.000 di interessi.
Small 15-year mortgage
- €100.000 al 3% per 15 anni → rata ≈ €691/mese. Interessi totali ≈ €24.400. Durata breve: costo del credito contenuto.
Frequently asked questions
Why is the payment high even with a low rate?
The payment depends on principal, rate and duration. A €200,000 mortgage at 3% over 10 years costs ≈€1,931/month; over 25 years it drops to ≈€948/month. Increasing the duration reduces the payment but increases the total interest paid — evaluate both before choosing.
What is the maximum recommended payment relative to income?
The prudential banking rule is that the payment should not exceed 30–35% of monthly net income. E.g.: net income €2,500 → maximum recommended payment €750–875. Exceeding 40% exposes to risk of default in case of unforeseen events (job loss, extraordinary expenses).
Should the rate be entered as annual or monthly?
Enter the annual TAN in decimal format. E.g.: TAN 3.5% = 0.035. The calculator automatically converts to the monthly rate (÷12) for the French plan. Don't confuse TAN (nominal rate) with TAEG (which also includes commissions and ancillary costs).