Mortgage Credit Simulator: How to Calculate and Compare Your Home Loan Before Talking to a Bank
A mortgage simulator does one thing that no bank brochure can: it lets you change the variables and immediately see how each change affects your monthly payment, total interest cost, and the full amortization schedule over the life of the loan. Before signing any mortgage deed, every borrower should run multiple scenarios — different amounts, terms, and rate assumptions — to understand exactly what they are committing to over 20 or 30 years.
The Core Mortgage Formula
All standard Spanish and Latin American mortgages use the French amortization method (amortizacion francesa), which produces a constant monthly installment where the ratio of interest to principal shifts over time. The formula is:
M = P x [r(1+r)^n] / [(1+r)^n - 1]
Where:
- M = Monthly payment
- P = Principal (loan amount)
- r = Monthly interest rate = TIN / 12
- n = Total payments = years x 12
This formula is deterministic: given the same inputs, every lender calculates the same monthly payment. Where lenders differ is in the TIN they offer, the fees they charge (which affect the TAE but not the monthly payment formula), and the conditions attached to the mortgage (mandatory insurance, linked products, early repayment penalties).
TIN vs TAE — The Most Important Distinction
TIN (Tipo de Interes Nominal) is the rate applied to the outstanding balance each month. It is the input to the payment formula above.
TAE (Tasa Anual Equivalente) is the standardized annual percentage rate that includes TIN plus all recurring costs: origination fees amortized over the loan term, mandatory insurance premiums, and any required linked products. The TAE is always higher than the TIN.
Spanish law (Ley Reguladora de los Contratos de Credito Inmobiliario, 2019) requires all lenders to publish the TAE in their marketing. When comparing offers across multiple banks, compare TAEs — not TINs. A mortgage advertised at 3.2% TIN with a mandatory life insurance premium and a comision de apertura of 1% may carry a TAE of 3.9%, while a competitor offering 3.5% TIN with no fees has a TAE of 3.6%.
In Mexico, the equivalent comparison rate is the CAT (Costo Anual Total), which functions like the TAE. Use CAT to compare Mexican mortgage offers across BBVA Mexico, Santander Mexico, Banorte, and INFONAVIT.
Key Variables That Determine Your Mortgage Payment
Loan Amount (Principal)
The loan amount is the purchase price minus your down payment, minus any seller or government subsidy. In Spain, banks typically finance up to 80% of the appraised value (LTV = 80%), requiring a 20% down payment. Budget an additional 10%–15% of the purchase price for closing costs: ITP (Impuesto de Transmisiones Patrimoniales) for second-hand properties, or IVA at 10% for new builds, plus notary fees, land registry fees, and the lender's mortgage tax.
In Mexico, INFONAVIT (Instituto del Fondo Nacional de la Vivienda para los Trabajadores) may contribute a portion of the loan for eligible workers, reducing the bank mortgage needed. The maximum INFONAVIT loan for 2026 is up to MXN 2,000,000 depending on accumulated points and salary.
Interest Rate
Run the simulator with three rate scenarios:
- Base case: The rate quoted in the lender's current offer
- Downside (variable mortgages): EURIBOR rises by 2 percentage points above today's level
- Upside: EURIBOR falls to 1.5%
For a EUR 200,000 mortgage over 25 years:
| TIN | Monthly Payment | Total Interest Over Life |
|---|---|---|
| 2.5% | EUR 897 | EUR 69,100 |
| 3.5% | EUR 1,001 | EUR 100,300 |
| 4.5% | EUR 1,111 | EUR 133,300 |
| 5.5% | EUR 1,228 | EUR 168,400 |
The difference between a 2.5% and a 5.5% rate on this loan is EUR 99,300 in total interest — nearly half the principal. Rate matters more than any other variable.
Loan Term
A longer term reduces the monthly payment but dramatically increases total interest paid. Using EUR 200,000 at 3.5% TIN:
| Term | Monthly Payment | Total Interest |
|---|---|---|
| 15 years | EUR 1,430 | EUR 57,400 |
| 20 years | EUR 1,160 | EUR 78,400 |
| 25 years | EUR 1,001 | EUR 100,300 |
| 30 years | EUR 898 | EUR 123,300 |
Extending from 25 to 30 years saves EUR 103/month but costs EUR 23,000 more in interest over the life of the loan. Most Spanish banks offer up to 30 years for primary residences, with some extending to 35 for younger borrowers. The Banco de Espana guideline is that the borrower's age at loan maturity should not exceed 75 years.
What the Amortization Schedule Reveals
The full amortization schedule (cuadro de amortizacion) breaks down every single payment for the life of the loan, showing:
- Payment number and date
- Principal component of each payment
- Interest component of each payment
- Outstanding balance after each payment
In the early years of a French amortization mortgage, most of each payment is interest. For a 3.5% TIN mortgage on EUR 200,000 over 25 years, the first payment of EUR 1,001 splits approximately as EUR 417 interest and EUR 584 principal. By year 20, the split reverses: roughly EUR 150 interest and EUR 851 principal.
This front-loading of interest has an important implication: early repayment is most financially advantageous in the first third of the loan term, when it eliminates the highest-interest portions of remaining payments.
Common Mistakes When Using a Mortgage Simulator
Using the advertised rate without reading the conditions. Many headline rates require maintaining a salary account, taking out the lender's life and home insurance, and meeting minimum annual transactions. These linked products have a real cost that the simulator cannot capture unless you add them manually.
Forgetting closing costs. The deposit (10%–15% of purchase price for taxes and closing costs in Spain, 3%–7% for mortgages in Mexico) is a cash requirement on top of the down payment. Budget for both.
Simulating only the base case. Always run the interest rate stress test. If the worst-case monthly payment exceeds 35% of your net household income, you are taking on excessive interest rate risk.
Ignoring the comision de amortizacion anticipada (early repayment penalty). Spain's 2019 Mortgage Law capped early repayment penalties: 0.25% for the first 3 years and 0% thereafter for variable mortgages; 2% for the first 10 years and 1.5% after for fixed mortgages. Some legacy mortgages still carry higher penalties — always check the FEIN.
Not comparing the notarial deed against the FEIN. The FEIN (Ficha Europea de Informacion Normalizada) is a binding offer document that lenders must provide at least 10 days before signing. The terms in the deed must match the FEIN. If they differ, you can walk away without penalty.
Spain vs Mexico: Key Differences in Mortgage Markets
| Variable | Spain (2026) | Mexico (2026) |
|---|---|---|
| Benchmark rate | EURIBOR 12m approx. 2.45% | Banxico policy rate approx. 9.0% |
| Average fixed mortgage rate | 2.9%–3.5% TIN | 10%–13% annual |
| Max LTV (standard) | 80% | 80%–90% (INFONAVIT) |
| Max term | 30–35 years | 20–30 years |
| Comparison rate | TAE | CAT |
| Key regulator | Banco de Espana | CNBV |
The large rate differential between Spain and Mexico reflects the difference in central bank policy rates and inflation environments. A Mexican borrower paying 12% annual on a mortgage is not being poorly served — that is the market rate given Banxico's rate environment. The comparison that matters is whether the offered rate is competitive within Mexico's market, not against European rates.
Using the Simulator to Prepare for Bank Negotiations
Before approaching any lender, run the simulator to establish three numbers:
- Maximum affordable monthly payment — the highest payment that keeps your DTI (Debt-to-Income ratio, or relacion cuota/ingresos) at or below 30%–35% of your net monthly income.
- Implied maximum loan amount — working backward from the maximum payment at a conservative rate (use a rate 1% above today's market) and your preferred term.
- True cost comparison — the total interest paid over the life of the loan for each offer you receive.
Armed with these numbers, you negotiate from a position of knowledge rather than optimism. The bank's job is to lend at the highest rate the market will bear; your job is to secure the lowest rate your profile supports.
Mortgage simulation is not a one-time exercise. Run it again when rates change, when you receive competing offers, and when you are considering early repayment to quantify the savings from paying down principal ahead of schedule.