How to Calculate Your Mortgage: Monthly Payment, Interest and Amortization

Understanding a mortgage: the key concepts before signing

Buying a home is often the biggest financial decision in life. And yet, many people sign their mortgage without fully understanding how the installments are calculated, how much they will pay in interest or what the difference is between one amortization system and another. Before going to the bank, you need to master some concepts: outstanding capital, nominal interest rate (TIN), equivalent annual rate (APR), term y amortization system.

The TIN is the percentage that the bank charges on the capital you still owe. The APR, on the other hand, includes commissions and expenses, so it reflects the real cost of the loan. To compare offers, always look at the APR. If you need to convert between percentages or calculate differences, the Percentage Calculator It will be of great help to you.

How the monthly payment of a mortgage is calculated

Most mortgages in Spain and Latin America use the French amortization system, where the monthly payment is constant throughout the life of the loan. The formula is:

Where C is the borrowed capital, r is the monthly rate (TIN/12) and n the total number of installments. For example, for a mortgage of €200,000 over 25 years with a TIN of 3%, the monthly payment is around €948.

You don't have to do the calculation by hand: enter your data in the Loan Calculator and instantly obtain the monthly payment, the total interest and the complete amortization table.

Table: example of amortization of a 25-year mortgage

Year	Amortized capital	Interest paid	Outstanding capital
1	€5,440	€5,936	€194,560
5	€29,900	€26,980	€170,100
10	€66,350	€47,430	€133,650
20	€154,700	€72,780	€45,300
25	€200,000	€84,380	€0

Notice how in the first years most of the payment goes to interest. As the loan progresses, that proportion is reversed and the principal is amortized more quickly. Understanding this is essential to assess whether it is advisable to pay off early.

Fixed, variable or mixed type: which one to choose?

The great dilemma of every mortgage. Each modality has advantages and risks:

• Fixed type: constant fee throughout the life of the loan. It offers security and predictability, but usually starts at a slightly higher rate.
• Variable type: referenced to the Euribor (or other index), it is reviewed periodically. It may be cheaper at first, but it exposes you to rate increases.
• Mixed type: fixed for the first years (3, 5 or 10) and then variable. It combines initial stability with the possibility of benefiting from downturns.

To simulate how your quota would change under different rate scenarios, go back to Loan Calculator and compare the results. If you also want to understand the effect of compound interest when you invest instead of paying debt, read our complete guide to compound interest.

Tips before taking out your mortgage

Before signing, keep these essential points in mind:

• Compare at least three offers. The difference of 0.5 points in the rate can cost thousands of euros over the years.
• Calculate your debt ratio. Experts recommend that the fee not exceed 30-35% of your monthly net income.
• Take into account the associated expenses: appraisal, notary, registration, taxes and related insurance.
• Negotiate the conditions. Many clauses (opening fees, early repayment) are negotiable.
• Check the links. Domiciliating your payroll, taking out insurance or pension plans with the bank can reduce the differential, but consider whether it really compensates you.

For complete financial planning including mortgage, savings and investing, check out our article on how to plan your personal finances with online tools. And if you want to calculate discounts on linked insurance, the Discount Calculator It will help you compare prices quickly.