How the repayment calculator works
This calculator uses the standard mortgage annuity formula, the same approach used by Irish banks and brokers for calculating repayment (capital & interest) mortgages. Given your loan amount, annual interest rate, and term, it computes a fixed monthly payment that will fully repay the loan — both principal and interest — by the end of the term.
The formula is: M = P × r(1+r)n / ((1+r)n − 1), where P is the principal (loan amount), r is the monthly interest rate (your annual rate divided by 12), and n is the total number of monthly payments (term in years × 12).
Worked example
Say you borrow €300,000 over 30 years at a fixed rate of 4.0%. The monthly interest rate works out to 4.0% ÷ 12 = 0.333%, and there are 360 monthly payments. Plugging these into the formula gives a monthly repayment of roughly €1,432. Over the full 30-year term, you'd repay approximately €515,600 in total — meaning around €215,600 of that is interest, not principal.
Early in the schedule, most of each €1,432 payment goes toward interest, because the outstanding balance is at its highest. As the balance falls year after year, a growing share of each payment goes toward principal instead — this is normal for any repayment mortgage and is visible in the amortisation table above.
Frequently asked questions
How is a mortgage monthly repayment calculated?
Repayment mortgages use the standard annuity formula shown above. It produces a fixed monthly payment that fully pays off the loan by the end of the agreed term, assuming the interest rate stays constant.
Why do I pay mostly interest at the start of my mortgage?
Interest is charged on the outstanding balance each month. Early on, the balance is largest, so more of each payment goes to interest. As the balance shrinks over time, more of each equal payment goes toward principal.
Does this calculator account for fixed vs variable rates?
This calculator assumes a single fixed interest rate for the full term you enter. Many Irish mortgages have a fixed rate for an initial period (e.g. 1–5 years) before reverting to a variable rate. Re-run the calculator with a new rate to model what happens after a rate change.