The repayment of a mortgage loan relies on a precise mechanism: each monthly payment combines a portion of capital, a portion of interest, and, in almost all cases, a borrower insurance premium. Understanding this breakdown allows you to anticipate the real cost of the loan even before signing a loan offer.
Anatomy of a Monthly Payment: Capital, Interest, and Borrower Insurance
In a classic fixed-rate loan (the most common in France), the monthly payment remains the same from the first to the last month. This fixed amount conceals an internal distribution that evolves: at the beginning of the loan, the portion allocated to interest is predominant, then it gradually decreases in favor of the repaid capital.
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The basic formula to obtain the monthly payment excluding insurance uses three variables: the borrowed capital, the monthly interest rate (annual rate divided by 12), and the total number of monthly payments. The result gives what the bank calls the “bare” monthly payment.
A common mistake is to stop there. Borrower insurance, often charged as a percentage of the initial capital or the remaining capital, can represent up to 30% of the total cost of monthly payments on a fixed-rate loan. To calculate the repayment of a mortgage loan realistically, this line must be included from the first calculation, not as a final adjustment.
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Debt-to-Income Ratio HCSF: The Constraint That Sets Your Maximum Monthly Payment
Since the standards of the High Council for Financial Stability, banks apply a ceiling of 35% debt-to-income ratio, including insurance. This threshold is now firmly controlled, which mechanically tightens the amount that can be borrowed compared to practices before 2020.
Specifically, the maximum monthly payment is calculated as follows: net monthly income multiplied by 0.35, then subtracting any ongoing credit charges. The result sets the limit that the bank will accept, regardless of the proposed arrangement.
Banks have a limited derogatory margin to exceed this threshold, but they reserve it for specific profiles: very solid first-time buyers, high-income earners, or investors with a comfortable remaining living allowance. Relying on this exemption to finalize a financing plan is a risky bet.
- The calculation of the debt-to-income ratio includes all ongoing loan payments (auto, consumer, other real estate), not just the new loan.
- Borrower insurance is included in the calculation of the 35%, which reduces the portion available for capital repayment.
- The maximum repayment period is capped at 25 years (27 years for new builds with a deferred start), which prevents extending the monthly payments indefinitely to stay below the threshold.
Amortization Schedule: Reading the Distribution of Capital and Interest Over Time
The amortization schedule is the document that details, month by month, the breakdown of each monthly payment between repaid capital, paid interest, and remaining capital. The bank is required to provide it with the loan offer.
Its reading reveals an often underestimated piece of information: interest weighs much more heavily in the first years. On a long-term loan, it can take until halfway through the total duration before the portion of repaid capital exceeds that of interest in each installment.
This mechanism has a direct consequence on early repayment. A partial repayment made early in the life of the loan reduces the total cost more than an identical repayment made in the later years, since the remaining interest is already low at that point.
Lemoine Law and Early Repayment: Two Levers to Reduce the Cost of Credit
Two mechanisms allow for modifying the cost of an ongoing loan without renegotiating the nominal rate with the bank.
The first is the Lemoine Law of 2022, which allows for the cancellation of borrower insurance at any time. Changing insurers for equivalent but cheaper coverage leads to a direct reduction in the monthly payment (or a shortening of the duration if the monthly payment remains the same). The savings can be substantial over the total duration of the loan, especially for young and healthy borrowers who had taken out their bank’s group contract without competition.
The second lever is early repayment, either total or partial. The bank may apply early repayment penalties (IRA), but these are regulated by law. The relevant calculation consists of comparing the amount of the IRA with the interest saved through early repayment.
- Check the clauses of your loan offer: some contracts provide for the exemption from IRA after a certain holding period or for specific reasons (sale of the property, professional relocation).
- A partial repayment can result in either a reduction of the monthly payment at a constant duration or a shortening of the duration at a constant monthly payment. The second option reduces the total cost of credit more significantly.
- In case of a change of insurance, the new contract must offer guarantees at least equivalent to the old one for the bank to accept the substitution.
The calculation of the repayment of a mortgage loan is not limited to dividing an amount by a number of months. The HCSF constraint, the weight of insurance in the actual monthly payment, and the possibility of adjusting the contract along the way profoundly alter the final cost. Including these variables from the initial simulation avoids discovering, at the signing, a discrepancy between the expected monthly payment and the one stated in the loan offer.