Loan duration: a key variable in your total mortgage cost

When you ask yourself whether to buy or rent, you usually look at the price of the property, the monthly payment and the advertised interest rate. But another factor weighs just as much on the total cost of your mortgage: the loan duration.

At the same interest rate, a 15, 20 or 25‑year loan does not cost the same at all. Duration directly affects:

With mortgage rates around 3.6% over 20 years in today’s European market (excluding insurance), choosing 15, 20 or 25 years changes the bill by tens of thousands of euros. The buy-or-rent.net simulator explicitly uses the taux_pret (loan rate) parameter and duration to show how this plays out in your case.

How loan duration changes monthly payments and interest

A standard amortizing mortgage works with constant monthly payments that include:

The longer the loan duration:

Numerical example: €250,000 borrowed at 3.6%

Let’s take a simple example, ignoring insurance to isolate duration:

Approximate results:

Between 15 and 25 years, at the same loan rate, the difference in cost exceeds €60,000. That’s roughly the equivalent of 6 years of rent at €800–900 per month. In a buy or rent comparison, that extra cost could instead have been invested in the stock market or other assets if you had chosen to rent.

Loan duration and borrowing capacity

Loan duration is also a lever to increase your apparent borrowing capacity. By extending the term, you reduce the monthly payment and therefore your debt‑to‑income ratio (often capped around 35%), which lets you:

But this flexibility has a price: your total mortgage cost goes up. You must compare this extra cost with:

The buy-or-rent.net simulator lets you test multiple durations (15, 20, 25 years) while adjusting the taux_pret to see the impact on total cost and on your net wealth over 10, 20 or 25 years.

Loan duration, mortgage insurance and overall cost

In practice, your mortgage cost is not just the loan rate. You also have to include:

A longer term automatically increases:

Example: impact of insurance on 15, 20 and 25 years

Let’s reuse €250,000 at 3.6% and add insurance at 0.30% of the initial principal (simple group contract calculated on the original amount):

Over the term:

Combining interest + insurance:

The gap between 15 and 25 years now tops €67,000. In a buy or rent framework, you have to compare these amounts with the rent you avoid, and with the potential gains if the same money had been invested at a reasonable investment rate of 3–5% per year.

Longer term vs investing the monthly savings

A lower monthly payment on 25 years frees up cash compared with a 15‑year mortgage. The key question becomes: what do you do with that difference?

Comparison: 15 years vs 25 years with invested savings

On €250,000 at 3.6%:

Assume that with the 25‑year loan, you invest those €520/month at a 4% net annual return (for instance in a diversified ETF portfolio):

The future value of this investment is above €245,000. In return, you have paid roughly €67,000 more in interest + insurance than with a 15‑year loan, but you may have built a much larger financial portfolio.

Of course, this is theoretical. Markets fluctuate, returns are not guaranteed, and you must also compare it with the scenario where you choose a 15‑year loan, finish your mortgage much earlier, then invest the full monthly payment afterward.

The buy-or-rent.net simulator is designed to model exactly these trade‑offs: an ownership scenario with different durations and taux_pret versus a rental scenario where your surplus cash is invested.

Loan duration, inflation and the real cost of debt

Another crucial parameter is annual inflation. If inflation runs at 3–4% while your loan rate is 3.6%, you are effectively repaying your mortgage with money that loses value every year. In practice:

Over a long term (25 years), this erosion effect is stronger: you pay more interest in nominal euros, but in real (inflation‑adjusted) terms, the burden is lower than it looks. Conversely, a shorter loan reduces the nominal cost but concentrates the repayment effort in the years when your euros are worth more.

In a buy or rent comparison, you also have to weigh this against the annual rent increase (often indexed to inflation or a rent index). If you rent, your rent tends to rise with inflation, while a fixed‑rate mortgage payment stays the same.

Loan duration and holding period: you may not keep the loan to the end

Many borrowers sell before the end of their mortgage term because of:

In practice, the actual holding period for a primary residence often ranges around 7–10 years. In this case, the impact of loan duration is mainly about:

Example: selling after 10 years

Again on €250,000 at 3.6%:

When you sell, this difference in remaining balance directly affects your net equity after repaying the bank. A shorter loan builds equity faster, but with a higher monthly payment and less room for parallel investing while the mortgage is running.

Where loan duration fits in the buy or rent decision

To compare buy or rent realistically, you must embed loan duration into a broader picture:

Loan duration then becomes a dial you can turn:

There is no one‑size‑fits‑all answer to the question of the “right” duration or whether it is better to buy or rent: it depends on your situation, your risk tolerance, your income stability and your investment habits. This article is for information only and is not personalized financial advice.

Using loan duration in the buy-or-rent.net simulator

On buy-or-rent.net, the taux_pret parameter is central. To analyze the impact of duration, you can:

At the same time, the simulator models the rental scenario:

You can then see, with numbers rather than intuition, how loan duration affects your net outcome in a buy or rent comparison. This does not mean the simulator tells you what you “should” do: qualitative factors like flexibility, personal plans and risk comfort remain essential.

Conclusion: loan duration is a powerful lever, use it with data

Loan duration has a major impact on the total cost of your mortgage. At the same taux_pret, moving from 15 to 25 years on a €250,000 loan can add over €60,000 in interest and insurance. In exchange, the lower payment can be used to build a financial portfolio that may offset or exceed that extra cost.

There is no universal rule telling you whether to choose a short or long term, or whether it is better to buy or rent: it depends on your situation, your discipline in investing any savings, and your long‑term plans. This is not personalized financial advice; for tailored guidance, consult a qualified professional and use detailed simulations.

To make an informed choice, test different loan durations and rates, and compare them with a rental + investing strategy. Simulate your situation on buy-or-rent.net.

⚠️ Disclaimer: This article is for informational purposes only and does not constitute personalized financial advice. Consult a professional for your situation.

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