How Long Should a Mortgage Be? The Term Trade-Off
The term is the most consequential number on the application and the one given the least thought. It is usually inherited from whatever the calculator defaulted to.
The trade-off is straightforward. A longer term makes each payment smaller and the total much larger, and beyond a certain point the payment stops falling meaningfully while the total keeps climbing.
What each decade costs#
| Term | Monthly payment | Total interest | Total repaid |
|---|---|---|---|
| 15 years | 1,562 | 81,200 | 281,200 |
| 20 years | 1,299 | 111,700 | 311,700 |
| 25 years | 1,146 | 143,800 | 343,800 |
| 30 years | 1,049 | 177,700 | 377,700 |
| 40 years | 941 | 251,700 | 451,700 |
200,000 at 4.8%, illustrative. Read the first and third columns together: going from 25 to 30 years saves 97 a month and costs an extra 33,900 in interest.
The diminishing return#
The payment relief per extra decade shrinks while the interest cost per extra decade grows. That is the shape of the whole decision, and it is why forty-year terms are a solution to a specific problem rather than a general improvement.
- 15 → 20 years: payment falls 263, interest rises 30,500.
- 20 → 25 years: payment falls 153, interest rises 32,100.
- 25 → 30 years: payment falls 97, interest rises 33,900.
- 30 → 40 years: payment falls 108 over two decades, interest rises 74,000.
Somewhere around thirty years the trade turns clearly unfavourable in most markets. Beyond that you are buying a small amount of monthly room for a great deal of money.
The flexible answer#
There is a middle path that most borrowers should take: choose the longer term and overpay. A long term with regular overpayments behaves almost exactly like a short term, with one crucial difference — you can stop in a difficult month, and a contractual shorter term will not let you.
| Approach | Payment obligation | Actual repayment speed | Flexibility |
|---|---|---|---|
| 15-year term | High, contractual | Fast | None |
| 25-year term with overpayments | Lower, contractual | Nearly as fast | High |
| 25-year term, no overpayments | Lower | Slow | High |
| 40-year term | Lowest | Very slow | High |
The second row is the arrangement to aim for, provided the overpayments actually happen. Automate them, and check that they reduce the term rather than the payment.
The constraints on the term#
- Retirement age. Lenders generally want the term to end at or before your expected retirement, which caps the term for older borrowers.
- Regulatory limits. Several markets cap the maximum term or apply extra scrutiny beyond a threshold.
- Affordability in reverse. A longer term can be the only way to pass the affordability test, which is a legitimate use of it.
- Product availability. The longest terms are not offered on every product or at every LTV.
- Mandatory amortisation in markets such as Sweden effectively sets a minimum repayment speed regardless of the nominal term.
Choosing, in order#
- Find the shortest term whose payment you could comfortably meet at a rate three points higher than today’s.
- Check whether that term is available at your LTV and within the lender’s age limits.
- If it is uncomfortable, take a longer term and commit to a standing overpayment instead.
- Confirm that overpayments will reduce the term rather than the monthly payment.
- Revisit at each remortgage: shortening the term at that point costs nothing and is easy to forget.
- Do not extend the term to fund something else without comparing the total cost.
Step five is where most of the practical benefit sits. Every remortgage is a free opportunity to shorten the term, and almost everyone rolls the remaining term forward without thinking about it.
Frequently asked questions
Is a 15-year or a 30-year mortgage better?
A shorter term costs far less in total and demands a much higher payment; a longer term is affordable and expensive. On 200,000 at 4.8%, fifteen years costs about 81,000 in interest and thirty years about 178,000. For most households the better answer is neither extreme: take the longer term for the lower contractual obligation, then overpay regularly. That behaves almost like the short term while leaving you the option of stopping in a difficult month.
Is a 40-year mortgage a bad idea?
It is a solution to a specific problem rather than a general improvement. Beyond about thirty years the payment stops falling meaningfully while the total interest keeps climbing sharply — the last decade of a forty-year term buys very little monthly room for a great deal of money. It is defensible when it is the only way to buy at all, and it should come with a plan to overpay or to shorten the term at the first remortgage.
Can I shorten my mortgage term later?
Yes, and remortgaging is the natural moment to do it — it costs nothing extra and is very easy to forget. Most people roll the remaining term forward without thinking, so a twenty-five-year loan quietly becomes twenty-five years again five years in. You can also shorten within an existing product by overpaying, provided the lender applies the overpayment to the term rather than to the monthly payment, which is worth asking for explicitly.
mortgage term length15 vs 30 year mortgage40 year mortgageshorten mortgage termmortgage term calculatortotal interest by term
