What a rate change does to your mortgage payment
Photo by Declan Sun · Unsplash
Central banks move rates in quarter points, and lenders pass them on in steps that look small on paper. Here is what each step is actually worth on a monthly payment, and why two borrowers with the same mortgage feel the same change differently.
When interest rates change, your mortgage payment changes by more than most people expect. A quarter-point rise on a $300,000 mortgage with 25 years to run adds about $44 a month; a full point adds around $179. Those numbers are the whole article in one line, but the useful part is why they are what they are — because the same headline rate change does very different things to two borrowers, and the difference is entirely predictable.
What each step is actually worth
Mortgage arithmetic is an annuity: a fixed payment that covers the interest due on the balance and chips away at the rest. Raise the rate and the interest half grows, so the payment has to grow with it to still clear the balance by the end of the term. Here is that worked out on a repayment mortgage of $300,000 over 25 years.
| Rate | Monthly payment | Interest over the full term |
|---|---|---|
| 3.00% | $1,422.63 | $126,790 |
| 4.00% | $1,583.51 | $175,053 |
| 5.00% | $1,753.77 | $226,131 |
| 6.00% | $1,932.90 | $279,871 |
| 7.00% | $2,120.34 | $336,101 |
Read the right-hand column twice. Going from 3% to 5% costs $331 a month, which is uncomfortable but survivable for most households. Over the life of the loan it costs $99,341 in extra interest, which is a deposit on another property. The monthly figure is what people budget against; the total is what the decision is actually about. Both come out of the mortgage repayment calculator, and the year-by-year split is in the amortisation schedule.
Why a quarter point is not a small number
Rate changes arrive in quarter-point steps because that is how central banks move, and both the European Central Bank and the Bank of England publish every decision and the dates the next ones are due. Stepping up from 5% on our example mortgage looks like this:
- 5.00% → 5.25%: +$43.97 a month
- 5.25% → 5.50%: +$44.52
- 5.50% → 5.75%: +$45.06
- 5.75% → 6.00%: +$45.58
Two things are worth noticing. The steps are not identical — each quarter point costs slightly more than the one before, because the interest is compounding on a balance that is now being repaid more slowly. And four of them in a row, which is an ordinary tightening cycle rather than a crisis, adds $179 a month: over $2,100 a year out of taxed income.
What decides how hard it hits you
The headline rate is the same for everyone. The impact is not, and it turns on three things.
The balance outstanding, not the original loan. A rate change acts on what you still owe. Someone eighteen years into a 25-year mortgage has a small balance left, so even a large rate move produces a modest change in payment. Someone two years in feels nearly the full force. This is why the same percentage move can be a rounding error to one neighbour and a serious problem to the next.
The years remaining. A long remaining term spreads the extra interest over more payments, which softens the monthly hit and worsens the total. A short remaining term does the opposite. If you are offered the option to extend the term to absorb a rate rise, that trade is exactly what you are making — the interest-paid calculator will show you the price of it before you agree.
How much of your payment is interest right now. Early in a mortgage almost all of it is. On our $300,000 example at 5%, the first payment of $1,753.77 is $1,250 of interest and only $503.77 of principal. Ten years in, the balance has fallen to about $221,773 — under a third of the loan repaid, despite a decade of payments. That front-loading is not a trick; it is what happens when interest is charged on a balance that starts large.
A rate change reaches you on a date, not on the news
This is the part that most often catches people out. A change in the base rate does nothing to your payment until your own rate resets, and when that happens depends on the product you hold.
- A tracker or variable rate moves with the market, usually within a month or two. You feel every step, in both directions.
- A fixed rate is untouched for its term. Nothing that happens in the wider market reaches you until the fix ends — and then all of it does, at once.
- A capped or collared rate moves within a band. The cap is the number to check against your budget, because it is the worst case you have actually agreed to.
The fixed-rate case is the one worth preparing for. A two-year fix taken at 4% that matures into a 6% market does not cost you one quarter-point step; it costs eight of them simultaneously. On our example that is the difference between $1,583.51 and $1,932.90 — about $349 a month, appearing in a single letter. Nothing about it is a surprise except the timing, which is printed on your original offer.
What is worth doing before the reset
None of this is advice about your circumstances — it is arithmetic, and the numbers below are the ones to put your own figures into.
Model the reset, not today. Take your current balance and the years left, not the original loan, and run it at two or three percentage points above your current rate in the repayment calculator. If that payment is affordable, the rest of this is noise. If it is not, you have found out while you still have time to act.
Check the affordability ratio, not just the payment. Lenders reassess when you remortgage, and the test is what proportion of gross income the housing costs take. The affordability calculator and the debt-to-income ratio use the same measures a lender does.
Compare the deal on APR, not on the rate. A lower headline rate with a large arrangement fee is frequently the more expensive product over a two-year fix. The APR calculator folds the fee back into the rate so the two can be compared honestly.
Know what overpaying buys. On the same $300,000 mortgage at 5%, paying an extra $100 a month clears the loan two and a half years early and saves about $25,613 in interest. Check your own figures with the payoff date calculator, and check your lender's overpayment limit before relying on it.
What not to panic about
Rates move in both directions, and a fix is a decision about certainty rather than a bet you win or lose. A borrower who fixed at 5% in a market that later fell to 3% did not make a mistake — they bought four years of knowing the number, which is worth something in itself and worth a great deal to a household with no slack in it.
What does deserve attention is a payment you could not meet at the cap, or at the market rate when your fix ends. That is a solvable problem two years out and a serious one on the month it lands. The arithmetic that tells you which of the two you have takes about five minutes.
This is general information, not financial advice. Figures are worked examples on a repayment mortgage and ignore fees, insurance and tax, which vary by lender and by country. For a decision about your own mortgage, speak to a regulated adviser or your lender.