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In October 2023 the twelve-month Euribor — the index to which almost every Spanish variable-rate mortgage is tied — closed the month at 4.160 %, its highest value in fifteen years. In May 2025 it closed at 2.081 %. Between those two dates the index shed two full points, and there are mortgage bills that moved only once in all that time, because in a variable-rate mortgage what rules is the day on which you are reset.
That is written into your deed, and it has five parts.
1. The index they apply to you is a whole month's average. Euribor is published every business day, but the number that goes into your instalment is a different one: the official reference rate that the Bank of Spain calculates as the monthly average of those daily values, and which is then published in the state gazette, where it takes legal effect. That is how Order EHA/2899/2011 sets it up. Your rate comes from a thirty-day average, not from the quote you read in the paper.
2. And your contract chose the month. Look up the reset clause in your deed. It usually says one of three things: that the value applied is the last one published before the reset date, or the previous month's, or the one before that. Between those three wordings there are weeks of Euribor, and in a fast fall those weeks are money.
3. The reset falls on a fixed day of the year. Most Spanish variable-rate mortgages reset every twelve months; some every six. On that day the relevant value is looked up, your spread is added —the points the bank puts on top of the index—, and the result stays nailed to your bill until the next reset. Twelve months on the same number, whatever the index does along the way.
4. The lag is paid in euros, and it comes out of a subtraction. A mortgage of 150,000 euros over 25 years at Euribor plus one point of spread: with the October 2023 index, 4.160 %, the instalment is 890.92 euros a month; with May 2025's, 2.081 %, it is 717.65. The difference is 173.27 euros every month. A whole year on the old instalment, 2,079 euros.
5. The same fall is worth different amounts depending on the term you have left, and this is where two different borrowers end up with sums that look nothing alike. With 150,000 euros outstanding and 25 years ahead, those two points cut the instalment by 19.4 %: from 890.92 to 717.65 euros. With 60,000 outstanding and five years ahead, the same fall cuts it by 5 %: from 1,136.68 to 1,080.28, fifty-six euros a month. The reason is arithmetic and has no trick in it: when little term remains, almost everything you pay each month is principal, and interest already weighs little on the total.
Where to look up the right number. The official series for the reference rate is published by the Bank of Spain on its statistics portal, month by month and to three decimal places, and it is the same one that reaches the state gazette. The daily Euribor quoted in the media will not do: what they apply to you is the monthly average, and the gap between the two can be a tenth of a point.
The lag works just as well in the other direction, and it is worth knowing before you ring the bank in anger. Between January 2022 and March 2023 Euribor went from −0.477 % to 3.647 %: four points in fourteen months. The same mechanism that delays the falls today delayed the rises then, and there were people who spent a whole year paying the previous year's Euribor. It is a slow system in both directions, and almost nobody knows which day is theirs.
So tomorrow morning, in this order: dig out the deed or the bank's last annual statement and look for three figures, which always sit together — the reference index, the spread and the reset date; check against the Bank of Spain's official series that the value they applied is exactly the one your clause requires and not another month's, because the error exists and can be challenged; and open the calendar on your phone and put that date in with a reminder a month beforehand.