A rate for a day, not a rate for a moment
Most currency converters answer a question this one deliberately does not: what is the rate right now. They stream a live mid-market quote, it moves while you read it, and the number you saw is gone by the time you write it down. That is the right answer if you are about to trade. It is the wrong answer, and an unusable one, if what you actually need is the rate for a particular day.
That second question is the common one in paperwork. An invoice issued in one currency and accounted for in another has to be converted at a rate somebody can point to afterwards. So does a customs entry, an expense claim, a tax return, a set of accounts. What makes a rate usable there is not that it is fresh but that it is published, dated and the same for everyone who looks it up. This page answers that question: it shows the European Central Bank's euro reference rates for a stated day, and it puts the day next to the figure rather than hiding it.
The rates are a dated file this site holds, not a request made while you are reading. A scheduled job fetches the Bank's figures once a day and writes that file; serving a page never touches the network. The consequence is visible and deliberate: everyone reading the page at the same moment sees the same number, the figure cannot shift between the moment you read it and the moment you write it down, and it is exactly as old as the date beside it says.
The twelve months behind the number
One dated rate answers what a conversion is worth today and leaves the obvious next question open: is that high or low? The chart under the converter answers it for whichever pair you have selected. It plots one point for every day the Bank published a rate over the last twelve months, from the same reference rates the figures above are worked out from, so the line and the number are the same source rather than two.
Its vertical axis does not start at zero, and that is worth knowing before you read the shape. A rate is a ratio rather than a quantity, so zero says nothing about it, and over a year most pairs move by a few per cent — on an axis that started at zero the whole year would be a flat line. The axis is labelled with real rates instead, and the caption under the chart gives the low, the high and the change across the window, so the numbers are there to check the picture against.
How a cross rate is worked out
The Bank quotes every currency on its list against the euro, and nothing else. A conversion where one side is the euro therefore uses a rate the Bank actually published. A conversion between two currencies that are neither the euro does not exist in the Bank's table at all: it is those two quotes divided, and this page says so on the result rather than letting a derived figure pass as a quoted one. The arithmetic is one multiplication and one division with no rounding in between, so nothing is lost at the intermediate step, and only the final amount is brought back to the smallest unit of the currency.
The table under the answer converts the same amount into every currency on the list at once, with the per-unit rate beside it. That is there because the question behind a converter is often comparative rather than singular — what a price looks like in several places, which of two quotes in different currencies is the better one — and doing it one pair at a time is slower than reading a column.
What is on the list and what is not
The list is whatever the Bank publishes a euro reference rate for, minus a small number of exclusions this site makes on purpose. Every amount on this site is held as a whole number of the smallest unit of its currency, with two decimal places assumed throughout. A handful of the world's currencies have no subunit at all: the whole unit is the smallest unit. Printing those with two decimal places would be printing a precision that does not exist, so they are left off rather than shown wrongly, and the file records which ones and why.
A reference rate is also not a rate anyone will deal at. Banks and payment services quote a rate with a spread built into it, and often a fee on top, so the amount that actually reaches the other side is smaller than the figure here. Treat this as the neutral midpoint that paperwork uses and that a quote can be measured against, not as a prediction of what you will be charged.