Watch accuracy is quoted in seconds per day, usually shortened to spd. It is a small unit for a small effect, and it is the one figure that lets two watches be compared honestly.
A watch at +4 gains four seconds every day. A watch at −2 loses two. The sign carries as much information as the number. Plus is fast, minus is slow, and a watch sitting near zero is keeping true time whichever way you look at it.
What the number is counting
A rate measures the error a watch adds in a day, rather than the error currently showing on its dial.
Those two get confused constantly, and the confusion runs in a particular direction. People look at a watch two minutes ahead and assume something is badly wrong. Two minutes is only alarming if it arrived quickly. Spread over three months it is about one and a third seconds a day, which is very good indeed for a mechanical watch.
So the first thing a rate needs is a span of time to be measured across. A number without one is not a rate, it is just a gap.
Why the unit is a day
A day is the interval a mechanical watch is naturally judged on, for two reasons that have nothing to do with convention.
The first is that a watch’s error accumulates steadily rather than arriving all at once. A figure per day scales cleanly to a week or a month by multiplication, which a figure per week does not scale downward nearly as usefully.
The second is practical. A day or two is long enough for a real rate to separate itself from the imprecision of reading a dial, and short enough that you will actually do it. Quoting accuracy per month would be more stable and would mean waiting a month to learn anything.
What a given rate looks like
The abstract number becomes concrete quickly once you multiply it out.
| Rate | After a week | After a month | When you would notice |
|---|---|---|---|
| ±1 spd | 7 seconds | half a minute | Practically never |
| ±5 spd | half a minute | two and a half minutes | Setting it against a phone once a month |
| ±15 spd | under two minutes | seven or eight minutes | Noticeably off by month’s end |
| ±30 spd | three and a half minutes | a quarter hour | Every couple of weeks |
Most people who measure a mechanical watch for the first time land somewhere in the middle two rows and are surprised, usually because they had assumed it was worse.
The sign is not a grade
A watch that gains is not doing something worse than a watch that loses, and the reverse is equally untrue. The direction is a consequence of how the watch was adjusted and how it is worn, and either can be moved.
There is one practical asymmetry worth knowing. A watch that runs slightly fast is easier to live with, because a watch that is a little ahead never makes you late. Some watches are deliberately regulated to gain a touch for that reason, which is also why one of the two common reference bands, 0/+5, allows gaining only.
How precise the figure really is
A rate carries the imprecision of both readings it was calculated from, divided by the number of days between them.
That division does the work. Read a watch twelve hours after setting it and any small misjudgment gets doubled on its way into the answer. Read it a week later and the same misjudgment is divided by seven. The longer you wait, the less your own hand matters and the more the figure is about the watch.
This is why a rate quoted to a tenth of a second means something over several days and very little over an afternoon. The calculator refuses spans under twelve hours for exactly that reason.
Quartz uses the same unit and different numbers
A quartz watch is quoted in seconds per day as well, but the figures are perhaps twenty times smaller, and often given per month instead because a day is too short to show anything.
The measurement works identically. It simply needs longer. A week or two for a quartz watch does the job that a day or two does for a mechanical one.
Where this leads
A single figure is where this starts rather than where it ends. What you do with one, and what makes a rate worth trusting, is the next question.
Why your watch runs fast or slow covers what moves the number between readings, and what is good accuracy for a mechanical watch covers what to make of the figure once you have one you trust.