Accuracy: what's normal, what's not.

Most complaints about a watch “running badly” are complaints about a watch running exactly as its specification promised. This lesson is about knowing the difference.

The unit: seconds per day

Mechanical accuracy is quoted in seconds per day, usually as a range with a sign: −10/+20 s/day means the watch may lose up to ten seconds or gain up to twenty in twenty-four hours. The sign matters. Makers deliberately regulate many movements to run slightly fast, because a watch that gains is a watch you are never late by.

Quartz is quoted per month instead, because the numbers would otherwise be absurd fractions.

What normal looks like

A standard, uncertified mechanical movement of the kind you find under $1,000 is typically specified somewhere in the region of −20 to +40 s/day, and commonly performs better than that once it settles. Citizen’s Miyota 8203, in the NY0086, is a good example of a movement people call inaccurate when it is in fact performing to its own published tolerance.

A quartz watch will usually be within about ±15 seconds a month. A radio-synchronised solar quartz, like the Casio GW-M5610U, corrects itself against a broadcast time signal and is effectively never wrong.

Positional error is the reason a mechanical watch can gain lying dial-up and lose standing on its crown: gravity pulls differently on the balance depending on orientation. Leaving a fast-running watch crown-down overnight is a genuine, free way to pull its daily average back toward zero.

Certification, and what it is worth

A chronometer is a movement that has passed independent testing to a defined standard. The best-known is COSC in Switzerland, whose mechanical standard is a mean daily rate of −4 to +6 s/day across a multi-day test in several positions and temperatures. Various makers run their own in-house standards, some stricter.

Certification is real and it is not free — but it certifies the movement, not the finished watch, and it says nothing about the case, the bracelet or whether you will enjoy wearing the thing. Treat it as one line on the spec sheet rather than a verdict.

When to worry

Worry when the rate changes suddenly, when the watch stops well short of its stated power reserve, or when it gains or loses in minutes rather than seconds. Those are symptoms of a movement that needs service, a magnetised hairspring, or damage. A watch that has quietly gained twelve seconds a day since you bought it is not broken; it is a watch, and a watchmaker can usually regulate it closer for a modest fee.