Chronometer (COSC)
A chronometer is a watch whose movement has been tested and certified for accuracy by an independent body. In Switzerland that body is COSC, and the movement must keep to about minus four to plus six seconds a day. It is an accuracy grade, not the same thing as a chronograph.
At a glance
- Certifying body (Switzerland)
- COSC, independent
- Allowed daily rate
- About minus four to plus six seconds
- What is tested
- The movement, not the whole watch
Calling a watch a chronometer is not a marketing claim the maker can grant itself; the accuracy has to be measured by a body independent of the manufacturer. In Switzerland that body is COSC, and the certificate is awarded to the movement and its daily rate, not to the finished watch as a whole.
What the certificate measures
The judgement rests on one thing: how far the movement runs fast or slow over a day. The window is narrow.
- Upper limit: about plus six seconds a day
- Lower limit: about minus four seconds a day
- The subject: the accuracy deviation, not the case or the looks
A mechanical watch that stays inside that band can be called a chronometer, sitting alongside other accuracy grades under standards.
A chronometer is not a chronograph
The two words look alike but mean different things. A chronometer is an accuracy grade; a chronograph is a complication that works as a stopwatch. For a look at how accuracy is approached in Japanese watchmaking, read our guide to the best Japanese watches.
Examples
The word "chronometer" printed on a dial or caseback means the movement was tested by an independent body such as COSC and held to about minus four to plus six seconds a day.
Comparison
Chronometer and chronograph are often confused but are separate ideas.
| Option A | Option B | Notes |
|---|---|---|
| Chronometer | Chronograph | A chronometer is an independently certified accuracy grade; a chronograph is a stopwatch complication. |
Related terms
Frequently asked questions
What is the difference between a chronometer and a chronograph?
A chronometer is an accuracy grade: the movement is tested by an independent body such as COSC and certified to about minus four to plus six seconds a day. A chronograph is a separate thing entirely, a complication that works as a stopwatch.
How accurate must a watch be to qualify as a chronometer?
The movement must keep to about minus four to plus six seconds a day. That is measured by an independent body, COSC in Switzerland, rather than by the maker, and the certificate is awarded to the movement rather than the whole watch.