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COSC VS Chronometer

In the watch community, COSC and chronometer are often used interchangeably, sometimes incorrectly. It’s like whiskey and bourbon: all bourbon is whiskey, but not all whiskey is bourbon…

“All COSC certified watches are chronometers, but not all chronometers are COSC certified” -Caliber Corner

What is a chronometer?

The dictionary definition of chronometer:

1) A timepiece or timing device with a special mechanism for ensuring and adjusting its accuracy, for use in determining longitude at sea or for any purpose where very exact measurement of time is required. 2) Any timepiece, especially a wristwatch, designed for the highest accuracy. –source

What is COSC?

The term COSC is an acronym for the Contrôle Officiel Suisse des Chronomètres. COSC is a Swiss testing institute that certifies the accuracy of precision instruments (a fancy way of saying watches). COSC is responsible for issuing certificates for watches tested in Switzerland that meet chronometer specifications, in accordance with ISO 3159.

COSC Chronometer Rating Requirements:

Among other requirements, a COSC certified chronometer must have the following basic attributes:

  • The watch must be Swiss Made
  • The movement must keep time within an average rate of -4/+6 seconds per day

Tests are conducted for at least 15 days, in 5 positions, in 3 different temperatures.

Paperwork and Identification:

COSC certified watches will often include a chronometer certificate. Some collectors simply refer to this as the COSC. The COSC paperwork will have a number which should correspond to a number on the movement. Just because the box and papers includes COSC paperwork doesn’t mean it is the correct certificate for the watch. To ensure that you have the correct papers, you should try to match it with the number on the movement.

According to COSC:

“Compliance with the definition of a chronometer is approved by a neutral official body, which carries out the checks on the watch or movement, and issues an official certificate. In Switzerland, each chronometer is unique, identified by a number engraved on its movement and a certificate number issued by the COSC.” –source

This Wikipedia entry claims that COSC issues over 1,000,000 certifications per year.

Examples of COSC paperwork:

Some watch companies add their own branding to the COSC certificate.

Others, like Rolex, even proudly print the designation on the dials of their watches and include a special COSC hangtag.

Like Whiskey and Bourbon

As mentioned above, not all chronometers are COSC.

Keep in mind that just because a watch is not COSC certified does not mean it doesn’t keep time within Chronometer specs, it just means that it is not officially tested and certified by COSC. For a Swiss Made watch to have the word Chronometer on the dial or case, it has to be certified by COSC.

You may find watches that were not made in Switzerland using the term Chronometer – this Ginault for example.

Additional Resources:

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Keith Nelson
Keith Nelson
7 months ago

This clarifies a lot for me. I was looking at a King Seiko 4502 with Superior Chronometer Officially Certified written on it, and I was wondering how a Japanese watch maker went about getting it certified by the Swiss. So basically the Swiss regulate the use of the word Chronometer within Switzerland but have no control over how it is used outside of Switzerland. This makes me want the 4502 even more now!

Donald Perley
Donald Perley
6 months ago
Reply to  Keith Nelson

My understanding is that COSC added the “Swiss made” requirement shortly after Seiko started submitting movements for certification, and were doing a little too well in accuracy competition. It wouldn’t look good if the majority of COSC chronometers were from Japan!

3 months ago
Reply to  Keith Nelson

Or course, the Swiss Watch Industrie has no grip on the rest of the world. The introduction of a certification in Germany is an illustration of that.

Sellita Caliber SW215-1 Watch Movement | Caliber Corner
4 months ago

[…] COSC VS Chronometer […]

3 months ago

Germany has it’s own certification, in Glashütte I believe.

And yes, other watches may be precise as well. Longines has it’s own tradition, stemming from the sports events. Now of course they have a line with COSC-Chronometers.

Seiko does not really need a COSC -Qualification. If it wanted, their association could set up a certification process, even though the association probably is all Seiko.

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