What does it mean when a watch is COSC Certified?

(Photo courtesy of: https://watchtime.me/the-knowledge/article/1251/all-you-need-to-know-about-cosc-chronometer-certification)

If you have read some of our previous posts, you know that the vocabulary within the watch world can be extensive and complicated. One of the terms, and in this case acronyms, that often comes up and is never really explained is COSC Certification. It sounds like something you might want for your watch because it sounds important and official, but we will dive a little into what it means for you and your timepiece.

COSC is an acronym for “Contrôle Officiel Suisse des Chronomètres”. In English, this translates to “Official Swiss Chronometer Testing Institute”. This is the body that is tasked with certifying accuracy and precision of watches made in Switzerland. Generally, this certification is reserved for higher-end luxury timepieces. Any chronometer made in Switzerland has to go through the COSC certification process in order to be labeled a chronometer. The COSC certification is relatively rare and hard to obtain as only about 3% of all Swiss watch production is given this title.

Interestingly, COSC is a non-profit organization that has been around since 1973, and today there are only three certified labs in Switzerland that can test and certify watches submitted by the watch brands. Of all the Swiss made watches, Rolex, Breitlingi, TAG Heuer, and Panerai are the ones that send the most watches for certification. Other countries like Germany and Japan have their own certification bodies, or even certify their watches internally. In Japan’s case, where they certify their watches internally, they actually aim for higher standards than COSC.

(Photo courtesy of: https://millenarywatches.com/what-is-cosc/) 

The testing for COSC certification is rigorous, detailed, and specific. Movements from the watches are removed and tested for over two weeks in five different positions at three different temperatures. The internals are left in each specific temperature for 24-hours in each position, then removed and the test repeats. Data is compiled using cameras to take measurements of the reactions of each movement. The data is calculated to find an average daily rate of accuracy. The COSC standards require the average to be between -4 seconds to +6 seconds off, but only once the following criteria are met, will certification be given:

1. Average daily rate
2. Mean variation rate
3. Greatest variation in rates
4. The difference between rates horizontal and vertical positions
5. Largest variation rates
6. Variation in rate depending on temperature
7. Rate resumption

As you can see, COSC Certification is not simply a fancy watch term. It ensures that the timepiece you purchase is the best of Swiss watchmaking.