COSC certification in watchmaking
Precision is one of the fundamental considerations when choosing a mechanical watch and, in this context, the designation ‘chronometer’ is tied to a very specific level of exactitude.
In this article we take an in-depth look at COSC certification in order to understand what this certification involves within watchmaking.
What is COSC certification?
COSC stands for Contrôle Officiel Suisse des Chronomètres, the independent Swiss body that verifies the precision of Swiss Made movements and watches.
When a piece passes its tests, it earns the official designation of chronometer in accordance with ISO 3159 in the case of mechanical wristwatches.
In the traditional format, known as ‘Chronometer Certified’, each movement is examined individually before it is fitted into the watch; for this reason, the certificate is awarded to that particular calibre and not to the fully assembled watch.
In 2026, the body also announced ‘Excellence Chronometer Certified’, a complementary certification that extends the assessment to the fully assembled watch.
In this article we focus on the classic certification that accompanies selected TUDOR calibres.
How is the certification obtained?
To obtain COSC certification, the watch houses themselves send their movements to the Contrôle Officiel Suisse des Chronomètres.
There, each unit is examined individually and, in the case of a mechanical movement, it undergoes 15 days of measurements in five different positions and at three temperatures: 8 °C, 23 °C and 38 °C. These tests make it possible to verify how factors such as position and temperature influence the precision and consistency of the movement.
During the process, seven performance criteria are analysed, among them the average daily rate, the variations in precision between different positions, the influence of temperature and the movement’s ability to recover its rate after being subjected to different conditions.
To pass certification, a mechanical movement must record, among other requirements, an average rate of between −4 and +6 seconds per day.
TUDOR, for example, goes beyond this requirement in numerous models equipped with its COSC-certified Manufacture Calibre, applying its own standard of −2/+4 seconds per day measured on the fully assembled watch, compared with the margin of −4/+6 seconds per day that COSC sets for the movement.
Only movements that meet all the required criteria officially obtain certification as a COSC chronometer.
In addition, each certified chronometer is identified by a unique number engraved on its movement, which makes COSC certification an independent, objective attestation of the precision achieved during testing.
COSC-certified watches at The Red House
At The Red House you will find different interpretations of TUDOR watchmaking with COSC-certified Manufacture Calibres.
TUDOR Black Bay 54
The TUDOR Black Bay 54 revives the aesthetic of TUDOR’s earliest diving watches with a 37 mm steel case and a bezel without minute graduations.
Its MT5400 Manufacture Calibre, water resistance to 200 metres and balanced proportions make it an elegant and versatile watch.
TUDOR Ranger
With a 36 mm satin-finished steel case and a domed beige dial, the TUDOR Ranger stands out for its functional design and excellent legibility.
Its water resistance to 100 metres reinforces its robust character, ready for everyday wear.
TUDOR Black Bay Pro
Designed for those who need to keep track of two time zones, the TUDOR Black Bay Pro features the MT5652 Manufacture Calibre with an integrated GMT function.
Its 39 mm case, fixed 24-hour bezel and distinctive yellow Snowflake hand combine functionality, robustness and an instantly recognisable aesthetic.
TUDOR Pelagos FXD
The TUDOR Pelagos FXD expresses one of TUDOR’s most technical facets.
Its 42 mm titanium case, fixed strap bars and 60-minute ceramic bezel reflect a professional orientation, accompanied by the MT5602 Manufacture Calibre and water resistance to 200 metres.
TUDOR Black Bay Chrono
The TUDOR Black Bay Chrono combines the legacy of diving watches with the language of sports timing.
Its 41 mm steel case, opaline dial with black counters and tachymeter bezel accompany the MT5813 Manufacture Calibre, equipped with a chronograph function and certified by COSC.
If you would like to compare these references, check their availability or receive personalised advice, our team at The Red House in Gibraltar will be delighted to assist you, answer your questions and guide you in choosing the watch that best suits your preferences.
