Let’s navigate the intricacies of precision timekeeping…

Ever wondered what the COSC status stands for in a luxury watch and what it means for your timepiece? Read this article to learn more.

When searching the market for a luxury timepiece, you may have come across four letters: COSC. The letters are found in many watches that have been tested and certified by an important institute. But what does it actually mean for a watch and you as the wearer? We’re going to uncover the secrets of a chronometer and how these timepieces guarantee a level of accuracy and precision that supersedes any standard watch.

What is a chronometer?

COSC-certified watches refer to timekeeping instruments that have been tested by the Contrôle Officiel Suisse des Chronomètres, which translates to the Swiss Official Chronometer Testing Institute. It is where chronometer watches are subjected to a rigorous testing schedule to determine their accuracy. Not all luxury watches are put to the test in this way but those that pass are classified as COSC-certified and can be called a chronometer. That said, not every chronometer is COSC certified, yet every COSC watch is a chronometer. The stamp is associated with a high level of quality craftsmanship and a reputable brand name that prides itself on the reliability of its timepieces. Hublot is an example of a manufacturer that does not produce movements that have been certified by COSC. Instead, its movements such as the UNICO calibre meet the brand’s own standards and are well within the same parameters.

The science behind precision timekeeping

The mechanism of a mechanical watch is moved by power that is generated as the mainspring unwinds. However, the fragile metal parts responsible for this are often influenced by environmental factors which can affect accuracy and precision. To measure how well some movements operate under environmental factors, they are put to the test.

COSC certification is not easy to acquire. Once the movement has been uncased it is subjected to tests that run for a total of 15 days. It is also tested in five positions and at three different temperatures. The movement is tested for 24 hours in each position and at each temperature. Once the process is complete the data is collected from the testing phase and the results are compiled where they must meet a set of criteria. To break the criteria down, we have listed the specifications that each movement must meet to be classified as a COSC-certified chronometer.

Standards and certifications for chronometers:

  • The movement must uphold an average daily rate of between -4 and +6 seconds
  • The mean variation in rate over the entire testing period must be 2
  • The greatest variation rate over any two days within the testing process must not exceed 5
  • The largest variation in rate must be no more than 10
  • The temperature variation must be +/- 0.60
  • The rate resumption must be +/- 1.5

Why is COSC certification important in a watch?

Why is COSC certification important in a watch?

The importance of owning a COSC-certified watch lies with the individual wearing the watch. While some collectors feel that the accuracy of a wristwatch is the most important aspect, others prefer to focus on other aspects, such as its design features or the history behind a particular model. In terms of evaluating the quality of a wristwatch before buying one, however, COSC certification gives a good indication of its reliability and accuracy.

Many serious watch collectors will consider nothing less than a COSC-certified chronometer. They believe that if a brand is going to the lengths of getting its movements precisely and stringently tested in this way, it is likely that the same values of quality are upheld across the entire design of a watch as a whole. This level of testing reassures you that the movement inside your watch, provided you care for it and take it for regular servicing, should last an entire lifetime and will always keep accurate time.

In terms of investment, the COSC-certified status of a chronometer can add value to your watch. Therefore, if you are looking for an investment piece and you see the COSC rating on a watch, you know you’re spending your money on a timepiece that will hold its value well. To give you some insight into the rarity of this kind of rating, only around 6% of all Swiss watches have COSC certification. It is therefore plausible to say that owning one of these chronometers automatically puts you in an exclusive group of watch owners.

Lastly, owning a watch that has been tasted and merited by the Contrôle Officiel Suisse des Chronomètres, dispels any worry you may have about the authenticity of a luxury watch. To demonstrate this, each movement is stamped with a unique number that relates to its certification process, verifying its legitimacy.

Notable chronometer watches and their features

Notable chronometer watches and their features

In many circumstances, a chronometer-certified watch will display other innovations as part of its movement. These features will help the engine run smoothly and precisely. Watch brands like Longines, for example, are powered by chronometer movements that have been fitted with a silicone balance spring. Made from this material, the hairspring guarantees longevity. As the watch’s regulating organ, it oscillates while maintaining a regular rhythm. Because it is made from silicone, the component is resistant to magnetic damage and atmospheric pressure. Both of these factors can inflict damage on the accuracy of the movement, affecting the way it performs.

There are currently two collections of Longines watches that offer a COSC chronometer status. These are the Record collection and the Spirit collection. The latter is designed for pioneers of the land and sky and offers a classic pilot’s watch design with a dial that is stamped with five stars above the 6 o’clock location, indicating chronometer verification and an improved warranty from two years to five.

The origins of the chronometer

Balancing the delicate mechanics of a traditional mechanical timepiece is no mean feat. In fact, it poses a whole different challenge to manufacturers compared to those who specialise only in making quartz watches. To create one single timepiece promising a high degree of precision is a painstaking challenge, let alone thousands of watches per year. It requires hands-on expertise. Back in 2019, it was estimated that around 2.15 million watches graduated with a COSC distinction. Much of this number was taken up by Rolex, closely followed by Omega. Other brands include Breitling, Tudor, Tissot, Zenith, Chopard, Bremont and Panerai.

Before the birth of COSC certification in 1973, watch brands were able to attend annual observatory trials where one or two selected timepieces could be submitted for intense scrutiny. The Key Observatory in London was a prestigious trial where the likes of Mr. John Harrison had proved that a robust and reliable chronometer was needed for navigating the rough seas. As a result, he won the Longitude Prize. Today, many brands employ their own testing process to determine the accuracy and reliability of their chronometers.

Popular chronometer manufactures

Brands that manufacture chronometer watches are loved the world over for their accuracy and their symbolic status within the industry. By upholding a reputation for creating high-quality timepieces, designed to last an entire lifetime, many are considered leaders in the world of horology.



Omega has been offering chronometer-certified watches in the form of its Master Chronometer watches since 2015. This new quality standard for chronometer watches was initiated by Omega to ensure each watch reaches a level of resistance and precision that has never been met before. Part of the tests that these watches undergo deem them resistant to magnetic forces of 15,000 gauss. Some 8 tests carried out over 10 days guarantee an additional level of reliability. At the end of these tests, a variation of 0-5 seconds per day is awarded to these timepieces. Some of the most accurate chronometers by Omega include the Speedmaster – an iconic watch known for its first appearance on the moon in 1969.

TAG Heuer

TAG Heuer

As a brand that is synonymous with timekeeping on the racetrack, the Swiss company upholds its strong roots to the motorsport industry with some movements that are COSC-certified. One model that has been awarded with this status includes the Autavia Flyback Chronometer. Being a flyback chronograph, the wearer can operate the stopwatch features with one single push of a button. All the while the COSC-certified movement guarantees a power reserve of 80 hours. The Aquaracer Professional 200 is another popular model with COSC certification. The dive watch upholds a 200-metre water resistance, while the Carrera chronograph Tourbillon shows a slice of its movement through a unique open heart window at 6 o’clock.



Guarding the chronometric precision of a Rolex watch is the Parachrom hairspring. You may have heard of this movement component before but were unsure as to what its role is. First released in 2005, it revolutionised the accuracy of a Rolex watch. In addition to this, all Rolex watches have a “Superlative Chronometer” status, which the brand formulated for itself during the 1950s. More rigorous than existing watchmaking norms, the criteria were reinforced once again in 2015 to establish new standards of excellence. This extensive criteria is an add-on to the already-awarded status of a COSC certification in each Rolex watch. It exceeds watchmaking norms and guarantees an accuracy of +/-2 seconds per day.

In a nutshell

Not all chronometer watches are COSC-certified but those watches that do carry this status are all chronometers. A chronometer, however, is a high-precision watch that has undergone extensive tests to determine its reliability and accuracy. Some chronometers are tested under the guidance of the Contrôle Officiel Suisse des Chronomètres, others a series of equivalent in-house tests. Watches that earn this certification have a deviation of just -4/+6 seconds per day yet those manufacturers who subject their movements to additional criteria can promise an even better level of accuracy.

Not just a fancy certificate, however, COSC certification in a watch can provide reassurance when exploring the market for an investment. Many can be found here at Watches World. For more news-related topics and articles on the latest chronometers here at Watches World, browse our expansive online collections today.