Guides Watches

What Does COSC Chronometer Certification Mean for Your Watch?


Whatever else your prized wrist candy may be, it’s first and foremost a timekeeper. And when it comes to doing the job it was originally built for, accuracy is everything. So how do we judge just how accurate a watch actually is? Well, you could sit there glancing at an atomic clock every hour to make sure your watch isn’t going out of time. You could hook it up to sensors to measure the frequency of the balance spring. Or you could do what most of the industry does and rely on COSC.

You’ll have seen COSC thrown about plenty of times, generally from watchmakers that want to shout about the precision of their movements. COSC-certified has become a byword for chronometric excellence, but who is it? What is it? And what does it actually mean?

What is COSC?

COSC, or the Contrôle Officiel Suisse des Chronomètres (Official Swiss Chronometer Control in English) is a major player in the watch world, but while it sounds like it was there in the early days of Swiss watchmaking, the actual organisation only cropped up in 1973. It came about for the sake of consistency. Each of Switzerland’s five main watchmaking cantons – Bern, Geneva, Neuchâtel,  Solothurn, and Vaud – were before then testing their watches independently. This means that there was no agreed-upon methodology, no central testing, and no way of comparing a watch from one region to a watch from another. So, the five cantons came together to set up COSC in La Chaux-de-Fonds.

COSC Certificate Example Omega

The idea was to provide a neutral, independent tester of Swiss movements to ensure a level of quality attached to the ‘Made in Switzerland’ moniker. This is why COSC only tests Swiss movements. And by Swiss I mean 60% of production costs and 50% of value must be Swiss. There’s obviously a lot of wiggle room there – I’m sure a fair few of us would have assumed that ‘Made in Switzerland’ would mean a much greater proportion – but it does exclude the accessible Japanese and Chinese movements, as well as a lot of the independent manufactures around the world. Fortunately it doesn’t matter too much for the former and the latter tend to go above and beyond COSC.

What Does COSC Test?


In brief, a lot. The idea is to test how a movement will work in any situation, but given that COSC has between 14 and 20 days to do it, they run a broad battery of different tests to gauge accuracy on their space-age machinery.

Each movement is placed in five different, pre-set positions: three o’clock, six o’clock, nine o’clock, with the dial facing up and with the dial facing down. This lets the technicians ensure that the balance spring is working the same no matter which way it’s facing in relation to gravity – gravity being a nefarious, slightly inconsistent issue when you’re talking about tolerances this small.

When we’re talking about such minute mechanics, temperature also comes into things and cold can affect a movement as much as heat. So, each movement is also tested at eight degrees, 23 degrees and 38 degrees Celsius. It’s a broad range of temperatures and designed to mimic the ranges in which most people will actually wear their piece. If you’re wondering why sub-zero temperatures on there, trust me, don’t wear your watch below zero.

COSC Certification Lab

It’s important to note that the staff at COSC are not watchmakers. They’re closer to scientists and as such can’t and won’t make adjustments to the movements. They just provide the data to help the actual watchmakers understand why their work either passed or failed, emphasising their independent, third-party nature.

As for what passes, there’s a specific range of timekeeping a movement needs to fall into: -4 to +6 seconds a day. So, it needs to lose no more than four seconds a day and gain no more than six. This is pretty damn accurate – and that’s just to start.

Tissot Heritage 1938 T142.464.16.332.00

Tissot Heritage 1938 with COSC-Certified calibre 2824-2 movement

Movements also have to meet other minimum requirements in their daily rate. For example, even if the average is, say -2, it’s still an issue if that’s averaged out from one insanely high number and one incredibly low. That would still indicate a serious problem. Rather than go through everything here though, check out the table, which has the various limits. Type 1 and Type 2 simply refer to watch movements of different sizes, with Type 1 being the larger. Smaller movements are given a bit more leeway in certification.

What Are The Problems With COSC?

Often COSC-certified (the coolest of watch nerds call it ISO 3159 standard) and chronometer are used interchangeably, but it has to be noted that while all COSC-certified movements are chronometers, not all chronometers are COSC-Certified. A movement can be perfectly capable of passing the tests, making it of chronometer standard. But it might just not have been sent to the organisation to get its paperwork done. It might be that it’s not Swiss or it might be that the maker just didn’t want to send their movement for testing.

This can be for any number of reasons, but the most common is the cost. While COSC is a non- profit, it still costs brands a fair bit of time and money to put their movements through the wringer and not every watchmaker agrees that those four little letters are worth it, no matter the prestige, especially at the more accessible end of the scale. It can be easier to just not bother. There are also a few issues with the certification itself. All the tests we’ve gone over are administered on the movement, not the fully- assembled watch. If any problems cropped up during casing or the chronometric get altered once you attach the various complication plates, you would never know.

Finally, there are the testing brackets. There’s only so many tests you can do before things start getting ridiculous, but a broader range of temperatures (under eight degrees is a regular occurrence in a lot of places) and positions would give a better indication of how good a movement is.

Does COSC Matter?

Tudor Pelagos FXD Navy

Yes and no. As I’ve mentioned, COSC is an independent organisation so cynically labelling them as a marketing tool is a tad disingenuous. And it is useful for the industry at large to have a benchmark to hit, both for consistency and so that we as consumers know what we’re buying. But at the same time, they are here to promote solely Swiss-made movements, which is good for all of the brands and cantons involved.

The bottom line however is that if you’re not particularly into browsing esoteric rate statistics, then COSC is an easy-to-understand seal of quality. It’s not necessarily that the testing itself matters, but that it makes the concept digestible.

Are There Alternatives To COSC?

Tudor MT5602 Master Chronometer Movement

Tudor’s Master Chronometer calibre MT5601 undergoes both COSC and METAS certification


While not as well-known as COSC, the Chronofiable standard has been around just as long. Created by the Federation of the Swiss Watch Industry, today the independent testing centre is the Laboratoire Dubois SA, also in La Chaux-de-Fonds. Largely they measure the same things as COSC, but include shock tests and simulate six months of wear in 21 days to really put each movement through its paces.

Geneva Seal

Before 2011, the Poinçon de Genève was entirely about the finishing of movements from Geneva-based watchmakers. It dates all the way back to the late 1800s, but that hasn’t stopped it modernising. Today, it will test functionality, accuracy, water resistance, and power reserve, making it one of the most all-encompassing tests – just for very few qualified watchmakers. And still with a major focus on decoration.

Besancon Observatory

The Besancon Observatory dates back to the early days of accuracy certification, when they handed out prizes to watchmakers in the 1800s. Now, they have an ISO Standard programme, but where COSC only tests movements, they only test cased watches, eliminating one of the main issues with COSC. They only do a relative handful of watches, however.



You’ll have seen this on many an Omega in recent years – and a Tudor – as Master Chronometer certified, METAS is essentially the next level of COSC. It came about in 2014 when Omega partnered with the Swiss Federal Institute for Metrology, but there’s no reason other watchmakers can’t sign up. Once a movement has been COSC-certified and cased into a finished watch, METAS put it through a battery of tests designed to check its resistance to shocks, water, magnetism and temperature, all while maintaining superb accuracy. In terms of magnetism, that means resistance to 15,000 Gauss; in terms of accuracy, that means 0 to +5 seconds a day.

Qualite Fleurier Seal

Like the Geneva Seal, this one is specifically for the resident watchmakers of Fleurier – Bovet, Parmigiani, Chopard, et al – and is intense. It has to pass five different tests. First, it has to be entirely manufacturers in Switzerland. Then, it has to pass the Chronofiable test and be COSC-Certified. THEN it must pass an inspection under a microscope before going through a 24-hour wear stress test where it must manage a 0 to +5 second accuracy. It’s something to be proud of, and it’s reassuring if you see the FQF logo on the movement.


Jaeger-LeCoultre calibre 759

Jaeger-LeCoultre’s calibre 759 with “1,000 Hour Control Certification” testing

Alongside the above independent seals and accuracy insurance tests, many watchmakers have their own, in-house standards. These vary from watchmaker to watchmaker and are largely tailored to what they can achieve. The best known is Jaeger-LeCoultre’s 1,000-Hour Control Program, a standard they’re so proud of it’s in the name of some of their watches. The most recent on the other hand is Rolex’s Superlative Chronometer Certification, which requires a movement to pass COSC and then maintain an incredible accuracy of -2 to +2 seconds per day.

There are many, many more these days, but they all have the same problem: they are all in-house rather than independent, so the slightly paranoid among us may assume that there are no checks and balances.

To Conclude

Rolex Superlative Chronometer Certification

There are a multitude of different tests, certifications, and seals out there to denote the accuracy of a watch. To some extent, it’s useful to know just how accurate your watch is, to appreciate the level of performance a hand-made mechanical timepiece can have.

But be careful not to assume a movement is terrible just because it’s not COSC-Certified. Just because someone doesn’t have a degree, doesn’t mean they’re not intelligent; it just means they didn’t think getting into tens of thousands of pounds of debt for a certificate was worth it. Or, if we’re pushing this painfully extended metaphor even further, they couldn’t get a VISA to study here.

I guess this makes METAS a Master’s. And the Fleurier Seal that one guy that never wants to leave university. Ever.

More details at COSC.

Leave a Comment



About the author

Sam Kessler

Legend has it that Sam’s first word was ‘escapement’ and, while he might have started that legend himself, he’s been in the watch world long enough that it makes little difference. As the editor of Oracle Time, he’s our leading man for all things horological – even if he does love yellow dials to a worrying degree. Owns a Pogue; doesn’t own an Oyster Perpetual. Yet.