Chronometer standards, explained

What COSC and METAS actually certify — the real criteria, and what they leave out.

"Chronometer" is a certification, not a compliment

In watchmaking the word has a legal meaning. A chronometer is a watch whose movement has been individually tested by an independent body and passed. It is not a marketing adjective, and it is not the same word as chronograph, which is a stopwatch complication and says nothing about accuracy.

COSC

The Contrôle Officiel Suisse des Chronomètres tests uncased movements over 15 days, in 5 positions and at 3 temperatures. Seven criteria must all pass; the one everyone quotes is the first.

Criterion	Requirement
Mean daily rate	−4 to +6 s/day
Mean variation in rate	2 s/day max
Greatest variation in rate	5 s/day max
Difference, horizontal vs vertical	−6 to +8 s/day
Largest variation from the mean	10 s/day max
Rate change per °C	±0.6 s/day max
Rate resumption	±5 s/day max

Note what this does not cover: the movement is tested before it goes into a case, and the certificate belongs to that movement, not to the watch you own today. A COSC movement that has been cased, knocked about for six years and never serviced is not running to COSC any more.

METAS Master Chronometer

Introduced by Omega with the Swiss Federal Institute of Metrology, METAS certification is applied to the fully cased watch, after COSC certification of the movement. Eight criteria, and two of them are the interesting ones.

Criterion	Requirement
Average daily rate	0 to +5 s/day
Magnetic resistance	Functions in 15,000 gauss
Rate after magnetic exposure	Within tolerance, before and after
Water resistance, power reserve, deviation across positions and wind states	All to the stated spec

The 15,000 gauss figure matters more in daily life than the extra second of rate does. Magnetism is the single most common cause of a suddenly and wildly inaccurate mechanical watch, and it comes from ordinary things — laptop speakers, magnetic clasps, tablet covers.

The others you will see

Grand Seiko Special / Standard — Seiko's own in-house standards, tested in more positions than COSC. The Special standard is tighter than COSC (+4/−2 s/day static).
Patek Philippe Seal, Rolex Superlative Chronometer — in-house standards applied to the cased watch after COSC, both quoting −2/+2 s/day. In-house, so self-certified, but both are stricter numbers than COSC.
Qualité Fleurier, Geneva Seal — these certify finishing and origin as much as rate. Do not read them as accuracy claims.

Can I tell if my watch would pass?

Not officially — certification requires the testing body, and for COSC it requires the movement out of the case. But you can get a well-founded estimate: measure your daily rate over several days, measure it in the standard positions, and compare against the windows above. That is exactly what Chronograde's Chronometer Score does — it grades your actual readings against the COSC and METAS windows and tells you which criterion is the one holding you back.

It is worth being clear that this is an estimate from your own measurements, not a certificate, and Chronograde says so in the app.