"Chronometer" on a dial is not a style of watch. It is a certificate: an independent body, or the maker itself, has tested the watch's accuracy against a published standard. A chronometer is not the same as a chronograph, which is a watch with a stopwatch function. Here is what each certificate guarantees.
The standards compared
| Standard | Who tests | What is tested | Accuracy allowed per day |
|---|---|---|---|
| COSC (Swiss chronometer) | Independent Swiss institute | The movement alone, before casing: 15 days, 5 positions, 3 temperatures | −4 to +6 seconds |
| METAS Master Chronometer | Swiss federal metrology institute's protocol, used by Omega and Tudor | The finished watch, after COSC: accuracy, magnetism up to 15,000 gauss, water resistance, power reserve | 0 to +5 seconds |
| Rolex Superlative Chronometer | Rolex, after COSC | The finished watch | −2 to +2 seconds |
| Grand Seiko standard | Grand Seiko | The movement: 17 days, 6 positions, 3 temperatures | −3 to +5 seconds (mechanical) |
| Patek Philippe Seal | Patek Philippe | The finished watch | −3 to +2 seconds on most calibres |
COSC
The Contrôle Officiel Suisse des Chronomètres has certified Swiss movements since 1973. Each movement is tested individually for 15 days. Breitling certifies every watch it makes; Rolex, Omega, Tudor and Longines (on the Spirit line) certify many or all. COSC is the baseline that the stricter standards build on.
METAS Master Chronometer
Omega introduced this with the Swiss Federal Institute of Metrology in 2015. The crucial differences: it tests the cased watch, not the bare movement, and it adds resistance to magnetic fields of 15,000 gauss. Magnetism from phones, speakers and bag clasps is one of the most common causes of a mechanical watch running badly, so this matters in daily life. Tudor now certifies several models as Master Chronometers too. See our Omega Seamaster buying guide.
Rolex Superlative Chronometer
Since 2015 every Rolex is tested twice: the movement by COSC, then the cased watch by Rolex to −2/+2 seconds a day. The green seal that comes with the watch is the sign of it, along with a five-year international guarantee. Our guide to reading Rolex reference numbers helps you date a watch against this change.
Grand Seiko
Grand Seiko tests to its own standard, which is stricter than COSC for mechanical movements. Its Spring Drive movements, which combine a mainspring with a quartz-regulated glide wheel, are accurate to about one second a day. Read the Grand Seiko buying guide.
What this means when you buy
- A chronometer certificate describes the watch when it left the factory. A pre-owned watch's accuracy depends on its condition and when it was last serviced. That is why we put every watch on a timing machine before it ships. See our inspection.
- A few seconds a day is excellent. Even −4/+6 seconds a day is 99.99% accurate. Do not choose between two watches on certification alone.
- Anti-magnetism is the practical benefit. If you work around electronics, a Master Chronometer or a Rolex with a Parachrom hairspring will stay accurate with less fuss.
- Quartz is more accurate than any of them. If accuracy is all you want, see automatic vs quartz.
Frequently asked questions
What is the difference between a chronometer and a chronograph?
A chronometer is a watch certified for accuracy. A chronograph is a watch with a stopwatch function. A watch can be both.
Is METAS better than COSC?
It is stricter and more complete: it tests the finished watch to 0/+5 seconds a day and adds magnetism, water resistance and power reserve tests. A Master Chronometer has passed COSC first.
How accurate is a Rolex?
A current Rolex is certified to −2/+2 seconds a day when new.
Does a chronometer need more servicing?
No. Service intervals are the same as for any mechanical watch. See how often to service a luxury watch.
