What is a chronometer?

A chronometer is a watch whose movement has been officially tested and certified for accuracy. In Switzerland the testing body is COSC, which certifies a mechanical movement only if it averages within −4 to +6 seconds a day over about two weeks of tests in different positions and temperatures. A chronometer is not the same as a chronograph, which is a watch with a stopwatch.

Swiss testing body	COSC (Contrôle Officiel Suisse des Chronomètres)
Mechanical standard	−4 to +6 seconds a day
Test length	About 15 days
Tested in	5 positions, 3 temperatures
Rolex standard	−2/+2 seconds a day, cased
Original meaning	Harrison's marine chronometer for navigation

How COSC certification works

COSC is an independent Swiss institute. Watchmakers send it uncased movements, each with a serial number, and it tests them for about 15 days in five positions (dial up, dial down, crown in different directions) and at three temperatures, roughly 8°C, 23°C and 38°C. To pass, a mechanical movement must average −4 to +6 seconds a day and meet limits on how much its rate varies. Quartz movements can also be certified, to a much tighter standard. Only movements that pass can be called "chronometers" and carry the word on the dial.

Stricter in-house standards

Rolex Superlative Chronometer: after COSC, Rolex tests the finished, cased watch to −2/+2 seconds a day, plus water resistance, self-winding and power reserve. See why Rolex costs so much.
Omega Master Chronometer: tested with the Swiss federal metrology institute METAS to 0/+5 seconds a day, including exposure to strong magnetic fields.
Grand Seiko: its own standard of −3 to +5 seconds a day for mechanical models.

Chronometer vs chronograph

	Chronometer	Chronograph
Meaning	A certified accurate watch	A watch with a built-in stopwatch
How to spot it	"Chronometer" on the dial, a certificate	Pushers beside the crown and small subdials
Example	Rolex Submariner	Omega Speedmaster
Can it be both?	Yes: a chronograph whose movement is chronometer-certified, like the Rolex Daytona

The original chronometer

The word first meant a marine chronometer: a clock accurate enough to find longitude at sea. John Harrison's H4, completed in 1759, solved that problem, and ships carried chronometers until satellite navigation replaced them. Read the story in the longitude problem and John Harrison.

Questions people ask

What is a chronometer watch?

A watch whose movement has passed an official accuracy test, in Switzerland by COSC, to within −4 to +6 seconds a day for mechanical movements.

What is the difference between a chronometer and a chronograph?

A chronometer is certified accurate. A chronograph has a stopwatch function. A watch can be both.

Is a chronometer more accurate than quartz?

No. Even a certified mechanical chronometer is less accurate than a typical quartz watch.

Is a Rolex a chronometer?

Yes. Rolex movements are COSC certified, and Rolex then tests the finished watch to its stricter −2/+2 seconds a day Superlative Chronometer standard.

Sources

Last updated October 18, 2026. Prices are auction hammer-plus-premium results or published list prices as reported by the sources below; retail prices change, so check an authorized dealer before buying. Brand and model names belong to their owners and are used here only to identify the watches; onlineclock.app is not affiliated with any watch brand. Found an error? Tell us.