You'll see "Chronometer" on many watch dials and marketing materials. It sounds impressive—but what does it actually mean? Is it worth paying extra for? This guide explains COSC certification clearly.
What is COSC?
COSC (Contrôle Officiel Suisse des Chronomètres) is an independent Swiss organization that tests watch movements for accuracy. A movement that passes becomes a "certified chronometer" and can display that term legally.
COSC Requirements
To earn certification, a movement must maintain accuracy of -4 to +6 seconds per day across 15 days of testing in 5 positions and 3 temperatures. About 3% of Swiss movements undergo COSC testing; most pass.
The Testing Process
- Duration: 15 days of continuous testing
- Positions: Tested in 5 different positions (dial up, dial down, crown up, crown down, crown left)
- Temperatures: 8°C, 23°C, and 38°C
- Criteria: Mean daily rate -4/+6 sec, mean variation 2 sec, max variation 5 sec, position deviation 10 sec
- Result: Certificate issued for each movement with individual serial number
What COSC Means Practically
A COSC-certified watch should gain or lose no more than about 2-3 minutes per month in normal wear. That's noticeably better than non-certified movements, which might vary ±15-20 seconds daily.
💡 The Reality Check
COSC tests movements, not complete watches. Once a movement is cased, accuracy can change. Many COSC watches perform better than certification requires; some perform slightly worse. It's a minimum standard, not a guarantee.
COSC vs Other Standards
Omega Master Chronometer (METAS)
Tests complete watches (not just movements) for accuracy AND magnetic resistance to 15,000 gauss. Stricter than COSC: 0/+5 sec/day. Currently the highest mainstream standard.
Grand Seiko Standards
GS Special Standard requires -3/+5 sec/day—tighter than COSC. Tested in-house to their own rigorous protocols. Spring Drive achieves ±1 sec/day, exceeding any mechanical certification.
Rolex Superlative Chronometer
After COSC certification, Rolex tests complete watches to -2/+2 sec/day—significantly stricter. All modern Rolex watches meet this standard.
Brands Using COSC
- Rolex: All movements COSC certified (then further tested)
- Omega: Most models (now many are Master Chronometer)
- Breitling: All in-house movements
- Tudor: In-house MT movements
- TAG Heuer: Select models
- Tissot: Certain models (Powermatic 80 Chronometer)
- Longines: Select models
- Mido: Chronometer models
Is It Worth the Premium?
COSC certification typically adds $100-300 to a watch's price. Worth it if:
- You notice seconds-per-day accuracy differences
- You set your watch precisely and want it to stay accurate
- The certification validates quality control
- Resale value matters (certified watches often retain value better)
Not worth it if:
- You set your watch by phone daily anyway
- A few seconds variation doesn't bother you
- The premium is better spent on other features
The Bottom Line
COSC certification is meaningful but not transformative. It guarantees a minimum accuracy standard, indicates quality control investment, and provides documentation. But a non-COSC watch from a good brand can perform equally well—they just haven't paid for the certificate.