How accurate should a mechanical watch be?
What seconds per day means, what COSC and other standards allow, and simple ways to measure and improve how your watch keeps time.
Typical numbers
- COSC chronometer certification allows -4 to +6 seconds a day for the bare movement.
- Many brands promise tighter in-house standards, such as -2 to +2 seconds a day.
- Everyday movements without certification often run within about -20 to +40 seconds a day.
How to measure it
A single day is noisy, because rate changes with position, temperature and how wound the watch is.
- Set the watch to the exact second.
- Wear it normally for at least three days.
- Note how far ahead or behind it is, and divide by the days elapsed.
Nudging the rate without a watchmaker
A watch runs at different speeds in different positions. Try leaving it overnight dial up, crown down or crown up, and note which position makes it gain or lose. Choosing the right overnight position can cancel out a few seconds a day. If it gains or loses minutes a day, it may be magnetised.