A mechanical watch depends on a balance wheel swinging on a very fine spring. Temperature, the watch’s position and the tension of the mainspring change its rate slightly, so every automatic gains or loses a little. What separates one movement from another is how much, and how consistently.
What the makers publish
- Seiko 4R36 (5 Sports, Turtle, Presage Cocktail Time): −35 to +45 seconds a day.
- Seiko NH35A and Miyota 821A: −20 to +40 seconds a day.
- Seiko 6R35 (Alpinist, Presage Sharp Edged): −15 to +25 seconds a day.
- COSC-certified chronometer: −4 to +6 seconds a day, tested in a laboratory for fifteen days.
These are tolerances, not typical rates. Many watches from these families run well inside the limit, but one gaining 30 seconds a day within the tolerance is not faulty.
Why your watch gains or loses
- Reserve: with the spring nearly unwound, many movements lose time. Worn daily, the rate is steadier.
- Position: the same watch runs differently dial up and crown down. Leaving it overnight in one position or another can offset a drift.
- Magnetism: if it suddenly gains minutes a day, it is most likely magnetised. A watchmaker fixes it in minutes.
- Age: without servicing, the oils dry out and the rate gets worse.
How to measure and correct it
Set the watch to the second, ideally with hacking seconds, and compare it with a reference clock at the same time each day for several days. If the drift is outside the maker’s tolerance, a watchmaker can regulate it by adjusting the balance. It is a quick job if the watch does not need a full service.
A higher frequency helps keep the rate stable, but on its own it does not guarantee better accuracy.