Compare your watch with an exact time source and note how many seconds it is ahead (+) or behind (−). Do it again a day or more later, then enter both readings and the hours between them.
How it works
The rate is the change in offset divided by the days between the readings. In the example above, (4.2 − 1.0) ÷ 1 day = +3.2 s/d. Over 30 days that adds up to 96 seconds: the watch would be 1 minute 36 seconds fast by the end of the month.
Tips for a reliable result
- Use an exact reference such as atomic time, not another clock or a computer that may itself be off.
- Leave a day or more between readings: the longer the gap, the less a slightly early or late reading matters.
- Wear the watch as you normally do, or note how it rested: position and how well it is wound both change the rate.
- Wind a hand-wound watch fully at the same time each day.
Reading the result
A certified chronometer is tested to stay within −4 and +6 seconds a day. Tolerances for other movements are wider: Seiko's 4R36, for example, is specified at −35 to +45 seconds a day. Quartz watches usually stay within half a second a day.
Gaining far more than usual, perhaps minutes a day? Magnetism is the likeliest cause: see why a watch runs fast or slow. For how to take the readings, see how to measure your watch's accuracy.