Free tool

Watch accuracy calculator

Compare your watch to an accurate clock twice, a day or more apart. We'll turn it into seconds per day and tell you whether that's within spec.

First check Seconds ahead (+) or behind (−)

Second check Seconds ahead (+) or behind (−)

Compare against Seiko 4R, NH and 7S movements (−35 to +45 s/day) Standard Swiss movements (about ±12 s/day) COSC chronometer (−4 to +6 s/day) METAS Master Chronometer (0 to +5 s/day) Rolex Superlative Chronometer (−2 to +2 s/day)

Use time.gov or your phone's clock as the reference, and note the difference at the same time of day each check.

How to measure it

1. Wind the watch fully and set it to the reference, or note how far off it is.
2. Wear it as normal for at least a day. Several days gives a steadier result.
3. Compare again and enter both readings above.

What counts as normal, and how to nudge the rate with overnight positions, is in How accurate should a mechanical watch be? A sudden jump to minutes a day usually means it's magnetized.

FAQ

How many seconds a day is normal for a mechanical watch?

Anywhere from a couple of seconds to about half a minute, depending on the movement. Seiko rates its 4R and NH calibres at −35 to +45 seconds a day; a COSC chronometer at −4 to +6.

How do I calculate my watch's accuracy?

Note how far ahead or behind the watch is against an accurate clock, check again a day or more later, and divide the change in seconds by the number of days.

Is it better if my watch runs fast or slow?

Most makers regulate slightly fast, because a small gain is easier to live with than being late. Consistency matters more than the exact number.

Why does my watch's accuracy change from day to day?

Position, how much it was worn, the state of wind and temperature all shift the rate a little. An average over several days is more reliable than a single day.