Accuracy Calculator
Seconds a day, and whether that figure is a fault or simply what the movement does.
+7.0 s
Within specification
Where a movement of this kind should be. Leave it alone.
Specification for this movement: -10 to +20 s/day
Measure over several days, wearing the watch as you normally would. Position, temperature and state of wind all move the figure, which is why a single overnight reading proves little.
Seconds a day, and whether yours is a problem
Mechanical watch accuracy is quoted in seconds per day, gained or lost. The measurement is simple — set the watch against a reliable reference, wear it normally for a few days, then read the difference and divide by the number of days. What people actually want to know is whether the figure they got is normal, and that depends entirely on what is inside the case.
An uncertified automatic is doing well within about ten seconds a day either way. A chronometer-certified movement is held to −4/+6 seconds a day. A quartz movement is quoted per month rather than per day, and a good one drifts less in a year than a mechanical watch does in a week.
How to time a watch properly
Set the seconds hand against an atomic clock or a phone's network time, note the exact moment, then leave the watch alone and wear it as you normally do for at least three days. Read the difference at the same time of day. A single overnight reading measures your eyesight as much as the watch.
Position matters more than most people expect. A watch resting dial up overnight often runs differently from one resting on its crown, so a watch that is fast on the wrist and slow on the nightstand can be brought closer simply by changing where you put it down.
What each kind of movement should hold
Specifications differ by movement, not by price. A chronometer-certified calibre is tested to −4/+6 seconds a day before it is cased; a good modern automatic is usually specified somewhere around −5/+10; an ordinary automatic or manual movement is doing what it was built to do inside −10/+20. Vintage movements are held to nothing at all, and twenty or thirty seconds a day on a fifty-year-old watch is not a fault.
Quartz is a different scale entirely. A standard quartz movement is quoted at about fifteen seconds a month, and a high-accuracy one at ten seconds a year. Comparing a quartz watch to a mechanical one on rate misses the point of both.
Magnetism is usually the culprit
When a mechanical watch suddenly starts gaining thirty seconds or a minute a day, magnetism is the first suspect, not wear. Laptops, speakers, tablet covers, handbag clasps and induction hobs all carry magnets strong enough to affect a hairspring. Demagnetising takes a watchmaker a couple of minutes and costs very little.
If it is not magnetism, a watch running well outside specification is usually asking for a regulation — a small adjustment to the balance — or, if it is also losing amplitude, a full service.
Amplitude, beat error and what a timing machine sees
Seconds per day is the outcome, not the diagnosis. A watchmaker puts the movement on a timing machine and reads three things: rate in seconds per day, amplitude (how far the balance swings, which shows the health of the movement) and beat error (whether the swing is symmetrical). A watch can keep excellent time on a failing movement, so a good rate is not a clean bill of health.
| Movement | Expected rate | Over a month |
|---|---|---|
| Chronometer certified (COSC) | −4 to +6 s/day | about 2 minutes at worst |
| Good modern automatic | −5 to +10 s/day | 3 to 5 minutes |
| Ordinary automatic or manual | −10 to +20 s/day | 5 to 10 minutes |
| Vintage mechanical | −20 to +40 s/day | 10 to 20 minutes |
| Quartz | ±15 s/month | about 15 seconds |
| High-accuracy quartz | ±10 s/year | under a second |
Rates are for a serviced movement worn normally. Position, temperature and state of wind all move the figure.
Questions we are asked
- How many seconds a day is acceptable for an automatic watch?
- Within about ten seconds a day either way is good for an uncertified automatic, and −10 to +20 is within most manufacturers' own specification. A chronometer-certified movement should hold −4 to +6.
- Why is my watch suddenly running fast?
- Magnetism, almost always. Laptops, speakers and magnetic clasps will do it, and a watchmaker can demagnetise the movement in minutes for very little money.
- Is a watch that gains time better than one that loses time?
- Gaining is generally the healthier sign. A watch losing time often indicates low amplitude — a movement short of power — which points to a service rather than a regulation.
- How accurate is a quartz watch?
- Typically within about fifteen seconds a month, which is roughly three minutes a year. High-accuracy quartz movements are specified to within ten seconds a year.
Beyond the RateRead the watch itself
A rate tells you how it runs. Photograph it and the instrument tells you what it appears to be, and roughly what its kind fetches.