How Accurate Should an Automatic Watch Be?
Running Fast or Slow, Explained

What "Normal" Looks Like

Accuracy for a mechanical watch is quoted in seconds per day, as a range: a watch "at -10/+20" may lose up to 10 or gain up to 20 seconds in 24 hours. The figures manufacturers publish for common calibers:

Two things stand out. First, the entry-level specification is wide, but it is a guarantee, not a prediction — most examples land well inside it. Second, certified accuracy costs money because every movement has to be adjusted and tested individually; that is what the "chronometer" word on a dial is actually paying for. For how quartz compares (around ±15 seconds a month), see quartz movement explained.

Why an Automatic Runs Fast or Slow

Position

A balance wheel behaves slightly differently lying flat than hanging vertically, because gravity pulls on the hairspring differently. That is why a watch can gain on the wrist and lose on the nightstand, and why watchmakers regulate in several positions. Watchmakers sometimes use this to fine-tune a watch, but the effect is small and differs from movement to movement.

Power reserve state

A mainspring delivers slightly more torque when fully wound than when nearly empty, and most movements run a little differently across that curve. A watch worn all day and measured in the morning is being measured at a different state than one wound once and left. Longer-reserve calibers like the Powermatic 80 are partly popular because they spend more of their life in the flat part of the curve. More on this in power reserve explained.

Temperature

Modern alloys reduce it, but a hairspring still changes stiffness with temperature. A watch kept on a warm wrist runs differently from the same watch on a cold windowsill. This is why manufacturers quote rates at a reference temperature.

Magnetism — the most common real fault

If a watch that used to run within a few seconds suddenly gains minutes per day, it is almost certainly magnetized. Laptop lids, tablet covers, handbag clasps, speakers and induction cooktops all carry magnets strong enough to do it. The hairspring coils stick, the balance swings short and fast, and the watch races. Demagnetization takes a watchmaker under a minute and typically costs little or nothing; silicon hairsprings and soft-iron inner cases are the engineering answers at the high end.

Shock

A hard knock can shift the regulator or, worse, the hairspring stud. The symptom is a step change in rate after a specific event (a dropped watch, a slammed door). Modern shock protection (Incabloc, Diashock and equivalents) protects the balance staff from breaking but does not keep the regulation perfectly put.

Age and lubrication

Oils thicken and migrate over years. A watch that gradually loses more and more, or whose amplitude drops, is usually telling you it is due for a service. Regulation alone will not fix it.

How to Measure Accuracy Properly

1. Set the watch to a reference — your phone's clock, or time.gov — by pulling the crown to the hacking position, setting the seconds hand to 12 and pushing the crown in exactly on the reference minute.
2. Wear the watch as you normally would for a full week. Do not judge it after one day.
3. At the same time each day, note how many seconds it has gained or lost against the reference.
4. Average the daily figures. A watch that gains 8 seconds on Monday and 12 on Tuesday is a +10 watch, not an unreliable one.

Smartphone apps that listen to the ticking through the microphone can give an instant rate and amplitude reading; they are useful for checking magnetism or comparing positions, but a week on the wrist is the number that matters for daily life.

Regulation vs Service — Which Do You Need?

Regulation on a healthy, recently serviced movement is a short job for any competent watchmaker and can bring an entry-level caliber to within a handful of seconds a day. Ask for it to be regulated "in the positions you wear it" rather than just dial-up on the bench.

Living With a Few Seconds a Day

Most owners settle into a rhythm: set the watch on Sunday, ignore the drift during the week, set it again next Sunday. If you rotate several watches, a winder keeps them running but does not keep them accurate — a watch on a winder for a month will still be a minute or two out. For the mechanics of setting the time, the date and hand-winding without damaging the movement, see how to wind an automatic watch.