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When will my automatic watch stop?
An automatic keeps running for a fixed number of hours after you take it off. Set your movement below and see precisely when yours runs out, and whether it will still be accurate when it does.
Figures are the manufacturer's stated reserve for a fully wound, recently serviced movement. A worn mainspring delivers noticeably less.
What power reserve actually measures
Power reserve is how long a fully wound mainspring can drive the movement with no further input. It is measured from a full wind in a laboratory, on a movement in good order, so it is a best case rather than a promise.
The number varies enormously by movement generation. A 1990s ETA 2824 gives you around 38 hours, which means a watch taken off on Friday evening is dead by Sunday morning. Modern movements have pushed towards 70 and 80 hours specifically so the watch survives a weekend in a drawer, which is why the specification suddenly became a marketing point.
Wearing the watch does not simply top it up. A rotor only winds when your arm moves, so a desk day contributes far less than a day walking around. Automatics worn by mostly sedentary people frequently run at partial wind without the owner realising.
Accuracy falls before the watch stops
This is the part most owners miss. As the mainspring unwinds, the torque it delivers drops, and below roughly a quarter of the reserve most movements start running noticeably slow. The watch is still ticking, so nothing looks wrong, but it is no longer keeping the rate it was regulated to.
This is why a watch that seems accurate when worn daily can look terrible when tested after sitting partly wound. If you are checking your watch against a time standard, do it on a full wind, otherwise you are measuring the mainspring rather than the regulation.
It is also the argument for a watch winder if you rotate between several watches, though a winder is a convenience rather than a necessity. Leaving a watch stopped does no harm at all.
Restarting a stopped automatic
Wind the crown roughly 30 to 40 turns in the forward direction to build a working reserve, then set the time. Shaking the watch to start it works but only puts in a token amount of power, and it puts a shock through the rotor bearing for no good reason.
Avoid setting the date while the hands sit between roughly 9pm and 3am. In that window the date change mechanism is engaged, and forcing it can break teeth on the date wheel. Move the hands to a safe hour first, set the date, then set the time.
Common questions
How long does an automatic watch run when you take it off?
Between roughly 38 and 80 hours depending on the movement. Older Swiss movements such as the ETA 2824 give around 38 hours, while modern Rolex, Tudor and Seiko 6R35 movements reach about 70, and the Powermatic 80 reaches 80.
Does an automatic watch lose accuracy as it winds down?
Yes, and it happens before the watch stops. Below about a quarter of the power reserve the mainspring torque drops and most movements start running slow. If you want to test your watch's real accuracy, do it from a full wind.
Is it bad to let an automatic watch stop?
No. A stopped watch causes no damage and no wear, since nothing is moving. The only downside is having to reset the time and date. The idea that a movement must be kept running comes from watch winder marketing rather than from watchmakers.
How do I fully wind an automatic watch?
Turn the crown forward roughly 30 to 40 times in the unscrewed but unpulled position. Most automatics use a slipping clutch that prevents overwinding, so you cannot damage the mainspring by continuing, though there is no benefit either.
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