Accuracy and Power Reserve: What the Numbers Mean
Two numbers describe how well a mechanical watch behaves: how many seconds a day it gains or loses, and how many hours it runs from a full wind. Neither is a fixed property of the watch — both move with how you wear it.
Understanding that is what separates a genuine fault from a tolerance you should stop worrying about. Most watches people take to a watchmaker are behaving exactly as designed.
What “seconds per day” actually describes
A rate is an average, and averages hide things. A watch quoted at plus or minus ten seconds a day might gain twelve lying flat and lose eight crown-up, and average out at something respectable.
Consistency matters more than magnitude for anybody actually using a watch. A watch that gains a steady five seconds a day is easy to live with; one that gains five one day and loses seven the next is annoying at the same average.
The published figure also comes from a bench rather than a wrist. Laboratory conditions are stable temperature and controlled positions, and your Tuesday is neither.
Quartz is quoted per month rather than per day for the same reason it needs to be: the numbers are an order of magnitude smaller, as the movement comparison guide sets out.
Typical rates, and what is normal
| Movement | Expected rate | Worry if |
|---|---|---|
| Basic mechanical | Minus 20 to plus 40 seconds a day | It changes suddenly |
| Well-regulated mechanical | Within about 10 seconds a day | It drifts steadily worse |
| Chronometer-certified | Minus 4 to plus 6 seconds a day | It leaves that band on the wrist |
| Standard quartz | Within about 15 seconds a month | It gains or loses minutes |
| Thermocompensated quartz | Within about 10 seconds a year | Anything more |
A watch running twenty seconds fast a day is not broken if it has always done that. A watch that used to run five seconds fast and now runs forty has changed, and change is the thing to investigate.
How to measure your own rate
Do it properly or the number tells you nothing.
- Set the watch against a reliable reference. Any network-synchronised clock will do, since it traces back to the standards described by the International Bureau of Weights and Measures.
- Wear it as you normally would for seven full days without adjusting it.
- Note the difference at the end and divide by seven.
- Repeat, but leave the watch in a different position overnight, and compare.
A single day tells you almost nothing, because one unusual day of activity moves it. A week averages that out.
If the two positional tests differ substantially, you have found something useful: your watch has a favourite resting position, and choosing it is free regulation.
The four things that move a rate
A balance wheel is a physical object, and four forces act on it.
- Position. Gravity loads the balance staff differently flat, on edge and crown down.
- Temperature. Metals expand, and a hairspring’s elasticity changes with them.
- Magnetism. A magnetised hairspring sticks to itself and the watch gains, often minutes a day.
- Amplitude. A nearly unwound mainspring delivers less torque, the balance swings less far, and the rate shifts.
Magnetism is the one that produces dramatic, sudden change, and it is cheap to fix — the troubleshooting guide covers testing for it with a compass.
Amplitude decline over months is the one that means a service is due. That is lubricant degrading, and it is covered in the service guide.
Power reserve: what the hours buy you
Power reserve is how long the watch runs from fully wound. Forty hours is common, seventy is generous, and the difference is entirely practical rather than technical.
Forty hours means a watch taken off on Friday evening is stopped by Sunday morning. Seventy means it is still running on Monday, which is the whole argument for a longer reserve.
Longer reserves are achieved by a bigger barrel, two barrels, or a lower beat rate. Each has a cost: size, complexity, or a slightly less stable rate.
The last hours of any reserve are the worst ones. Torque falls as the mainspring unwinds, so a watch running on its final few hours keeps notably worse time than a freshly wound one.
Keeping an automatic properly wound
The commonest cause of poor timekeeping in an automatic is not a fault at all: the watch is simply not being wound enough.
Desk work, driving and holding a phone are low-amplitude motions that barely turn the rotor. A watch worn through a sedentary day can end it with less stored power than it started with.
The fix is to wind it by hand — thirty to forty turns — when you put it on after a rest, and to accept that a watch worn three days a week will always be at the weak end of its reserve.
If you rotate several watches, either wind them before wearing or read the storage and travel guide on whether a winder is worth it.
Beat rate, amplitude and beat error
Three numbers appear on a timing machine printout, and knowing what they mean turns a service conversation from guesswork into a discussion.
Beat rate is how fast the balance swings, quoted in vibrations per hour. Most modern movements run at 28,800, which is eight per second and why the seconds hand appears to sweep.
Amplitude is how far the balance swings, in degrees. Healthy is roughly 270 to 310 fully wound; a movement well below that has a torque problem, usually degraded lubricant or a tired mainspring.
Beat error is how uneven the two halves of the swing are, in milliseconds. Under about 0.5 is fine, and it is adjustable — a high figure is a setup issue rather than a wear issue.
Quartz drift, and why it is predictable
Quartz watches drift too, but they do it consistently, which makes them easy to live with.
The crystal’s frequency changes slightly with temperature, and the effect is a curve rather than a random walk. A watch that runs fifteen seconds a month fast will keep doing so.
Thermocompensated movements measure the temperature and correct for it, which is how a quartz watch reaches ten seconds a year rather than a month.
A quartz watch that has become erratic rather than consistently off usually has a failing battery or has taken a knock, and the battery is the cheap thing to eliminate first.
When to have a watch regulated
Regulation adjusts the effective length of the hairspring, and a watchmaker can usually bring a healthy movement to within a few seconds a day.
It is worth doing when the rate is consistently outside what you can live with, when the watch has just been serviced, or when it has been demagnetised and the rate has settled somewhere new.
It is not worth doing on a watch whose rate is getting worse month by month. That needs a service first, because regulating a movement running on degraded oil is adjusting a symptom.
And it is not a job for a screwdriver at the kitchen table. The adjustment is fine and position-dependent, and it is easy to turn ten seconds a day into forty.
Frequently asked questions
How accurate should a mechanical watch be?
A well-regulated movement holds within about ten seconds a day, and a certified chronometer within minus four to plus six. A basic movement can run twenty seconds out and still be working exactly as designed.
Why does my watch run differently overnight?
Position. Gravity loads the balance differently depending on whether the watch lies flat, sits on its side or rests crown down. Finding the position that suits your watch and using it is free regulation.
What is a good power reserve?
Forty hours is normal and means a watch taken off on Friday will have stopped by Sunday. Seventy hours or more carries it through a weekend, which is the practical reason to want one.
How do I measure my watch’s daily rate?
Set it against a network-synchronised clock, wear it normally for seven days without adjusting it, then divide the total difference by seven. A single day is meaningless because one unusual day of activity distorts it.
Measure over a week rather than a day, treat sudden change as the signal and steady drift as a tolerance, and wind an automatic by hand when it has been resting. Most timekeeping complaints resolve themselves at that point.