What a good daily rate actually is, how to measure one properly, and what to do about a bad number.
A mechanical watch is not a clock, and it is not supposed to keep clock time. What matters is not whether it is right now — it is whether it gains or loses the same amount every day. A watch that runs a steady +6 seconds a day is a good watch. A watch that runs +2 one day and −9 the next is not, even though its average looks better.
So the number you want is the daily rate: seconds gained or lost per 24 hours, measured over several days, against a time source that is actually correct.
| Daily rate | What it means |
|---|---|
| 0 to +5 s/day | METAS Master Chronometer territory. Excellent. |
| −4 to +6 s/day | COSC chronometer territory. Excellent. |
| within ±10 s/day | Very good for an uncertified mechanical watch. |
| within ±20 s/day | Normal and perfectly serviceable. A regulation would tighten it. |
| beyond ±30 s/day | Worth a service or a regulation. |
A slightly fast watch is generally preferred to a slow one, which is why COSC's window is asymmetric: you would rather be early than late.
There are two different measurements and people constantly conflate them.
Note the exact time your watch shows against a correct reference, wait a day or more, and note it again. The difference divided by the elapsed days is your rate. This is the number that matters, because it includes everything: your wrist, the temperature, how you set the watch down at night, the state of the mainspring.
The catch is the reference. Your phone's clock is usually right but drifts between network syncs, so a careful measurement queries a time server directly. Chronograde uses SNTP (RFC 4330) against public NTP servers, which is accurate to well under a tenth of a second — far better than your reaction time pressing a button, which is the real limit here. Press consistently and the error averages out over several readings.
A timegrapher listens to the escapement and computes the rate from the interval between ticks. It gives you an answer in about ten seconds instead of a week, and it also gives you beat error, which a daily-rate measurement cannot. But it is a snapshot: it tells you how the watch is running at that instant, in that position, at that state of wind. It does not predict your wrist.
Use both. The timegrapher tells you whether the movement is healthy and points at what to adjust. The daily rate tells you whether the watch is any good to live with.
The balance wheel swings both ways. Beat error is how unequal those two swings are, in milliseconds. Under about 0.5 ms is good; much above 1.0 ms and the watch loses amplitude and becomes position-sensitive, and it is worth having corrected. Unlike rate, you cannot fix beat error by turning a regulator arm — it needs the hairspring collet moved, which is a watchmaker's job.
If the rate is consistent but off — steadily +25 s/day, say — that is a regulation, and it is a small adjustment. If the rate is inconsistent, or positional variation is wide, or amplitude is low, regulating it only hides the problem: the movement wants servicing first, then regulating. And if the watch suddenly started gaining minutes, try a demagnetiser before you book anything.
Chronograde does both measurements on an iPhone or iPad. The acoustic Timegrapher is free — rest the phone on the caseback and read live rate and beat error. Drift tracking against atomic time, position analysis and chronometer grading are part of the app. See also what COSC and METAS actually certify and reference specs for common calibers.