Mechanical watch accuracy calculator
Compare your watch with a trusted clock twice. This calculator turns the change in offset into the number of seconds it gained or lost per day.
Your watch gained 6.0 seconds per day.
+12.0 seconds over 48.0 hours · +6.0 s/d
A one-second offset-reading error would change this result by 0.5 s/d.
What this measures: the average rate over the whole interval, including every position, motion, temperature, and state of wind the watch experienced. It is not the same as a short timegrapher reading in one position.
The formula
First find how much the watch's offset changed. If it began 2 seconds behind and ended 10 seconds ahead, it gained 12 seconds. Then scale that change to one day:
seconds per day = offset change × 24 ÷ elapsed hours
| Start | End | Elapsed | Rate |
|---|---|---|---|
| −2 s | +10 s | 48 h | +6.0 s/d |
| +8 s | −1 s | 72 h | −3.0 s/d |
| +4 s | +4 s | 24 h | 0.0 s/d |
A positive result means the watch gained time. A negative result means it lost time. The calculator does not label a result good or bad because the appropriate comparison depends on the movement, manufacturer specification, service condition, and how the watch is worn.
How to make the comparison useful
- Fully wind the watch and note its position relative to a trusted clock.
- Write the offset as a signed number: ahead is positive, behind is negative.
- Wear or store the watch normally without resetting it.
- After at least a day, compare it with the same reference and record the new offset.
- Enter both offsets and the exact elapsed interval above.
- Repeat the observation across several intervals before treating one result as typical.
Why a longer interval helps
Reading an analog seconds hand or tapping at the exact reference second can easily introduce a one-second observation error. Over 6 hours, one second of error changes the calculated rate by 4.0 s/d. Over 24 hours it changes the rate by 1.0 s/d; over 72 hours, about 0.33 s/d. Longer tracking does not remove real changes in the watch, but it reduces the leverage of small reading errors.
Daily accuracy and timegrapher rate answer different questions
This calculator measures the watch's average behavior during real use. A timegrapher listens to the escapement for a short interval and estimates the current rate in one position and state of wind. Daily wear blends many positions; a timegrapher helps isolate them.
Use the long-run number to learn what the watch does on your wrist. Use a timegrapher to investigate why: compare dial up with crown down, test full wind against later in the reserve, and repeat a measurement to see whether the result is stable. See how to read the rate, timing trace, beat error, amplitude, and BPH before treating a snapshot as a verdict.
Measure the watch by sound with Hairspring
Hairspring is an iPhone timegrapher for mechanical watches. It measures rate, beat error, live amplitude, and common beat rates; states statistical fit uncertainty; withholds results the signal cannot defend; and compares two fresh runs to tell you whether they agree.
One $9.99 purchase. No subscription, account, ads, analytics, or automatic uploads.
Frequently asked questions
How do I calculate a watch's seconds per day?
Subtract the starting clock offset from the ending offset, multiply by 24, and divide by the elapsed hours. A positive result means gain; a negative result means loss.
How long should I track a mechanical watch?
At least 24 hours makes a one-second observation error equivalent to 1.0 s/d or less. Several days reduce that sensitivity further and sample more of your normal wearing pattern.
Why does daily wear differ from a timegrapher reading?
A timegrapher measures a short snapshot in one position and state of wind. Daily wear combines changing positions, motion, temperature, and mainspring torque over many hours.
Can I use this calculator for a quartz watch?
The offset formula works for any clock or watch. Hairspring itself is designed for the acoustic escapement signal of a mechanical watch, not a quartz watch.