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Accuracy Standards
COSC Chronometer
-4 to +6 s/day
The Swiss Official Chronometer Testing Institute (COSC) tests movements over 15 days in 5 positions and 3 temperatures. A movement must fall within -4 to +6 seconds per day averaged across all tests to receive certification. Roughly 3% of Swiss movements are submitted for COSC testing.
Rolex Superlative Chronometer
-2 to +2 s/day
Rolex tests movements after casing (not just the bare movement like COSC), which accounts for real-world conditions. Their standard is tighter than COSC at plus or minus 2 seconds per day. This is verified by Rolex's own testing protocol using proprietary equipment.
Grand Seiko VFA
-1 to +3 s/day
Grand Seiko's Very Fine Adjusted standard applies to their high-end mechanical calibers. Movements are regulated in 6 positions over 17 days. The asymmetric range (-1/+3) reflects that it is easier to regulate a movement to run slightly fast than perfectly neutral.
METAS Master Chronometer
0 to +5 s/day
The Swiss Federal Institute of Metrology (METAS) tests the complete watch (not just the movement) for accuracy, magnetic resistance up to 15,000 gauss, water resistance, and power reserve. Used exclusively by Omega. The magnetic resistance testing is the distinguishing feature.
How to Measure Accuracy
Synchronize to a reference
Use time.is or a similar atomic clock reference. Set your watch's seconds hand to match the reference exactly. Note the exact time you synchronized.
Wait at least 24 hours
Wear the watch normally. The longer you wait between measurements, the more accurate your rate calculation will be. A single day gives you a rough rate. A week gives you a reliable average.
Compare again
Check your watch against the same atomic clock reference. Count how many seconds your watch is ahead (fast, positive) or behind (slow, negative). Divide by the number of days elapsed to get your rate in seconds per day.
Track over time
A single measurement tells you the rate at that moment. Repeated measurements over weeks reveal the trend. A consistent rate (even if not zero) is a sign of a healthy movement. An erratic rate that changes significantly between measurements may indicate a need for service.
What Affects Accuracy
Position
A mechanical watch keeps slightly different time depending on its orientation. Dial up, dial down, crown up, crown down, and crown left each produce a different rate. This is called positional variation. A well-regulated movement minimizes the difference between positions.
Temperature
The hairspring's elasticity changes with temperature. Cold temperatures make the spring stiffer, causing the watch to run faster. Warm temperatures soften it, causing the watch to run slower. Modern hairsprings made from silicon, Parachrom (Rolex), or Spiromax (Omega) are designed to minimize thermal sensitivity.
Magnetism
Magnetic fields can magnetize the hairspring, causing its coils to stick together and dramatically increasing the rate (sometimes by minutes per day). If your watch suddenly gains significant time, magnetism is the most likely cause. A watchmaker can demagnetize the movement in seconds.
Power Reserve State
A fully wound mainspring delivers more consistent torque than one nearing depletion. Many watches gain time when fully wound and lose time as the reserve drops. This is normal and is one reason why daily wearing (which keeps the reserve topped off) often produces better accuracy than intermittent use.