Watch Accuracy Standards Explained: COSC vs METAS (What the Numbers Really Mean)

Quick Answer

COSC certifies a movement as a chronometer based on the ISO 3159 standard. For mechanical watches, the well-known target is −4 to +6 seconds/day, tested over 15 days, in 5 positions and 3 temperatures.
METAS “Master Chronometer” is a tougher, more real-world style test on the fully cased watch, typically requiring daily precision around 0 to +5 seconds/day, plus strong anti-magnetic performance and other checks.

“Better” depends on what you care about: pure timekeeping consistency, real-world anti-magnetism, or simply a trusted baseline.

1) The only number most people misread: seconds per day

When you see something like +5 s/day, it means the watch gains 5 seconds in 24 hours.

A practical way to think about it:

+5 s/day ≈ +35 seconds/week
−4 s/day ≈ −28 seconds/week

This matters more than “chronometer” as a label.

If you want to set time precisely before measuring accuracy, use:
Hacking Seconds Explained: How to Set Time Precisely (and When It Matters)

2) COSC vs METAS (simple comparison)

Standard	What’s being tested	Where it’s tested	Typical accuracy target	What it proves
COSC	the movement (chronometer test)	lab test, movement-based	−4/+6 s/day for mechanical; 15 days, 5 positions, 3 temps	the movement meets an accepted chronometer standard
METAS (Master Chronometer)	the fully assembled watch	cased watch tests (more real-world oriented)	often 0/+5 s/day (tighter window)	real-world performance checks including anti-magnetism and precision

Key idea: COSC is a strong baseline. METAS tends to be more “real watch on your wrist” oriented.

3) What COSC actually does (why it’s respected)

COSC is a widely recognized Swiss chronometer certification. For mechanical watches, the standard is based on 15 days of testing, across positions and temperatures, with the well-known −4/+6 s/day average daily rate requirement.

What it’s good for:

a reliable “minimum bar” for regulated movements
consistency across conditions (position/temperature)

What it doesn’t guarantee:

your watch will always run in that range on your wrist (real life varies)
strong anti-magnetism (magnetism is a separate issue)

Magnetism guide:
Magnetized Watch Symptoms: Why Your Watch Runs Fast & How to Fix It (Safely)

4) What METAS adds (why it feels more “real-world”)

METAS Master Chronometer testing is known for:

tighter daily precision (often 0/+5)
strong anti-magnetic performance in the test regime
testing on the fully cased watch (more reflective of actual wear)

Why this matters:

magnetism is a very common real-life cause of sudden fast running
cased testing better matches “what you actually wear”

5) How accurate should a mechanical watch be in real life?

Even a great watch can vary depending on:

position (dial up, crown down, etc.)
how fully wound it is
magnetism exposure
whether it needs service

If your watch suddenly changes behavior, use these:

fast/slow causes:
Why Is My Watch Running Fast or Slow? 9 Common Causes (And Fixes)
power reserve (stops early):
Power Reserve Explained: Why Your Watch Stops Early (and How to Fix It)
how to wind properly:
How to Wind a Mechanical Watch Properly (Manual vs Automatic + Mistakes to Avoid)

6) Simple “at-home accuracy test” (no tools)

This is the most useful thing for readers.

Step 1 — Set the watch precisely

Use stop-seconds (hacking) if you have it:
Hacking Seconds Explained: How to Set Time Precisely (and When It Matters)

Step 2 — Wear normally for 3 days

Don’t change your routine. Record daily gain/loss at the same time each day.

Step 3 — Compare the average

If it’s stable (same direction, similar amount), that’s usually good.
If it swings wildly, suspect magnetism or service needs.

Magnetism:
Magnetized Watch Symptoms: Why Your Watch Runs Fast & How to Fix It (Safely)

7) When the “standard” matters for buying

COSC matters if you want:

a credible baseline of regulation
a widely recognized chronometer certificate concept

METAS matters if you want:

tighter daily rate window + real-life anti-magnetism emphasis
more reassurance under modern magnetic exposure conditions

8) When to service (the part people skip)

If your watch used to run fine but now:

drifts much more than before
becomes inconsistent day-to-day
has winding/crown issues

Service reference:
How Often Should You Service a Mechanical Watch? A Practical Maintenance Timeline

FAQ

1) Is METAS “better” than COSC?

Not always—METAS is typically stricter and more real-world oriented (cased watch + anti-magnetism emphasis), but COSC is still a respected baseline.

2) What does −4/+6 seconds/day mean?

It’s the allowed average daily rate range for COSC mechanical chronometers.

3) Why does my watch run fast suddenly?

A very common reason is magnetism:
Magnetized Watch Symptoms: Why Your Watch Runs Fast & How to Fix It (Safely)

4) My watch is accurate some days and not others—why?

Position, power reserve level, and magnetism are the big three. Start here:
https://www.globalwatchindustry.com/blogs/watch-running-fast-or-slow-causes-fixes

5) Does hacking seconds improve accuracy?

It improves setting precision, not the movement’s inherent accuracy.
Hacking Seconds Explained: How to Set Time Precisely (and When It Matters)