Behind the numbers

How the accuracy tool works

Two photos of your watch, some time apart, each compared with the exact time: the difference between them is how fast or slow your watch runs. Here's how we get each number, and how much to trust it.

1. The exact time

When the page opens it asks our server for the time ten times; the server keeps its clock right against internet time servers. The quickest answer wins, because a quick round trip leaves the least room for error. Your phone's own clock doesn't need to be right: we measure how far off it is.

Every camera frame carries the moment it was captured, so each photo gets its own exact time, usually to within a few hundredths of a second. That's far finer than anyone can read a seconds hand, so the time isn't what limits a reading.

2. Taking the photo

With auto capture on, a circle follows the dial in the camera view:

• Yellow: we've found the dial and we're waiting until it's close enough, centred, held still (about a second) and as sharp as it gets.
• Green: all four are true. We take a quick burst of up to 8 frames in half a second and keep the sharpest view of the dial. Each frame has its own time, so choosing the sharpest costs no timing accuracy.

The colours are about framing and sharpness. How square-on the photo was is checked next.

3. Square-on, tilt and parallax

A round dial photographed at an angle comes out as an ellipse. We fit the dial's outline, which tells us how tilted the camera was, and we straighten the dial back into a circle before reading the hands. So tilt doesn't bend the hands' angles.

What straightening can't undo is parallax. The seconds hand sits a fraction of a millimetre above the dial, so seen from an angle its tip looks shifted against the markers. For a typical hand (about 0.8 mm up, 17 mm long), the most it can shift a reading is:

• Green (up to 15°): square-on enough that parallax is about a tenth of a second at most. Most wrist shots land here.
• Amber (15–25°): fine for tracking; for your most careful readings, retake it square to the dial.
• Red (over 25°): we suggest a retake. You can always keep it; it just counts for less.

Every reading's uncertainty includes its parallax, so a tilted photo weighs less when the rate is worked out. You're never stopped from keeping a reading.

4. Reading the hands

We find the dial's centre, 12 o'clock and each hand, then show you the line-up to check. You put a ring on the pin, and for the seconds hand an arrow on its tip and a ring further in on the hand.

That gives two lines for the seconds hand: one from the centre through the tip, one through the hand's own two dots. Where they agree within 0.15 s, we use both together, the longer line counting more. Up to 0.5 s apart, we still use both and let you know. Further apart, something is off the hand (or the hand is bent, like a lightning-bolt hand), so we ask you to fix it, or read it from the centre in one tap.

A watch with its running seconds on a small dial is read the same way, around that dial's own centre.

5. From readings to a rate

Each reading is how far your watch is ahead or behind the exact time. The rate is how that changes over time: +16.7 s six days after you set it is about +2.8 s a day. Setting the watch starts a new run.

Readings days apart beat perfect photos: an error of 0.3 s spread over one day is 0.3 s a day, but over a week it's 0.04 s a day. A watch also runs a little differently on the wrist, face-up on a table, or when its power reserve is low, so a few readings over a few days give its real average.

6. For the best reading

• Hold the phone square to the dial. That matters most.
• Keep it steady and sharp: bracing your elbow or the phone helps more than taking the watch off.
• Tilt the watch away from the light if there's glare across the hands.
• Let the dial fill most of the circle.
• Take your next reading a day or more later.

Your photos

Without an account, your readings stay on your phone. With one, they and their photos are saved privately to your account, and shared only if you choose.