Chip or crack: can it be repaired?
A chip smaller than a 10c coin can usually be repaired; a crack longer than about 15 cm usually means a new windscreen. Most damage from a stone starts as a chip, and the trick is catching it before it turns into a line.
What a chip looks like
A chip is damage around one impact point. It might be a bullseye (a round mark with a dark centre), a star (short legs running out from a point), a half moon, or a combination of a bullseye with legs coming off it. They look different, but they are all repaired the same way: the air is drawn out of the break and replaced with resin.
The 10c coin is the quick test. Put one over the damage. If the coin covers it, that is almost always a repair. A 10c coin is about 24 mm across, which sits inside the size most repairers will take on.
What a crack looks like
A crack is a line. It has a direction and it can keep travelling, pushed along by heat, cold and the flex of the body as you drive. Once a chip has turned into a long line, resin cannot pull the glass back together, and most of the time the answer is a new windscreen.
Short cracks coming out of a chip are a grey area, and that is where a photo settles it. I would rather tell you honestly from a photo than drive out to a job that is not a repair.
Where it is matters too
Near the edge of the glass, a chip is usually still a repair. The edge is where the windscreen is bonded to the car, so I ask for a photo first to confirm before I come.
In front of the driver, a repair still leaves a faint mark, because it fills the break rather than making it disappear. If you are about to sell the car, the safety certificate inspector decides what passes, so it is worth fixing the chip before the inspection rather than after.
Damage that has gone through both layers of the glass cannot be repaired, wherever it is and whatever its size.
What to do now
If the coin covers it, get a fixed price in three taps. If you are not sure, the calculator asks for a photo and I tell you honestly. Either way, sooner is better: a chip that is left for weeks collects water and dirt, and that lowers the odds of a clean repair.
How to take a photo that helps
If you are not sure, a photo answers most of it. Take two. The first one square on, close enough that the chip fills a good part of the picture, with a coin next to it for size. The second one from a low angle, so the light catches the legs of the break and shows how far they run.
Wipe the glass first if it is dusty, and take the photo from outside the car, not through the glass from the inside. Shade helps: in direct sun the reflection hides the detail. For a crack, step back far enough to get the whole line in, from end to end.
What a repair can and cannot do
A good repair fills the break with resin, restores most of the strength of the glass around it and makes it far less likely to spread. What it does not do is make the chip disappear. You will usually still see a faint mark if you look for it, more on a star with long legs than on a small bullseye.
If you want the glass to look new, that is a replacement, which is a different job at a different price. For most chips the repair is the sensible choice: it keeps the original windscreen, its seal and anything mounted on it, and it takes about 40 minutes.
Questions
Can a crack be repaired?
A long crack usually needs a new windscreen. Short legs coming off a chip can often be filled with the chip. Send a photo and I will tell you honestly.
Does the size limit change in front of the driver?
The repair works the same anywhere, but in front of the driver it still leaves a faint mark, and whether a car passes a safety certificate is the inspector's call.
How long can I leave a chip?
There is no safe number of days. Heat, cold and potholes can turn it into a crack at any time, and dirt and water in the break make the repair less clean.