Short answer: about a quarter. Take a quarter out of the cupholder and hold it over the damage. If the coin covers the whole chip, you have good odds of a repair. That is the rule technicians actually use, and it is easy to check yourself before you call anybody.
Worth being clear up front, because these get mixed up constantly: the quarter rule is for chips. Cracks use a different number, roughly six inches, and that is a separate question with a separate answer.
Where the Quarter Comes From
It is not arbitrary. A chip is a void where glass used to be, and the repair fills that void with resin under pressure. Two things get harder as the void gets wider.
First, resin has to travel farther from the injection point to reach the outer boundary, and pressure falls off with distance. Past roughly quarter size the outer edge of the damage is the part that does not fill completely.
Second, a larger void means more of the outer layer is unsupported, so the repaired area carries less of the strength the original glass had. Around a quarter is where the industry landed as the point past which results stop being consistently good.
How the Rule Shifts by Chip Type
- Bullseye: quarter rule applies cleanly. Measure across the outer ring, not just the center.
- Half moon: same as a bullseye. Measure the full crescent.
- Star break: you get a little more room, up to about a half dollar, but measure tip to tip across the legs and not just the middle.
- Combination break: measure the total footprint including legs. Somewhere between the quarter and the half dollar depending on how the legs run.
The most common measuring mistake is sizing the impact point instead of the damage. On a star or combination break the legs are usually the larger dimension, and the legs are what decide it.
Three Things That Matter as Much as Size
Depth
Your windshield is two layers of glass with a plastic layer bonded between them. Nearly every rock chip only damages the outer layer, which is exactly what repair is designed for. If an impact was hard enough to reach through the plastic layer and damage the inner glass, no amount of resin fixes it and the windshield needs replacing. That is uncommon with typical road debris, but it happens.
Location
A quarter sized chip two inches from the edge of the glass is a worse candidate than a slightly larger one out in the open field, because the perimeter is under constant flex load and repairs there are less reliable. A chip sitting directly in the driver's line of sight is also judged more strictly, since any faint remaining trace lands right where your eye naturally goes.
Age and Contamination
A chip is an open void until it is sealed. Rain, car washes, and washer fluid put water and grime inside it. The vacuum step pulls most of that back out, but a chip that has been open since March fills less cleanly than one from last Tuesday. Size being equal, fresher repairs better.
If Yours Is Borderline
Borderline is worth a phone call rather than a guess, and it is worth making that call sooner rather than later. A chip sitting right at the edge of repairable today can be past it after one hot week in a parking lot.
Describe the size against a coin, roughly where it sits on the glass, and whether you can see any lines running off it. That is enough for Patrick to give you a straight read over the phone. Text a photo if it is easier.
Not sure what you are looking at? Call or text a photo to (352) 234-4412 and Patrick will tell you straight whether it can be repaired. We come to you across Citrus, Marion and Hernando County.
Call (352) 234-4412