How small does
a chip have to be?

Almost everybody asks the size question first, because everybody has heard the £2 coin rule. It is a decent starting point and it is not the whole answer. A chip the size of a pinhead in the wrong place needs a new screen; a chip the size of a five pence piece in the right place will repair and hold for the life of the car.

The four tests, in order

Run these in order and you will land on the right answer nearly every time.

1. Size. Damage that fits under a £2 coin is usually repairable. Cracks longer than about 25 millimetres generally are not.
2. The wiper sweep. If the chip sits where the wiper blade passes, the repair has to be near perfect or the blade will catch and chatter over it.
3. The driver’s eyeline. Damage in the area you look through gets replaced, whatever its size. A repair leaves slight distortion, and distortion in front of your eyes at night is not acceptable.
4. The edge. Anything within about 50 millimetres of the edge of the glass is replaced. That is where the screen carries structural load, and a repair there is a weak point in the bond.

Not all chips are the same shape

Fitters use a handful of names and they do genuinely matter, because some types stabilise and some spread.

• Bullseye. A clean circular break with a dark centre. The easiest thing we repair and the most likely to disappear almost entirely.
• Star break. Short legs radiating out from the impact. Repairable, but the legs are what run when the temperature swings, so it is worth doing sooner.
• Combination. A bullseye with legs. Usually fine, depending on how long those legs already are.
• Crack. A single line. Once it is past about 25 millimetres, or it has reached the edge, the screen is going to be replaced.

Why waiting makes the decision for you

Glass moves. It expands when the sun hits it and contracts on a cold night, and every one of those cycles works at the tip of a crack. A star break that passes all four tests in October regularly fails them by December, and the difference is a repair against a full replacement plus, on most modern cars, a camera recalibration.

Two things accelerate it more than anything else. The first is blasting the heater straight onto frozen glass, which is the single most common way a stable chip becomes a screen-wide crack. The second is dirt and water getting into the break, which stops the resin bonding properly and can turn a repairable chip into a non-repairable one within a fortnight.

If you cannot get it looked at quickly, put a piece of clear tape over the damage. It keeps the break clean and costs nothing.

What a repair actually does

The break is cleaned out, air is drawn from it under vacuum, and a clear resin is forced in and cured hard under ultraviolet light. That restores most of the strength and stops the damage spreading.

What it does not do is make the chip vanish. Expect a faint mark where the impact was, a bit like a watermark. Anyone promising an invisible repair is overselling it, and we would rather tell you that now than have you disappointed on the day. If you want to see honestly what one removes, there is a before and after on our recent work page.

The money side

On most comprehensive policies a chip repair is covered in full with no excess, because insurers would far rather pay for a repair now than a replacement plus recalibration later. It usually sits outside your no-claims bonus too, though it is worth checking your wording — there is more on that in will a glass claim cost me my no-claims.

If you are paying yourself, a repair is a fraction of a replacement. That gap is the whole reason this article exists.

What to do today

• Photograph it with a coin beside it, from outside the glass.
• Put clear tape over it if the break is open.
• Keep the heater off the glass and scrape ice rather than pouring anything on it.
• Send us the photo and the registration, and we will tell you which it is.

If it turns out to need replacing, that is covered in detail on our windscreen replacement page, including why the camera behind it has to be recalibrated the same visit.