Can You Drill Tempered Glass? Why It Shatters

No, you can’t drill tempered glass. Tempered glass is heat-treated so its surface is held in compression and its core in tension. That locked-in stress is what makes it about four times stronger than ordinary glass, but as soon as a bit cuts through the surface layer, the stress is released and the whole pane breaks into small pieces. Holes, notches and cut-outs in tempered glass are always made before it’s tempered. Ordinary (annealed) glass, on the other hand, can be drilled at home with the right bit and water.

How tempered glass is made

1. The glass is cut to its final size and shape, and every hole and notch is drilled and finished.
2. It’s heated in a furnace to about 620°C (1,150°F).
3. Jets of air cool both faces quickly. The surfaces harden and shrink first, which locks them in compression while the center stays in tension.

Because the stresses balance across the whole pane, any cut, deep scratch or hole made afterward upsets the balance and the pane shatters. The same is true of cutting or grinding it: you can’t cut tempered glass to size, and scoring and snapping it doesn’t work. If you need a different size, order a new piece from a glass shop.

How to tell if glass is tempered

• Look for the stamp. Tempered glass is usually etched in one corner with a small mark that says “tempered”, “toughened” or a safety standard number.
• Look through polarized sunglasses. Tempered glass often shows dark stripes or spots, the strain pattern left by the cooling jets.
• Check the edges. Tempered glass always has finished, smooth edges, because it’s processed before tempering. A raw, sharp cut edge means annealed glass.
• Where it’s used. Shower doors, glass tabletops, oven doors, patio doors and car side windows are usually tempered by code or by design.

If you can’t tell, treat the glass as tempered and don’t drill it.

How to drill ordinary (annealed) glass

Annealed glass, the kind in picture frames, bottles, jars and many older windows, can be drilled:

1. Use a glass bit: a diamond core bit or a diamond-tipped bit for clean holes, or a carbide spear-point glass and tile bit for small holes. Diamond drill bits for porcelain tile work the same way on glass.
2. Support and secure the glass flat on a soft, even surface such as a towel over a board, so it can’t flex or slide.
3. Mark the spot with masking tape to stop the bit skating, and keep the hole at least about twice the glass thickness away from the edge.
4. Keep it wet. Make a small ring of putty around the spot and fill it with water, or drip water on constantly. Heat cracks glass.
5. Drill slowly with light pressure, holding the drill square to the glass. Let the bit do the work.
6. Ease off as you break through. For thick glass, drill halfway from one side, turn the glass over and finish from the other side so the exit doesn’t chip.
7. Wear safety glasses and gloves, and smooth the hole edge with fine wet sandpaper if needed.

Related guides

• 10 Best Diamond Drill Bits for Porcelain Tile
• How to Drill Holes in Plexiglass Without Cracking It

For the full method on ordinary glass, see how to drill glass.