Drilling glass looks risky—and it is, if you don’t know the technique. But with the right bit and method, it’s straightforward and gives professional results. The golden rule: low speed, constant water cooling, and patience.
Drill Bits for Glass: What Works
Diamond Core Bits (Best Choice)
Diamond core bits are the premium option. They have a thin ring of industrial diamond embedded in the tip, with a hollow centre. As they cut, water runs through the centre to cool the bit and wash away debris. Result: clean hole, no cracking, long bit life.
Why they work: Diamond is harder than glass and abrades it away gradually rather than crushing it. The hollow core means less friction and heat.
Best for: Mirrors, windows, tile (can also be used on tile), decorative glass. Sizes typically 6mm to 50mm.
Speed: Very slow—200 to 600 RPM depending on bit diameter. Larger bits need slower speeds.
Cost: £15–£50+ per bit, depending on size. They last years if used correctly.
Amazon: Diamond Glass Drill Bit Sets on Amazon UK
Carbide Spear-Point Bits (Budget Option)
Carbide bits cost less than diamond and work fine if you’re careful. They have a spear-shaped carbide tip that abrades the glass. They’re not as efficient as diamond (more heat, more risk of cracking), but they work on occasional jobs.
Best for: One-off holes, light drilling, mirrors and window glass. Sizes 4mm to 16mm typically.
Speed: 300–800 RPM. Slower than twist bits but not as slow as diamond.
Cost: £5–£15 per bit.
Downside: They generate more heat than diamond bits, so you need more water cooling and patience. Risk of cracking increases if you’re not careful.
What NOT to Use
Masonry bits: These have a tungsten carbide point designed to crush, not abrade. They’ll crack glass.
Regular twist bits: Same problem—they’re designed to cut metal, not abrade glass. They’ll cause splintering and cracking.
Wood bits: Too aggressive. They’ll shatter the glass.
Technique: Step-by-Step Drilling Glass
1. Secure Your Glass Properly
The most important step. Clamp the glass to your bench or workbench with rubber padding on both sides of the glass (to avoid pressure points that cause cracking). The glass must not move—any vibration or flex will cause cracks.
If you’re drilling a window in a frame, clamp it to a piece of timber first, then clamp the timber to the bench. This prevents the frame from flexing.
2. Mark the Hole with Tape
Place masking tape or painter’s tape over the spot where you want to drill. Mark the centre point of the hole with a felt-tip pen on the tape. The tape serves two purposes:
- It prevents the bit from slipping as you start
- It reduces chipping and cracking on both sides
Don’t skip this step—the tape makes a huge difference.
3. Set Up Water Cooling
You need a constant supply of water at the drilling point. Options:
- Water bottle: Squeeze water onto the bit every few seconds as you drill. Simple and effective for small holes.
- Hose and clamp: Rig a hose or tubing to drip water continuously onto the hole. Ideal for larger projects.
- Wet tile method: For small glass, wet a tile or sponge and set the glass on top. The wet surface cools the bit.
The water does three things: cools the bit (glass heats up quickly), lubricates the drilling action, and flushes away glass dust.
4. Use a Backer Board
Place a wooden backing board underneath the glass, about 10–20mm away from the surface. When the bit breaks through, it exits into the wood instead of pushing into the back of the glass, which can cause large chips or cracks on the underside.
Clamp the backing board and glass together to keep them aligned.
5. Start at 45 Degrees
Set your drill to the correct low speed (200–600 RPM depending on bit size). Position the bit on the marked spot at a 45-degree angle to the glass. Apply light pressure and let the bit start cutting. The angle helps the bit establish itself and prevents slipping.
Once the bit has cut a small dimple (3–5mm deep), straighten the drill to perpendicular (90 degrees to the glass) and continue.
6. Drill Slowly with Light Pressure
This is not a race. Apply gentle, steady pressure. Let the bit do the work—if you push hard, you increase the risk of cracking dramatically. You should hear a gentle grinding sound, not a whine or screech.
Keep water flowing constantly. Stop every 15–20 seconds and pour water on the hole. The bit should stay wet.
7. Don’t Use Hammer Action
Switch off any hammer or percussion modes on your drill. This is critical. Hammer action will shatter glass. You need smooth rotation only.
8. Final Breakthrough
As the bit nears the bottom, the pressure from below increases. Slow down even more and reduce pressure. Apply constant water. When the bit finally breaks through, stop immediately. Don’t push the bit through hard—let it exit gently.
The backing board prevents the glass from chipping on the exit side.
9. Smooth the Edge
After drilling, the hole has sharp glass edges. Smooth them with fine sandpaper or a diamond tile file (80–120 grit). Work gently around the edge until it feels smooth. This prevents cuts and makes the hole look finished.
Types of Glass: Special Considerations
Window Glass (Ordinary Annealed Glass)
Standard single-pane window glass. Easy to drill if you take your time. The main risk is thermal shock—if the glass is cold and the hot bit touches it, it can crack. Keep it cool with water and you’ll be fine.
Tempered Glass (Safety Glass)
DO NOT TRY TO DRILL TEMPERED GLASS. Tempered glass is heat-treated to be stronger and shatter into small, harmless pieces when broken. The internal stresses that make it safe mean that drilling it causes the entire pane to shatter violently. No drilling method will work safely.
Tempered glass is commonly used in car windows, shower enclosures, and doors. If you need a hole in tempered glass, have it drilled during manufacturing or replace the panel.
Mirror Glass
Usually ordinary annealed glass with a reflective coating on the back. Drill from the reflective side (back) if possible, so any chips are hidden by the backing. Use the same technique as window glass. Water cooling is essential to prevent the coating from flaking.
Toughened / Borosilicate Glass
More heat-resistant than annealed glass. Drill slowly with plenty of water. Otherwise, the same technique applies.
Common Problems and Solutions
| Problem | Cause | Solution |
|---|---|---|
| Glass cracks from centre outward | Too much pressure or speed. Thermal shock from starting too aggressively. | Slow down further. Start at 45° and increase water cooling. Let the bit establish itself gently. |
| Large chips on exit side | No backing board or glass moved during drilling. | Always use a backing board 10–20mm away. Clamp both glass and board securely. |
| Bit binds and sticks | Insufficient water or glass dust jamming the hole. | Keep water flowing constantly. Back the bit out every 15 seconds to clear dust. Never force the bit. |
| Slow progress or bit seems dull | Bit is dulling or clogged with glass dust. | Increase water flow. Back out the bit to clear debris. If progress doesn’t improve, the bit may need replacing. |
| Hole not perpendicular to glass | Drill not held straight or glass shifted during drilling. | Use a drill guide (shop-bought or homemade from wood). Clamp glass down firmly. |
Tools and Setup You’ll Need
- Drill: Cordless is fine, but variable speed is essential. You need to run at 200–600 RPM.
- Diamond or carbide bit: The right size for your hole (typically 6–16mm).
- Water supply: Bottle, hose, or spray bottle.
- Masking tape: To prevent bit slip and reduce chipping.
- Backing board: Scrap timber clamped behind the glass.
- Rubber padding: To clamp glass without pressure points.
- Clamps: To secure glass and board to your bench.
- Sandpaper: 80–120 grit to smooth the edge afterward.
Size Guide
Diamond bits are typically sold individually or in sets. Common sizes:
- 6mm: Small holes for cable or light fixings
- 8mm: Handles, small brackets
- 10mm: Medium brackets, fixing points
- 12–16mm: Larger fixings, handles, hinges
- 20–40mm: Large holes for pipes or ducts (specialist work)
Most DIY projects use 8–16mm bits.
When to Call a Professional
Some jobs are better left to specialists:
- Tempered glass: Cannot be drilled safely.
- Large panes or mirrors: Risk of breakage is high. Professionals have proper equipment and experience.
- Multiple precise holes: A professional glass service can drill multiple aligned holes accurately.
- Antique or valuable glass: Not worth the risk.
Amazon: Glass Drill Bit Kits on Amazon UK
See also: What Drill Bit for Plastic
See also: What Drill Bit for Masonry
See also: Drill Bit Size Chart
Watch: Video Guide
Frequently Asked Questions
Can I drill tempered glass?
No. Tempered glass is under internal stress and will shatter violently if you drill it. Any stress—including a drill bit—triggers shattering. The only way to get a hole in tempered glass is to have it drilled during manufacturing or replace the pane.
What speed should I use?
For diamond core bits, 200–600 RPM depending on hole size. Larger bits need slower speeds. For carbide bits, 300–800 RPM. Most cordless drills have variable speed—dial it down to the low end. Check your bit manufacturer’s recommendation.
Do I really need water cooling?
Yes. Water is critical. It cools the bit (glass heats rapidly and can crack), lubricates the cutting action, and flushes away glass dust. Without water, you’ll overheat the bit and very likely crack the glass.
Why did my glass crack?
Most likely: too much speed, too much pressure, or no water cooling. Glass cracks from thermal or mechanical shock. Slow down, apply gentle pressure, and keep the bit wet. Also ensure the glass is clamped securely and won’t vibrate.
Can I use a masonry bit?
No. Masonry bits are designed to crush stone. They’ll crack glass. You need a bit designed to abrade glass (diamond or carbide spear-point).
How long does a diamond bit last?
Years, if used correctly. A quality diamond core bit will drill hundreds of holes. Carbide bits last shorter (dozens of holes) but cost less. Proper water cooling and speed control extend bit life.
