What size drill bit and plug do I need for a screw?

The rule that the bit matches the plug rather than the screw, the standard UK plug colour code, and how deep the hole has to be.

Difficulty: beginner
Time: 10 min
Read: 3 min

Short answer

The drill bit size matches the plug, not the screw. Yellow plugs take a 5mm bit, red take 6mm, brown take 7mm and blue take 10mm. The screw only has to be thick enough to expand the plug and long enough to reach its far end, plus the thickness of whatever you are fixing.

Most fixing failures in masonry come from one of two sizing errors: drilling to the screw diameter rather than the plug diameter, or using a screw too short to expand the whole plug. Both leave a fixing that feels tight and is not.

Matching wall plug colour, drill bit and screw size

Step by step

Choose the plug for the load, not the screw you happen to have.Heavier loads need a bigger plug with more surface area against the masonry. The screw follows from the plug.
Match the bit to the plug.The colour code is standard: yellow with a 5mm bit, red with 6mm, brown with 7mm, blue with 10mm. This is printed on the box and it refers to the drill, not the screw.
Choose a screw within the plug's stated gauge range.Yellow takes gauge 4 to 8, red 6 to 10, brown 10 to 14, blue 14 to 18. Too thin and it does not expand the plug; too thick and it splits it.
Work out the screw length.The thickness of the item being fixed, plus the plug length, plus a few millimetres. A screw that stops halfway down the plug only expands half of it.
Drill deeper than the plug is long.Ten millimetres of spare depth gives the drilling dust somewhere to go. A plug bottoming out on packed dust will not sit flush and will not grip properly.
Check the plug is a firm push fit.It should need a light tap with a hammer to go in, and its collar should finish flush with the wall surface, never proud.
Allow for the plaster.The plug must sit in the masonry, not in the plaster. On a wall with 15mm of plaster, a 30mm plug set flush is only half in brick — use a longer plug.
Clear the hole before inserting the plug.Brush and vacuum. Dust in the hole is the difference between a fixing that holds its rated load and one that holds a fraction of it.

Tips

If you remember one thing: the drill bit size is the plug size, never the screw size.
A plug that goes in with finger pressure alone means the hole is oversized. Move and drill a fresh one.
Buying plugs and screws as a matched pack removes the sizing decision entirely for light work.

Common mistakes

Drilling to the screw diameter — The hole ends up too small for the plug to enter, or the plug is hammered in damaged. Either way the expansion is uneven and the fixing is weak.
Using a screw that is too short — The screw only expands the part of the plug it reaches. The rest of the plug is dead length doing nothing, and the fixing carries far less than it should.

If it doesn't work

Plug pushes in with no resistance

Cause: Hole drilled oversize or wallowed out — Fix: Move 75mm and drill fresh. Alternatively use a chemical anchor, which does not depend on hole size.

Plug will not go in at all

Cause: Bit too small, or hole not clear of dust — Fix: Check the bit is the plug's stated size, clear the hole, and try again rather than hammering harder.

Plug turns as you drive the screw

Cause: Hole oversized, or the plug not seated fully before the screw was driven — Fix: Pull the plug and start again with a fresh hole; a spinning plug never recovers.

Screw goes in easily then feels loose at the end

Cause: Screw too long for the hole and bottoming out beyond the plug — Fix: Shorten the screw or drill the hole deeper — the head must pull the item tight, not sit on a bottomed screw.

Questions people ask

What is a screw gauge?

The old British diameter designation, still printed on boxes. Gauge 8 is roughly 4mm, gauge 10 roughly 5mm and gauge 12 roughly 5.5mm. Plug packaging quotes gauge, so it is worth knowing.

Can I use a plug in plasterboard?

No. Expansion plugs need something to expand against and plasterboard crumbles. Use a cavity fixing designed for board, or fix into a stud.

Do I need to use the plug at all if the screw feels tight?

Yes. A screw driven straight into masonry grips on a few crumbling threads and will pull out under load or over time.