Free size finder· UK units

Fixings Finder

Pick what you are fixing into and the fixing itself. Get the pilot hole, plug, masonry bit, hole depth and SDS bit in millimetres.

Data checked 19 September 2026 · No sign-up · Feeds rotary-hammers, drills, impact drivers

Step 1

What are you fixing into?

Fix into the brick body, not the mortar, for anything that carries weight. The age of the house decides how it drills: a property built before roughly 1919 is very likely to have lime mortar, which is deliberately soft, crumbles under hammer action and will not hold an expanding plug. Post-war houses are cement mortar and behave the way the charts assume. If the dust coming out of the hole is pale and powdery rather than grey and gritty, you are in lime: move the hole into the brick, use a longer plug, and consider a resin anchor instead. Start on rotary to seat the bit, then switch to hammer, and blow or vacuum the dust out before the plug goes in.

Step 2

Which fixing?

Step 3

What size?

Your sizes

6 mm wall plug in solid brick

Masonry bit

6mm

same as the plug

Hole depth

40mm

plug 28 mm plus dust room

Screw

No.6 to No.10

3.5 to 5.0 mm

Drill mode

Hammer

TCT masonry bit, or an SDS-plus bit in a rotary hammer

Read the millimetre size on the packet, not the colour: this one is red, and the traditional UK chart and the Rawlplug UNO range disagree about blue, which is 10 mm on one and 8 mm on the other.
The screw has to reach past the end of the plug so the plug expands along its whole length, and it wants to be about the plug diameter or a size under it.
6 mm is a common UK SDS-plus size, so a rotary hammer drills this quickly.
In a house built before about 1919 the mortar is probably lime: pale, powdery dust out of the hole means the joint will not hold a plug at all. Move into the brick, or use a resin anchor with a sieve sleeve.

Best SDS-plus rotary hammers for this material→

How it works

Method and assumptions

Timber pilot holes are sized from the screw's core diameter: about the core in softwood so the thread still bites, and a touch larger in hardwood so the screw neither snaps nor splits the board. UK screws are named No.4 to No.14, and increasingly just by metric diameter and length on the box, so a No.8 and a 4.0 x 40mm screw are the same thing. The clearance hole in the top piece matches the shank, so the two pieces pull together instead of jacking apart.

Masonry holes match the plug: a 6mm plug needs a 6mm hole, and the depth is the plug length plus a margin for dust so the fixing never bottoms out before it is tight. Do not size a plug by its colour. Two UK colour schemes are in circulation and they disagree: on a traditional chart blue is 10mm, while on a Rawlplug UNO packet blue is 8mm and grey is 10mm. The millimetre size printed on the packet is the only figure to trust.

Drilling mode follows the material: rotary for timber, hammer for solid brick and dense block, an SDS-plus rotary hammer for concrete and lintels, and rotary only with the hammer off for aircrete, hollow block, lath and plaster and tiles, where hammer action shatters the material or opens the hole out.

Which wall you have matters more in the UK than the fixing does, because the housing stock is so mixed. Drill a 3mm test hole first and read what comes out: pale powdery dust in a pre-1919 house means lime mortar, soft board then a void then block means dot-and-dab, soft board then timber means a stud wall, and a hole that suddenly runs free with a light grey dust means aircrete.

Sizes come from fastener manufacturer and supplier tables and are a starting point, not an engineering specification. For anything structural, the anchor manufacturer's datasheet for your exact product governs hole size, embedment depth, torque and load, and the through-bolt, sleeve anchor and concrete screw tables on this page are deliberately empty until we have transcribed UK datasheets rather than converting figures from another market. Test a fixing in an offcut or a hidden spot first.

Reference charts

The tables behind the finder

All sizes in millimetres. Pilot holes are for the piece the thread grips; clearance holes are for the piece the screw passes through. Diameters, clearance holes and head sizes come from the UK wood screw gauge table; softwood pilots follow the 60 to 70% of shank rule, rounded to bits you can buy.

Wood screws by UK gauge

Gauge	Diameter	Pilot, softwood	Pilot, hardwood	Clearance	Countersink
No.4	3.00	1.5	2.0	3.0	6.0
No.6	3.50	2.0	2.5	4.0	7.1
No.8	4.00	2.5	3.0	4.5	8.7
No.10	5.00	3.0	3.0	5.5	9.9
No.12	5.50	3.0	3.5	6.5	11.1
No.14	6.50	3.5	4.0	7.0	12.7

A No.8 and an Australian 8g are the same gauge, but the two markets publish slightly different metric equivalents: 4.0 mm here, 4.2 mm there. Most UK boxes now print the diameter and length rather than the gauge, so a No.8 x 40 is sold as 4.0 x 40.

Coach screws

Size	Pilot, softwood	Pilot, hardwood	Clearance	Spanner
M6	3.0	4.5	7.0	10 mm
M8	4.0	6.0	9.0	13 mm
M10	5.0	7.5	11.0	17 mm
M12	6.0	9.0	13.0	19 mm

Sold as coach screws in the UK and as lag screws elsewhere; the sizes and spanner flats are the same either way.

Wall plugs

Plug	Masonry bit	Hole depth	Screw	Colour on the packet
5 mm	5	plug length + 10	No.4 to No.8 (3.0 to 4.5 mm)	yellow
6 mm x 28	6	40	No.6 to No.10 (3.5 to 5.0 mm)	red
7 mm x 30	7	40	No.8 to No.12 (4.0 to 5.5 mm)	brown
8 mm x 32	8	42	No.10 to No.14 (5.0 to 6.0 mm)	blue (Rawlplug UNO)
10 mm	10	plug length + 10	No.12 to No.16 (5.0 to 8.0 mm)	blue (traditional chart) or grey (Rawlplug UNO)

Two UK colour schemes are in circulation and they disagree about blue: traditional charts read blue as 10 mm, Rawlplug's UNO range reads it as 8 mm and uses grey for 10 mm. The millimetre size on the packet is the only figure to trust. Lengths are the Rawlplug UNO figures where they are published; the 5 mm and 10 mm plugs have no datasheet length we could verify, so those rows take the length off the packet and add about 10 mm for dust.

Plasterboard and dot-and-dab fixings

Fixing	Colour	Rated load	Hole	Wall	Notes
GripIt	Yellow	25 kg	15	Stud wall, between the studs	Takes a 4mm screw, minimum 25mm long
GripIt	Red	74 kg	18	Stud wall, between the studs	–
GripIt	Brown	93 kg	20	Stud wall, between the studs	–
GripIt	Blue	113 kg	25	Stud wall, between the studs	The heaviest plasterboard fixing in common UK retail
Corefix Connect 100	Steel core in a nylon sleeve	55 kg	10	Dot-and-dab plasterboard on blockwork	55kg safe working load per fixing at a safety factor of 2, bridging up to 45mm of void from the face of the board to the blockwork. Not a cavity fixing: the load goes into the masonry behind, so it is the wrong tool on a stud wall and the right one on dot and dab. A 120mm version covers deeper voids

Manufacturer published figures. Drill a GripIt with a flat bit or a hole saw, never a twist drill, which tears the back face out of the board as it breaks through and leaves the wings nothing to bear on. A GripIt needs a void for its wings to open, so it is a stud-wall fixing; on dot and dab, use a Corefix. Cheaper options for light loads are self-drilling nylon anchors, spring toggles and metal cavity anchors.

Sleeve anchors, through bolts and concrete screws

No table here yet, on purpose. The UK equivalents of these anchors are Rawlplug (Rawlbolt, R-XPT, R-LX, R-HPTII), fischer (FSA, FAZ II, ULTRACUT FBS II) and Spit, plus resin systems such as Rawlplug R-KEM and fischer FIS V for aircrete, hollow block and lime-mortar brickwork where an expanding anchor cannot work. Drill diameter, embedment depth and installation torque are specific to the individual product and cannot be carried across from another brand or another market, so until we have transcribed the UK datasheets this page will not print numbers for them. Work from the datasheet that comes with the anchor you have bought.

SDS-plus bit sizes

Common UK sizes: 5, 5.5, 6, 6.5, 7, 8, 10, 12, 14, 16, 18, 20, 22, 24, 25 mm

Odd sizes outside this list exist but are ordered rather than stocked, so a 7 mm brown-plug hole is usually drilled with a 7 mm TCT masonry bit in a combi drill. SDS-max starts at 16 mm and takes over for big holes; SDS-plus shanks are 10 mm, SDS-max 18 mm.

Drilling mode by material

Material	Mode	Bit	Notes
Softwood: redwood and whitewood, CLS, treated softwood, plywood, MDF, OSB	Rotary (hammer off)	HSS twist or brad-point bit	Redwood and whitewood are the UK's generic softwoods and take a screw easily. Small screws, No.4 to No.6, usually drive straight in; modern UK screws such as TurboGold, GoldScrew, Spax and Reisser have a cutting point that does its own pilot in softwood. Pre-drill anyway within 50mm of a board end, where softwood splits, and in MDF, which splits worse than timber does.
Hardwood: oak, ash, beech, sapele, iroko	Rotary (hammer off)	Brad-point or HSS bit, plus a countersink	UK and European hardwoods are far milder than the dense tropical species, but they still split at board ends and still snap screws, so pre-drill within 25mm of every end and for anything above No.14. Oak needs one more decision: it is acidic, and its tannins corrode plain steel and ordinary zinc-plated screws, staining the timber blue-black around every head. Use A2 stainless indoors and A4 outdoors or near the coast, and the same goes for green oak, which is wetter and worse.
Solid brick: Victorian and pre-war walls, or the outer leaf of a cavity wall	Hammer drill or SDS-plus	TCT masonry bit, or an SDS-plus bit in a rotary hammer	Fix into the brick body, not the mortar, for anything that carries weight. The age of the house decides how it drills: a property built before roughly 1919 is very likely to have lime mortar, which is deliberately soft, crumbles under hammer action and will not hold an expanding plug. Post-war houses are cement mortar and behave the way the charts assume. If the dust coming out of the hole is pale and powdery rather than grey and gritty, you are in lime: move the hole into the brick, use a longer plug, and consider a resin anchor instead. Start on rotary to seat the bit, then switch to hammer, and blow or vacuum the dust out before the plug goes in.
Mortar joint: between bricks	Rotary or light hammer	TCT masonry bit	Mortar is weaker than the brick and crumbles under heavy hammer action, so use a joint only for light fixings, keep the hole no wider than the joint and use a longer plug. In a pre-1919 house the joint is probably lime mortar and will hold almost nothing, so move the hole into the brick. For anything that carries weight, go into the brick anyway.
Dense block: breeze block and medium-density concrete block	Light hammer, or rotary if the block is hollow	TCT masonry bit	Dense blocks take hammer action and a normal plug. Hollow and cellular blocks do not: hammer action breaks the thin webs. If the bit suddenly drops through into a void you are in a hollow block, so stop, back out, and switch to a cavity fixing, a longer plug that reaches the far web, or a resin anchor with a sieve sleeve.
Aircrete block: Thermalite, Celcon, Durox	Rotary only, hammer off	Ordinary twist or masonry bit on rotary; aircrete is soft enough that almost any bit will cut it	The substrate that catches people out most, and the one place where the usual answer is simply wrong. Never use hammer action: aircrete is soft enough that virtually any drill will make a hole through it, and hammering turns it to powder and opens the hole out. Drill slowly, because the bit wanders. Ordinary nylon wall plugs fail here too, spinning in the soft block as you tighten the screw instead of gripping, so use a purpose-made aerated-concrete plug (fischer make one with helical vanes that cut into the block and stop it rotating, in an 8 or 10mm hole) or a resin anchor. For anything heavy, spread the load across a timber batten rather than trusting a single point.
Concrete: slab, footing, precast panel	SDS-plus rotary hammer	SDS-plus masonry bit	A combi drill gets through a slab slowly; an SDS-plus rotary hammer does it in seconds and drills a rounder hole, which anchors need. Clean the hole with a blower or brush before setting anything; dust halves the hold. For a through bolt, sleeve anchor or concrete screw, work from the manufacturer's own datasheet for hole size, embedment and torque, because those figures are specific to the product and not interchangeable between brands.
Concrete lintel: over a door or window	SDS-plus rotary hammer	SDS-plus masonry bit	Prestressed concrete lintels are common over UK openings and are much harder than the brickwork around them, so you will feel the bit stop making progress. An SDS-plus rotary hammer gets through where a combi drill will struggle and burn the bit. The warning that matters: a prestressed lintel has steel tensioning wires running through the bottom of it, and cutting one can compromise the lintel. Fix into the face rather than the underside, and if something has to be fixed into the soffit of an opening, fix into the masonry either side instead. We have not found a manufacturer or NHBC figure for a minimum edge distance, so treat that as avoid the underside rather than as a measured rule.
Render: rendered or pebbledashed masonry	Rotary through the render, then hammer	TCT masonry bit or SDS-plus	Render is a skin. Go through it on rotary so it does not crater, then hammer into the brick or block behind, and size the plug and the hole depth for the masonry rather than the render. On a solid-wall Victorian or Edwardian property the render or pebbledash may be lime based and will blow out in lumps under hammer action, so keep the hammer off until the bit is through it.
Dot-and-dab plasterboard: drylined masonry wall	Rotary through the board, then hammer into the block behind	Twist bit through the board, then a TCT masonry bit for the blockwork	The most common wall in UK homes built or refurbished since about 1980, and the one every fixing chart gets wrong. The plasterboard is stuck to the blockwork on dabs of adhesive, leaving a void of up to about 45mm behind the board. A plasterboard anchor has nothing to grip once it is through the board, an ordinary plug spins in the void, and a plain frame fixing crushes the plasterboard inwards as you tighten it, so the fitting ends up loose and the board ends up cracked. The answer is a purpose-made dot-and-dab fixing that bridges the void and transfers the load to the masonry behind: Corefix is the best known, with a steel core inside the plug, and TIMco sell a Corefix-branded version. Corefix Connect 100 needs a 10mm hole, is rated 55kg safe working load per fixing, and bridges up to 45mm; a 120mm version covers deeper voids.
Stud wall: plasterboard on timber studs	No pilot for self-drilling anchors	Self-drilling nylon anchor, or a flat bit or hole saw for a GripIt	UK timber studs are usually at 400mm or 600mm centres and UK plasterboard is 9.5mm or 12.5mm thick, 12.5mm in most of the house. Find the stud and screw into it for anything heavy; a stud detector works here, unlike on lath and plaster. Between the studs, light loads take a self-drilling nylon anchor (Easyfix and Rawlplug both sell them, and the Easyfix 32mm self-drill is the Screwfix own-brand version), and heavier loads take a spring toggle, a metal cavity anchor or a GripIt. Drill a GripIt with a flat bit or hole saw rather than a twist drill, which tears the back face of the board out as it breaks through and leaves the fixing nothing to bear on.
Lath and plaster: pre-1930s walls and ceilings	Rotary only, slow, no hammer	Sharp twist bit, small pilot first	The wall or ceiling of a Victorian or Edwardian house: thin timber laths with lime plaster pushed through the gaps to form nibs on the back. It is brittle, and it is the one substrate where a stud detector is useless, because the laths and their nails set it off everywhere. Find the joist or stud by tapping for a solid note or by drilling small 3mm test holes until you hit timber, and expect to test-drill for each fixing point, because the joists in a Victorian building are not accurately spaced. Never hammer-drill it: the vibration cracks the plaster key off the back of the laths and you lose a whole panel of ceiling. Fix into the joist or stud wherever you can and do not hang anything significant from the laths alone. On a ceiling, a spring toggle through the plaster carries a light fitting at best; anything heavier needs a noggin fixed between the joists from above.
Tile: ceramic or porcelain	Rotary only, slow speed	Spear-point tile bit (ceramic) or diamond core bit with water (porcelain); tape the spot	Masking tape stops the bit skating; drill slowly and never on hammer, which cracks tiles. Once through the tile, switch to a masonry bit with hammer for the wall behind, sized for the plug, and keep holes at least 30mm from tile edges. Check what is behind before you start: tiles on a bathroom wall are often on plasterboard rather than masonry, which changes the fixing completely.
Steel: RHS, angle, brackets, sheet	Rotary, low speed	HSS or cobalt twist bit with cutting fluid	Self-drilling (tek) screws need no pilot up to their rated thickness. For bolts, drill the clearance size, which is 7mm for M6, 9mm for M8, 11mm for M10 and 13.5mm for M12, and deburr the hole.

Fixings Finder — Frequently Asked Questions

What size drill bit for a No.8 screw?+

A No.8 wood screw is 4.0mm across the thread on the UK gauge table. Drill a 2.5mm pilot in softwood and 3.0mm in hardwood such as oak or beech. If the screw passes through a top piece first, drill that piece at 4.5mm so it pulls down tight. Into masonry, a No.8 is the screw a 6mm red plug is made for.

Is a No.8 screw the same as an 8g screw?+

Yes, the same gauge under two naming conventions: the UK says No.8, Australia and New Zealand say 8g. The published metric equivalent differs slightly between the two markets' tables, UK charts giving No.8 as 4.0mm and Australian ones 4.2mm, so use the UK figure here. In practice the gauge number is fading out anyway: Screwfix and Toolstation label modern screws 4.0 x 40mm and the number on the box is the diameter.

What size drill bit for a 6mm wall plug?+

A 6mm masonry bit, drilled about 10mm deeper than the plug, so roughly 40mm for a 28mm plug. The red 6mm plug with a No.8 screw is the standard picture-hanging and shelf-bracket answer in a UK brick wall. Blow or vacuum the dust out before the plug goes in, and make sure the screw is long enough to reach past the end of the plug so it expands along its full length.

Why is the blue plug 8mm on one chart and 10mm on another?+

Because there are two UK colour schemes and they clash on exactly that colour. On a traditional chart the sequence is yellow 5mm, red 6mm, brown 7mm and blue 10mm. On a Rawlplug UNO packet it is red 6mm, brown 7mm, blue 8mm and grey 10mm. Yellow, red and brown agree; blue does not. Go by the millimetre size printed on the packet and ignore the colour, whichever chart you learned first.

How do I fix a TV to a dot-and-dab wall?+

Not with plasterboard anchors and not with a plain frame fixing. The board is stuck to the blockwork on dabs of adhesive with a void behind it, so a plasterboard anchor has nothing to grip, an ordinary plug spins in the void, and a frame fixing crushes the board inwards as you tighten it. Use a purpose-made dot-and-dab fixing that bridges the void and carries the load on the masonry behind: a Corefix Connect 100 takes a 10mm hole, is rated 55kg safe working load per fixing at a safety factor of 2, and bridges up to 45mm from the face of the wall to the blockwork. There is a 120mm version for deeper voids.

How do I tell dot-and-dab from a stud wall?+

Drill a 3mm test hole and feel what the bit goes through. Soft, then hard, then a void, then hard again is dot-and-dab plasterboard on blockwork. Soft, then hard, then timber is a stud wall. Soft the whole way through and then nothing is a stud wall with no stud where you drilled. A stud detector settles the second case but not the first, and on lath and plaster it is useless because the laths and their nails set it off everywhere.

Why will a wall plug not hold in my 1900s house?+

Most likely you are in lime mortar rather than brick. A house built before roughly 1919 almost certainly has soft lime mortar joints, which crumble under hammer action and will not hold an expanding plug at all. If the dust coming out of the hole is pale and powdery rather than grey and gritty, that is what you have. Move the hole into the body of the brick, drop the hammer action, use a longer plug, and for anything carrying weight use a resin anchor with a sieve sleeve instead of an expanding one.

Can I use hammer mode in Thermalite or aircrete?+

No. Aircrete is soft enough that almost any bit will cut it on rotary, and hammer action just turns it to powder and opens the hole out so nothing grips. Drill slowly with the hammer off, and expect the bit to wander. Ordinary nylon plugs also fail in aircrete because they spin in the block instead of expanding against it, so use a purpose-made aerated-concrete plug or a resin anchor, and spread anything heavy across a timber batten rather than trusting a single point.

Do I need to pre-drill oak?+

Yes, and use stainless steel while you are at it. Oak is dense enough to split at board ends and stiff enough to snap a screw, so pre-drill 3.0mm for a No.8 and countersink. The bigger issue is chemical: oak is acidic, and its tannins corrode plain steel and ordinary zinc-plated screws, staining the timber blue-black around every head. Use A2 stainless indoors and A4 outdoors or near the coast. The same applies to any oak-framed or oak-clad work, including green oak, which is worse.

What is the difference between a pilot hole and a clearance hole?+

The pilot hole is drilled in the piece the thread grips; it is smaller than the screw so the thread cuts into the timber. The clearance hole is drilled in the piece the screw passes through; it is the shank diameter or a little more, so the head pulls the top piece down instead of the thread holding it away.

Useful? Send it to the mate who keeps asking.