Anchor Bolt Drill Sizes: What Size Hole Does Each Fixing Need?

Get the hole wrong and the best anchor in the world won’t hold. Too small and you’ll be hammering and torquing until your arms give up — or snapping the bolt. Too big and the anchor spins instead of gripping. And the trap that catches people out: the drill bit size doesn’t always match the thread size. A sleeve anchor with an M8 thread needs a 10mm hole, not an 8mm one.

Here’s the whole thing on one table, then the detail for each fixing:

Quick reference — drill bit size by fixing

Through bolts — drill the same diameter as the anchor

Through bolts (torque-controlled expansion anchors for concrete) are the simplest to remember: the drill bit matches the anchor. An M6 through bolt takes a 6mm masonry bit, an M8 takes 8mm, right up to M20 and beyond. We stock the full range M6–M24.

The drill bit rule:

Drill slightly deeper than the anchor’s embedment depth so the anchor doesn’t bottom out before it’s fully home — the dust at the bottom of the hole needs somewhere to go. Then blow or vacuum the hole out, insert the anchor through the fixture, and tighten until the torque setting expands the clip.

One reminder: through bolts are concrete-only. Brick, block and general masonry need a different fixing.

Shop the range: Through Bolts for Concrete

Thunderbolts — the size on the box IS the drill size

Thunderbolts — the masonry anchor screws — are labelled by drill diameter, not thread. That’s why our range runs 5mm, 6mm, 8mm, 10mm, 12mm and 16mm in odd-looking increments: each number is the hole you drill, in concrete, brick or block. An “8mm” thunderbolt goes into an 8mm hole; a “5mm” thunderbolt into a 5mm hole.

Because the anchoring happens on the screw’s threads cutting into the base material, there’s no expansion clip and no torque wrench — just drill, blow out the dust, and drive it in. The head sits on a clearance hole through the fixture, which is bigger than the drill hole (an 8mm-diameter thunderbolt uses a 12mm clearance hole in the material you’re fixing, per our product specs).

Drill slightly deeper than the embedment so the tip has room, and keep the hole straight — thunderbolts don’t forgive angled holes.

Shop the range: Thunderbolts · Full explainer: What Is a Thunderbolt Anchor?

Sleeve anchors — the sleeve sets the hole, not the thread

This is the one that catches people out. Sleeve anchors are sold by thread size, but the hole is set by the sleeve — the metal jacket that expands against the base material as you tighten the nut.

Take our M8-thread sleeve anchor: the sleeve is 10mm across, so the drill bit is 10mm, not 8mm. The clearance hole through the fixture is bigger again — 12mm in that example.

Sleeve anchors are a medium-duty through-fixing for non-cracked concrete, solid brick and masonry: mark through the fixture, drill, insert through the item, and tighten the nut to expand the sleeve.

Shop the range: Sleeve Anchors

Drop-in anchors — sized by the body, with a setting punch

Drop-in anchors are internally threaded sockets for pre-positioned fixing: drill, drop the anchor in, then knock a setting punch down inside until the base expands at the bottom of the hole. Once set, they sit flush and take a bolt or threaded rod.

Again, the drill bit follows the body diameter, not the internal thread — verified from our own product specs:

Notice M12 needs a 15mm bit, not 16mm — another reason to check the spec rather than guess. The anchor can be set accurately independent of hole depth, so it suits jobs where the insert depth matters.

Shop the range: Drop-In Anchors

Resin fixings — bigger hole, follow the data sheet

Resin anchors work differently: the hole is deliberately larger than the threaded stud so resin can bond the stud into the base material. The exact diameter and depth come from the resin product’s data sheet and depend on the stud size and substrate — there’s no one-size rule like the fixings above.

We stock Fischer resin in stud, capsule and injection formats. For when to choose resin over a mechanical anchor (and the hole-prep that goes with it), see our dedicated guide:

Resin vs Mechanical Anchors: Which Do You Need?

Five mistakes that ruin an anchor fixing

1. Not clearing the dust. Drill dust packs the bottom of the hole and stops expansion anchors seating fully. Blow it out or vacuum it — every time.
2. Not drilling deep enough. If the anchor bottoms out, it can’t expand at the right depth. Drill slightly deeper than the embedment and check the depth against the anchor before you fix.
3. Assuming thread = drill size. Sleeve anchors and drop-in anchors are both sized by their outside diameter, not the thread. The quick-reference table above is your friend.
4. Skipping hammer mode in concrete. A masonry bit in rotary mode will take forever and wear flat. For concrete, use an SDS+ hammer drill and bit.
5. Angled or oversized holes. Anchors need a straight, correctly sized hole to grip. If a hole’s oversize, move along the wall — don’t try to fill the gap with resin unless the fixing’s designed for it.

And the right drill bit for the job?

For concrete and masonry fixings, use a masonry drill bit — an SDS+ bit in a hammer drill for concrete, or a standard masonry bit in rotary-hammer mode for brick and block. We stock Dart masonry and SDS+ bits in the sizes every anchor above needs.

For metal (drilling steel plate for set screws and bolts), you’ll want HSS bits instead — masonry bits won’t cut steel cleanly.

Browse: Concrete & Masonry Drill Bits · Wood Drill Bits

Quick answers

What size drill bit for an M10 through bolt? 10mm — through bolts take a hole the same diameter as the anchor, drilled slightly deeper than the embedment.

What size drill bit for an M8 sleeve anchor? 10mm — sleeve anchors are sized by the sleeve, not the thread. An M8-thread sleeve anchor has a 10mm sleeve.

What size drill bit for an 8mm thunderbolt? 8mm — thunderbolts are labelled by drill diameter, so the size on the pack is the hole.

Do I need a hammer drill for anchor bolts? For concrete, yes — an SDS+ hammer drill with a masonry bit. For brick and block, a hammer drill setting or good rotary drill with a masonry bit.

Founded in 1999, Fastco (Fasteners & Fixings) Ltd is a leading supplier of fasteners and fixings to trades, rated 4.9/5 on Trustpilot. Order online for fast UK delivery or click & collect from our Crewe counter — and if you’re not sure which fixing or drill size the job needs, speak to an expert on 01270 252326 or email [email protected].