A 1/2-inch bolt needs a 33/64-inch clearance bit for a close fit, while a 1/2-inch tap needs a 27/64-inch or 29/64-inch tap drill and a 1/2-inch concrete anchor uses a 1/2-inch bit.
Clearance holes, tap drills, anchor holes, and lag-bolt pilot holes all share the “1/2-inch” label but need four different bits.
Short answer: 33/64 in. for a close-fit bolt clearance hole, 27/64 in. for a 1/2-13 tap, 1/2 in. for a common concrete anchor, and the manufacturer’s chart for a lag bolt.
What Size Drill Bit for a 1/2 Inch Bolt Clearance Hole?
A 1/2-inch bolt through steel or wood needs a clearance hole slightly larger than the bolt. Standard references list 33/64 in. for a close fit and 17/32 in. for a free fit. A 1/2-inch bit is usually too tight and is not the recommended clearance size. Use 17/32 in. when you need tolerance for misaligned plates or want the bolt to slip in by hand. If you only buy one twist bit, the close-fit 33/64 in. covers most bolted connections.
What Size Drill Bit for Tapping a 1/2-Inch Thread?
Tapping needs a hole smaller than the bolt, because the tap cuts threads into the surrounding material. Bolt Depot’s US tap drill size table lists 27/64 in. for 1/2-13 UNC and 29/64 in. for 1/2-20 UNF.
- Too small — the tap binds, flutes clog, and you risk snapping the tap.
- Too large — threads come out shallow and the bolt strips under load.
- Too deep — a bottoming tap may not reach; keep depth to about 1.5 times the bolt diameter.
Double-check the thread pitch before drilling: 1/2-13 and 1/2-20 take different tap drills, and the difference between 27/64 in. and 29/64 in. is easy to miss.
What Size Drill Bit for a 1/2-Inch Concrete Anchor?
For common wedge and sleeve anchors, the bit matches the anchor diameter — a 1/2-inch anchor gets a 1/2-inch bit. Eugene Fast’s concrete anchor drill bit chart confirms this for standard wedge and sleeve designs. Bit size alone does not guarantee a strong hold: base material, embedment depth, and hole depth all matter. A bit too small prevents proper expansion; too large reduces holding strength. Drill to specified depth, blow out dust, and drive the anchor so the sleeve expands fully. Screw-type or drop-in anchors may differ — check that anchor’s own chart.
What Size Drill Bit for a 1/2-Inch Lag Bolt?
A lag bolt pilot hole is not a clearance hole, and the size comes from the manufacturer’s chart, depending on lag screw diameter and wood species — pine takes a different pilot than oak. Hardware bolted to a deck or framing almost always ships with a drilling recommendation; follow it. Using a 1/2-inch bit as a “pilot” defeats the purpose: without a proper pilot, the lag screw splits the wood and the threads bite poorly. For the clearance side of that project, a matched pair of 1/2-inch drill bit options covers both needs.
The Common Mistake That Wrecks All Four Jobs
The biggest error is assuming every “1/2-inch” fastener needs the same bit.
| Application | Recommended Bit | Why It Works |
|---|---|---|
| Bolt clearance (close fit) | 33/64 in. | Lets the bolt slide in without binding |
| Bolt clearance (free fit) | 17/32 in. | Extra tolerance for misaligned plates |
| Tap drill, 1/2-13 UNC | 27/64 in. | Cuts full-thread depth for the pitch |
| Tap drill, 1/2-20 UNF | 29/64 in. | Matches the finer thread pitch |
| Wedge/sleeve concrete anchor | 1/2 in. | Bit matches anchor diameter for expansion |
| Lag bolt pilot hole | Chart per size and wood | Prevents splitting, ensures thread bite |
Two habits prevent almost every mistake: read the thread pitch before drilling a tapped hole, and treat “1/2-inch” as the fastener diameter, not the bit size.
FAQs
Can I use a 1/2-inch bit for a 1/2-inch bolt?
Usually no. A 1/2-inch bit is too tight and is not a recommended clearance size. Standard references list 33/64 in. for a close fit or 17/32 in. for a free fit. Exceptions are where the hole matches the fastener on purpose, such as concrete anchors.
Why is a tap drill smaller than the bolt diameter?
Because the tap cuts threads into the surrounding material rather than clearing it away. For 1/2-13 UNC the tap drill is 27/64 in., and for 1/2-20 UNF it is 29/64 in. A full-diameter hole would leave no material for threads to bite into.
Do all 1/2-inch concrete anchors use a 1/2-inch bit?
Wedge and sleeve anchors typically match bit to anchor diameter, so a 1/2-inch anchor gets a 1/2-inch bit. Screw-type and drop-in styles can differ, so confirm against your anchor’s chart. Embedment depth and hole depth also affect holding strength.
References & Sources
- Bolt Depot. “US Tap Drill Size” Tap drill dimensions for 1/2-13 UNC and 1/2-20 UNF threads.
- Eugene Fast. “Concrete Anchor Drill Bit Chart” Shows the 1/2-inch bit match for common wedge and sleeve anchors.
