Drill Bit + Tap Size Reference
Why this matters
Wrong drill bit for the application = stripped fastener, broken bit, customer damage, OR no-thread tap. The field card every tech wishes were in the truck: bit size for tap match, bit size for screw thread, bit size for anchor. Most service calls eventually need a drilled hole; getting size right first time saves frustration + parts.
Drill bit sizing systems
Three sizing conventions used in U.S. trades:
Fractional inch (1/16" - 2"): most common for general construction.
Number (wire gauge) (#80 - #1 = 0.0135" - 0.228"): for precision + small fastener pilot holes.
Letter (A - Z = 0.234" - 0.413"): for tap drill sizes + medium precision.
Metric (1mm - 25mm): increasingly common; metric fasteners + Euro equipment.
Tap drill sizes (UNC/UNF threads - most common U.S.)
| Thread Size | Tap Drill (75% thread) |
|---|---|
| #4-40 | #43 (0.089") |
| #6-32 | #36 (0.106") |
| #8-32 | #29 (0.136") |
| #10-24 | #25 (0.149") |
| #10-32 | #21 (0.159") |
| #12-24 | #16 (0.177") |
| 1/4-20 | #7 (0.201") |
| 1/4-28 | #3 (0.213") |
| 5/16-18 | F (0.257") |
| 5/16-24 | I (0.272") |
| 3/8-16 | 5/16 (0.3125") |
| 3/8-24 | Q (0.332") |
| 7/16-14 | U (0.368") |
| 1/2-13 | 27/64 (0.4219") |
| 1/2-20 | 29/64 (0.4531") |
| 9/16-12 | 31/64 (0.4844") |
| 5/8-11 | 17/32 (0.5313") |
| 3/4-10 | 21/32 (0.6563") |
| 7/8-9 | 49/64 (0.7656") |
| 1-8 | 7/8 (0.875") |
75% thread engagement = standard. 50% for soft materials. 100% for critical / stainless.
Metric tap drill sizes
| Thread | Tap Drill (mm) |
|---|---|
| M3 × 0.5 | 2.5 |
| M4 × 0.7 | 3.3 |
| M5 × 0.8 | 4.2 |
| M6 × 1.0 | 5.0 |
| M8 × 1.25 | 6.8 |
| M10 × 1.5 | 8.5 |
| M12 × 1.75 | 10.2 |
| M14 × 2.0 | 12.0 |
| M16 × 2.0 | 14.0 |
| M20 × 2.5 | 17.5 |
Pilot + clearance hole sizes for wood screws
Pilot holes:
| Screw # | Hardwood | Softwood |
|---|---|---|
| #4 | 3/32 | 5/64 |
| #6 | 7/64 | 3/32 |
| #8 | 1/8 | 7/64 |
| #10 | 9/64 | 1/8 |
| #12 | 5/32 | 9/64 |
| #14 | 11/64 | 5/32 |
Clearance bits (screw passes through one piece + threads into another): #4 → 7/64; #6 → 9/64; #8 → 11/64; #10 → 13/64; #12 → 7/32; 1/4 → 9/32; 5/16 → 11/32; 3/8 → 13/32.
Concrete anchor holes
Specific to anchor type:
Wedge anchor (Hilti, Powers, Red Head):
- 1/4" anchor: 1/4" drill
- 3/8" anchor: 3/8" drill
- 1/2" anchor: 1/2" drill
- 5/8" anchor: 5/8" drill
Plastic anchor (basic):
- Match anchor size to drill bit
- Wall: drill 1/16" smaller than anchor
Tapcon (concrete screw):
- 3/16" Tapcon: 5/32" drill
- 1/4" Tapcon: 3/16" drill
Adhesive anchor (epoxy):
- Per manufacturer spec
- Usually 1/16" larger than rod
Use carbide-tipped masonry bit for concrete, brick, block, stone.
Drill bit types + applications
- HSS: standard for wood, soft metals, plastic - most common.
- Cobalt: stainless, harder metals.
- Carbide-tipped: concrete, stone, tile.
- Brad-point: precise wood drilling.
- Spade: large holes in wood (3/8" - 1.5"); rough.
- Forstner: precision wood holes (3/8" - 2"); clean.
- Hole saw: large holes (3/4" - 6") in wood/metal/drywall.
- Step bit/Unibit: sheet metal, thin material, multi-size in one bit.
- Auger: long-shank wood (through framing).
- Glass/tile: spear-point carbide.
- Annular cutter: precise large holes in metal.
Drill speed recommendations
| Material | RPM (1/4" bit) |
|---|---|
| Wood (soft) | 3,000 |
| Wood (hard) | 1,500 |
| Drywall | 2,500 |
| Mild steel | 1,000 |
| Stainless | 350 - 500 |
| Cast iron | 700 |
| Aluminum | 3,500 |
| Brass / bronze | 2,000 |
| Concrete | 1,500 + impact |
| Tile / glass | 100 - 500 + water |
Slower for larger bits: 1/2" bit at half the speed of 1/4" bit in same material.
Cutting fluids
- Steel + stainless: cutting oil OR coolant
- Aluminum: WD-40 OR dedicated aluminum lubricant
- Brass: dry OR very light oil
- Cast iron: dry
- Tile + glass: water (constant flow)
- Plastic: dry; let chips clear
Common drilling mistakes
Wrong bit material: HSS in concrete = burned bit + no hole.
Wrong speed: hot bit, smoking material, dull bit fast.
No pilot for large hole: chipped or wandering bit.
Skipping cutting fluid: hot bit, dull edge, work-harden steel.
Bit dull from previous use: doesn't cut; spins + burns.
Wrong RPM for material: heat damage + dulling.
Pushing too hard: bit breaks; for harder materials, less pressure + slower speed.
Quick-reference for trades
HVAC:
- Sheet metal: step bit OR HSS
- Refrigerant line through wall: 1" hole saw
- Outdoor unit mounting bolts: 3/8" carbide
Plumbing:
- Studs: 1" - 1-1/4" auger for 3/4" pipe
- Floor: 1-1/2" - 2-1/4" hole saw for drain pipe
- Tile: glass/tile bit + masking tape
Electrical:
- Drywall: 5/8" - 1-1/4" hole saw for box
- Studs: 3/4" auger for cable
- Concrete penetration: 3/4" - 1" carbide
- Floor (above-grade): 1" auger
Roofing:
- Decking: 1/4" - 5/16" pilot for screws
- Through ridge: 7/8" - 1" for ridge vent
Truck stock + chuck capacity
Wood: 3/32, 1/8, 5/32, 3/16, 1/4 pilots; 3/8, 1/2, 5/8 utility; spade set 3/8 - 1-1/4; hole saw 1-1/2 - 3. Metal: 1/16 - 1/2 in 1/64ths; step bit/Unibit. Concrete: 1/4, 5/16, 3/8, 1/2, 5/8 carbide. Specialty: tile/glass spear, 1" auger, 4" hole saw.
Chuck capacity: 3/8" chuck = max 3/8" shank; 1/2" chuck = max 1/2" shank; SDS-Plus max ~1-1/4"; SDS-Max up to 1-3/4"+.
Sharpening: most techs replace vs sharpen; Drill Doctor / bench grinder / send out. Sharpen at 118° general work; 135° hardened materials.
The single most-time-saving truck-stock item is the COMPLETE STEP BIT (Unibit) - drills 1/8" through 1-3/8" in one bit, perfect for sheet metal, thin material, OR when you don't know which exact size you'll need. Customer's electrical panel knockout, HVAC duct hole, plumbing access hole - one bit + done. Worth + saves carrying 12 individual bits. Most pros carry one or two; cheap enough to lose without crying.
References
- ANSI / ASME B94 (cutting tool standards)
- Manufacturer drill bit catalogs (DEWALT, Milwaukee, Bosch)
- Machinist Handbook (industry reference)
- Manuall internal: Fastener Torque Reference, Common Switches + Receptacles