The tap drill for 4-40 UNC is #43 (0.0890 in, 2.26 mm), which gives about 71% of full thread depth. The exact 75% hole is 0.0876 in (2.23 mm). The table lists every number, letter, fractional and metric drill that gives between 55% and 90% thread, so you can work from the bits you own.
| Drill | Type | in | mm | Thread | Note |
|---|---|---|---|---|---|
| #44 | number | 0.0860 | 2.18 | 80% | deep, hard to tap |
| 2.2 mm | metric | 0.0866 | 2.20 | 78% | |
| #43 | number | 0.0890 | 2.26 | 71% | chart drill |
| 2.3 mm | metric | 0.0906 | 2.30 | 66% | easier to tap |
| #42 | number | 0.0935 | 2.37 | 57% | easier to tap |
| 3/32" | fraction | 0.0938 | 2.38 | 56% | easier to tap |
About the 4-40 thread
4-40 is the coarse #4 machine screw thread, 0.112 in nominal. It is the jack-screw thread on D-sub (serial, VGA and similar) connectors, a common thread for electronics standoffs and small panel hardware, and a staple of radio-control models, model railways and scientific instruments. At this size a tap is about the thickness of a drill bit shank, so most of the advice on this page is about not breaking it.
The chart drill is #43 (0.089 in), about 71% thread. One size larger, #42, drops to about 57%, which some people accept in hard steel to save the tap; the table below shows what each nearby drill gives.
4-40 thread dimensions
| Dimension | Value | Also |
|---|---|---|
| Nominal diameter | 0.1120 in | 2.84 mm |
| Threads per inch | 40 | pitch 0.0250 in (0.635 mm) |
| Basic pitch diameter | 0.0958 in | 2.432 mm |
| Basic minor diameter (nut) | 0.0849 in | 2.157 mm |
| Tensile stress area | 0.0060 in² | 3.89 mm² |
| #4-48 UNF (same diameter): tap drill | #42 (0.0935 in) | 68% thread |
Clearance hole for #4
The tapped hole is in the part that holds the thread; the part being clamped gets a clearance hole the bolt passes through. For #4 a normal fit is #30 (0.1285 in). Never drill the clearance part with the tap drill: the screw will not pass.
| Fit | Drill | Diameter |
|---|---|---|
| Close fit | #32 | 0.1160 in |
| Normal fit | #30 | 0.1285 in |
| Loose fit | #27 | 0.1440 in |
How deep to drill a blind 4-40 hole
| Part material | Full thread wanted | Drill depth (full diameter) | Depth to drill tip (118°) |
|---|---|---|---|
| Steel or cast iron (1 × diameter) | 0.112 in | 0.287 in (about 9/32″) | 0.313 in (about 5/16″) |
| Aluminium or brass (2 × diameter) | 0.224 in | 0.399 in (about 13/32″) | 0.425 in (about 7/16″) |
Wrench size for #4
- Machine screw hex nut: 1/4 in wrench
- Socket head cap screw: 3/32 in hex key
SAE bolt grades start at 1/4 in, so there is no graded torque for #4 machine screws; tighten to the maker’s figure or until snug. Over-tightening is the usual way small tapped threads strip.
Threads easily mistaken for 4-40
| Thread | Diameter (in) | Diameter (mm) | Pitch |
|---|---|---|---|
| #4-40 UNC (this page) | 0.1120 | 2.84 | 40 TPI (0.64 mm) |
| #4-48 UNF | 0.1120 | 2.84 | 48 TPI (0.53 mm) |
| M3 x 0.5 | 0.1181 | 3.00 | 0.5 mm (50.8 TPI) |
Tips for tapping 4-40
- Use a spiral-point (gun) tap in through holes; it pushes chips ahead and is far less likely to break than a hand tap.
- Clean and lubricate often. A #4 tap jammed by chips in a blind hole usually breaks before you feel it bind.
- M3 x 0.5 is the nearest metric thread (0.118 in): a 4-40 screw will drop loosely into an M3 hole, and an M3 screw will not enter a 4-40 hole.
- In soft plastic, a 4-40 tapped hole strips easily; use a deeper hole or a brass heat-set insert.
Frequently asked questions
What size drill bit for a 4-40 tap?
#43 (0.0890 in), which gives about 71% thread. The exact 75% size is 0.0876 in (2.23 mm).
How deep should a blind 4-40 hole be?
For a full thread of 1 × diameter in steel, drill about 0.287 in (about 9/32″) at full diameter with a plug tap, or 0.313 in (about 5/16″) to the drill tip; in aluminium, about 0.399 in (about 13/32″).
What is the tap drill for 4-40 in mm?
2.3 mm is the closest 0.1 mm drill to #43 (2.26 mm) and gives about 66% thread; 2.2 mm gives a deep 78%.
What is a 4-40 screw used for?
Electronics: D-sub connector jack screws, standoffs and small panels, plus models and instruments.
Sources and method
Every dimension and percentage on this page is computed from the formulas listed, not copied from another chart. Values are basic (nominal); real parts are made to tolerance classes (2A/2B for inch, 6g/6H for metric).
- ASME B1.1 and FED-STD-H28/2B (Unified inch), ISO 68-1 and ISO 261 (metric): basic thread profile and stress area formulas.
- Machinery’s Handbook (Industrial Press): percentage-of-thread relation used by our tap drill size calculator.
- ASME B94.11M twist drill series (number, letter, fractional); 0.1 mm metric drill steps.
- ISO 273 metric clearance holes; inch close, normal and loose fits as published in the ASME B18.2.8 clearance classes.
- SAE J429 and ISO 898-1 proof strengths; T = K × D × F torque relation (see our bolt torque chart for the full method).
- Wrench and hex key sizes: ASME B18.2.1, ASME B18.3, ASME B18.6.3, ISO 4014/4017, DIN 933 and ISO 4762 values as widely published.
More thread and fastener tools
- All thread and fastener charts
- UNC and UNF thread chart
- Metric thread pitch chart
- Tap drill chart (inch and metric)
- Clearance hole chart
- Nail size chart
- Wood screw size chart
- Lag screw pilot hole chart
- Bolt torque chart
- Wrench size chart
- Drill bit size chart
- Thread engagement chart
- Nail gun nail sizes by gauge
- Screw and nail size calculator
- Tap drill size calculator
- Clearance hole size calculator
- Tapped hole depth calculator
Last reviewed: 24 October 2026. How we build and check our data: methodology and corrections. Spotted an error? Please let us know.