Tap Drill Size: US Imperial & Metric Chart

The tap-drill size is the hole size you need to drill before using a tap to cut accurate internal threads. This drill bit size is generally slightly bigger than the minor diameter of the screw or male fastener.

UNC Tap Drill Size Chart

This is the is the standard American thread widely used in the US for most general-purpose jobs. UNC stands for Unified National Coarse and the thread sizes are specified in inches.

💡 Pro Toolmaker Tip: When tapping difficult-to-tap materials such as stainless steel and alloy steels (including Grade 8 fastener steel and tool steels), using a slightly larger drill from the optional “For Difficult Material” column can make tapping easier and reduce the risk of tap breakage.

The caveat is that a larger hole size lowers thread engagement. While they will provide enough holding for general use, the correct drill bit size for tapping depends on the material, accuracy and application.

The drill-tap sizes mentioned on this page are primarily for hard materials like metals, hard plastics, etc. For woodworking, you will be drilling pilot holes for driving long and thick screws in dense wood. You can see the drill bit size for wood screws here.

UNF Tap Drill Size Chart (Unified National Fine)

Filter or search this table to find the correct tap drill size for standard American fine threads. Fine threads have a shallower pitch and require highly precise hole sizing to prevent stripped threads or broken taps.

📌 Note: For 1/2″ – 20 UNF the nearest larger size inch drill bit is 15/32″ (0.4688″) which is significantly larger. The metric 11.70 mm bit is a better choice for optional lower-engagement size for difficult materials.

💡 Pro Toolmaker Tip: UNF fine threads have finer pitch and shallower thread depth compared to UNC coarse threads. This means there is less room for error and drilling the correct tap-drill size hole in important. When tapping fine threads, set the workpiece perpendicular to the drill press axis and properly aligned to prevent cross-threading. Use tapping oil (cutting fluid) to reduce friction and get clean cut threads without damages.

Metric Tap Drill Size Chart

This chart provides the drill bit sizes for metric coarse threads which is the standard thread size under the metric system.

• The above formula is applicable only for metric coarse threads.

Metric Fine Tap Drill Size Chart

Fine metric threads have a smaller pitch than standard coarse threads, providing higher stripping strength and finer adjustment precision.

📌 Note: Nearest Imperial sizes are practical drill-size equivalents. Where two sizes are similarly close, the slightly larger drill may be shown to help reduce tapping torque and the risk of tap breakage.

💡 Pro Tip: Unlike imperial fasteners where the fine thread series (UNF) is highly standardized to one pitch per diameter, metric diameters frequently feature multiple different fine pitches (e.g., M10 can come in 1.0mm or 1.25mm pitch). Always verify both the bolt diameter and thread pitch before drilling. A thread-pitch gauge can help identify the pitch, while a caliper or micrometer can be used to check the screw/bolt’s diameter. Verify your thread before drilling, as using the wrong drill bit size can result in tap breakage (if the hole is too small), or reduced thread engagement if the hole is too large.

NPT Tap Drill Size Chart (National Pipe Taper)

Below, you can find the correct tap drill size for standard American tapered pipe threads. Because NPT threads taper at 3/4 inch per foot (1:16 ratio), choosing the right pre-tap preparation is vital to achieve a leak-proof, pressure-tight seal.

📌 Note: The tap-drill sizes shown for NPT are practical general-purpose values.
NPT threads have a 1:16 taper (3/4″ per foot).
The Tap Drill (No Reaming) column is for drilling a cylindrical hole directly before tapping. The Pre-Drill (for Taper Reamer) column gives a practical starting drill size when the hole will subsequently be tapered with a 1:16 reamer before tapping. Exact drilling and reaming requirements can vary with the tap, reamer, material, and application. Always follow the tooling manufacturer’s recommendations for critical work.

💡 Pro Tip: NPT threads can be tapped directly after drilling to the recommended tap-drill size. Where a more accurately tapered bore is desired, a 1:16 taper reamer can be used after drilling and before tapping. The reamer establishes the tapered hole geometry and can reduce the amount of material the tap must remove. For critical or high-pressure applications, follow the tap and reamer manufacturer’s recommended drilling, reaming, tapping, and gauging procedures.