Tap Drill Size Chart

How to Use This Chart

Use this tap drill size chart to find the drill bit diameter needed before cutting internal threads. The standard and metric tables also show clearance drill sizes for holes where the screw or bolt should pass through without engaging threads.

What Each Column Means

Screw Size is the fastener callout (a number size, fraction, or metric M designation). On the standard table, Major/Minor Diameter and Threads Per Inch describe the screw's actual thread geometry; the metric table lists Thread Pitch in millimeters in place of threads per inch. Tap Drill is the hole diameter to drill before tapping, split by material class since softer and harder metals call for different thread engagement. Clearance Drill is a larger hole for a fastener to pass through freely into a nut, insert, or a second tapped part.

Picking the Right Drill Size for a Tap

Find your fastener's screw size in the leftmost column, then read across to the Tap Drill section and pick the drill size under the thread-engagement column that matches your material. That diameter is what to drill before running the tap. If the fastener instead needs to pass through the hole without threading, such as into a nut or a second part, use the Clearance Drill columns instead. See our guide to designing parts for tapping for how hole size, thread engagement, and material thickness fit together when you're laying out a part.

Thread Percentage: Strength vs. Tapping Difficulty

Thread percentage is how much of a full thread profile the tap actually cuts. A larger tap drill leaves less material, so it cuts a lower percentage of thread; a smaller tap drill leaves more material for a higher percentage. Higher engagement holds more load, but it also takes more torque to tap, wears taps faster, and raises the risk of breaking a tap off in the hole, especially in tougher materials. That's why the chart defaults to 75% thread for aluminum, brass, and plastics and 50% thread for steel, stainless, and iron: those percentages balance holding strength against how hard the material is to tap. Requirements outside typical loads may call for adjusting up or down from those defaults.

Common Mistakes Reading the Chart

• Drilling to the Clearance Drill size when the hole was meant to be tapped. The hole ends up too large for the tap to cut any usable thread.
• Reading across the wrong table, matching an inch screw size to the metric table or a metric size to the standard table.
• Ignoring thread pitch on metric sizes that list more than one pitch. Coarse and fine pitches for the same screw size use different tap drills.
• Using the same tap drill size across materials without checking the thread-engagement column, which produces overtight aluminum holes or undersized, tap-breaking holes in stainless.

Material Considerations

Aluminum, brass, and plastics tap easily and tolerate the higher 75% thread engagement, but thin sheet is still prone to stripped threads under repeated assembly. Steel, stainless, and iron are harder on tools: stainless in particular work-hardens as it's cut, which raises the odds of a broken tap if the drilled hole runs undersized, so the chart's 50% engagement column and steady cutting lubricant both matter more there. Material thickness limits thread engagement too: thin sheet often can't hold enough thread depth for a reliable tapped connection, which is when PEM hardware is worth considering instead. If you're ordering parts through Fabworks, our tapping service automatically sizes tap drill holes to the fastener and material you select.