Tap & Drill Size Calculator
Tap drill sizes and thread data across imperial and metric thread standards.
Tap & Drill Size Calculator
Pick the right tap drill for any UNC, UNF, UNEF, or metric thread, with live % thread engagement, forming-tap support, clearance holes, and a full chart.
How the tap drill size is calculated
A tap drill leaves just enough material for the tap to cut a thread. Too small and you snap the tap; too large and the threads are shallow and strip out. The size depends on the percent of thread engagement you want.
Worked example, 1/4-20 UNC at 75%: pitch = 1/20 = 0.050". Drill = 0.250 − (1.299 × 0.050 × 0.75) = 0.2013". The nearest real drill is a #7 (0.201"), which is exactly what every shop chart lists.
Picking your thread percentage
75% thread is the old textbook target, but it is rarely the smart one. Going from 75% to 60% engagement costs only about 5% of pull-out strength while cutting tapping torque and tap-breakage risk dramatically. That trade is why production shops live around 60 to 65%.
Common questions
Why does my drill chart sometimes disagree by one drill size?
Charts are built around different default thread percentages (some use 75%, some 70%, some 65%) and some round to the nearest fractional drill while others allow number and letter drills. This tool shows the theoretical diameter and the actual % thread of the drill it picks, so you can see exactly what you are getting instead of trusting a single fixed row.
Cutting tap vs forming tap, which should I use?
Forming (roll) taps make no chips, produce stronger threads by cold-working the metal, and last far longer, but only in ductile materials like aluminum, mild steel, and copper. Cutting taps work in nearly everything including hard and brittle metals. If you are tapping stainless above 30 HRC, hardened steel, or cast iron, use a cutting tap.
What is a clearance hole and why is it different?
A clearance hole is for the bolt to pass through the top piece freely, not to be threaded. It is slightly larger than the bolt's major diameter. Close, normal, and loose fits give you progressively more room for misalignment. This tool lists all three for the selected size.
Can I trust these numbers for production?
The formulas are the standard Machinery's Handbook relations and the drill diameters are ANSI standard sizes. They are accurate for laying out and selecting tooling. For tight-tolerance or safety-critical threads, verify with a thread gauge on a test piece in your actual material, since material hardness and tap wear shift real results.
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