Tap Size Calculator
Recommended: 65–75% for most applications
Select a Thread Size
Choose a thread standard and size above to see the recommended tap drill size, nearest standard drill bit, and thread engagement details.
How to Use This Calculator
Choose Your Thread Standard
Select the thread standard that matches your tap: UNC for coarse inch threads, UNF for fine inch threads, Metric Coarse or Fine for metric fasteners, or NPT for pipe threads. If you are unsure, check the markings on your tap — they will show the size (e.g., M8×1.25 or 3/8"-16).
Select the Thread Size and Tap Type
Pick your thread size from the dropdown, which auto-fills the major diameter and pitch. Then choose Cutting Tap or Forming (Roll-Form) Tap. Forming taps are used for ductile materials like aluminum and require a slightly larger pilot hole. Use the quick-select buttons for the most common sizes.
Adjust Thread Engagement Percentage
The default 75% thread engagement is the industry standard for most applications. For soft materials like aluminum or brass, try 55–65% to reduce tapping torque and tap breakage risk. For hard steels or critical load-bearing threads, 75–85% gives additional strength. The engagement bar shows whether your setting is in the optimal range.
Read the Drill Size and Nearest Standard Bit
The calculator shows the exact mathematical tap drill size plus the nearest standard drill bit available in fractional, wire/letter, and metric sizes. Switch to the Clearance Hole tab to find the correct hole size for a bolt to pass through without threading. Toggle units between inches and millimeters to match your drill index.
Frequently Asked Questions
What happens if I use the wrong tap drill size?
Using a drill that is too small makes the pilot hole undersized, which forces the tap to remove too much material per flute. This generates excessive torque that can snap the tap, especially in hard materials or blind holes. A broken tap is one of the most frustrating shop situations because removing it is very difficult. Using a drill that is too large produces a shallow thread that may strip under load. The calculator gives you the exact theoretical size plus the nearest standard drill so you can choose the closest available bit while understanding the actual engagement percentage you will achieve.
Why is 75% thread engagement the standard, not 100%?
This tradeoff was studied extensively by tooling manufacturers including Guhring. Their research shows that a 100% thread engagement specification produces only about 5% more shear strength than a 75% engagement thread, but requires approximately three times the tapping torque. The extra torque dramatically increases tap breakage rates, shortens tap life, and makes hand tapping much harder. For this reason, most standard tap drill charts are based on 75% engagement. In applications with very soft materials or oversize tapping situations, dropping to 60–65% is common. Only in critical load-bearing joints with hard materials is engagement above 75% generally justified.
What is the difference between a cutting tap and a forming (roll-form) tap?
Cutting taps (also called fluted taps) remove material from the hole wall as they advance, producing chips that need to be cleared. Forming taps, also called roll-form or thread-rolling taps, have no cutting edges — they plastically deform the workpiece material to create thread flanks without removing any metal. Because forming taps displace material radially outward, they require a larger pilot hole than cutting taps for the same nominal engagement. Forming taps produce stronger threads (cold-worked surface) and no chips, but they only work in ductile materials (aluminum, mild steel, copper, brass) and cannot be used in cast iron, hardened steel, or other brittle materials.
What is the metric shortcut for tap drill size?
For metric coarse threads at standard 75–100% thread engagement, a well-known shop shortcut is: tap drill diameter = major diameter minus pitch. For example, an M10×1.5 thread needs a 10 − 1.5 = 8.5 mm drill. An M6×1.0 thread needs a 6 − 1.0 = 5.0 mm drill. This shortcut gives approximately 100% theoretical thread engagement, but in practice the actual engagement is slightly less because the drilled hole is rarely perfectly on-size. It works well for standard metric coarse threads in most materials and is easy to remember without a calculator.
What is a clearance hole and when do I use it?
A clearance hole is a hole drilled through one part of an assembly so a bolt or screw can pass through freely without threading — the bolt is then threaded into a tapped hole in the mating part, or secured with a nut. Clearance hole diameters are always larger than the bolt major diameter. ISO 286 defines three fits: close fit (approximately 1.04× major diameter) for precision assemblies with tight positional tolerances, normal fit (approximately 1.10×) for general use, and loose fit (approximately 1.15×) for applications where slight misalignment is expected or hole-to-hole position tolerances are loose. Use the Clearance Hole tab to find the right diameter for your application.
Why does the NPT pipe thread calculator work differently?
NPT (National Pipe Taper) threads are fundamentally different from straight machine threads. They have a 1-in-16 taper so that wrench-tightening causes the thread flanks to wedge together, creating a pressure-tight seal without a separate sealer on the threads — though PTFE tape or pipe compound is still normally used. Because the diameter changes along the thread length, the 'tap drill size' for NPT refers to the hole diameter at the start of engagement (the pipe end). The NPT data in this calculator is a fixed reference table from ASME B1.20.1, not a calculated value, because the taper geometry makes formula-based calculation impractical for a standard lookup.