For a standard 3/8 tap, the drill bit size depends on the thread type you’re using.

Quick Scoop

  • For 3/8-16 UNC (coarse thread) : use a 5/16 inch drill bit (0.3125 in). This is the common choice for general-purpose 3/8 coarse threads and gives about 75% thread engagement.
  • For 3/8-24 UNF (fine thread) : common recommendations are around 0.302–0.332 in depending on your target thread engagement and material; one typical chart calls for a “Q” drill (0.332 in) for 3/8-24.

If you’re unsure and just see “3/8-16” stamped on the tap (most common in general shop work), reach for a 5/16 in drill bit.

Rule of thumb: the harder the material, the slightly larger the drill you can get away with; the softer the material, the slightly smaller you can go for stronger threads.

At the bottom of many tap-drill charts, you’ll see this formula for imperial threads:
Drill size ≈ major diameter − (1 ÷ threads per inch), which for 3/8-16 gives 0.375 − 1/16 = 0.3125 in (5/16).

Information gathered from public forums or data available on the internet and portrayed here.