What drill is listed for 2-4½ UNC?

1 25/32 in (1.78125 in), also listed in the ICS chart. The geometric D − pitch estimate is 1.7778 in; a 1 inch drill is not a suitable rounding of that estimate.

Why is #2-56 listed with a #50 drill?

This row uses the ICS wire-gauge lookup, #50 at 0.0700 in, rather than the nearest fractional drill. Alternative columns in a supplier chart are not exact equivalents.

Is a tap drill the same as a clearance drill?

No. A tap drill leaves material for an internal thread. A clearance hole lets the fastener pass without threading. Select clearance dimensions from the drawing and applicable fit requirements.

Does this chart apply to forming taps?

No. These are cutting-tap starting sizes. Forming taps displace material and require a different hole range specified by the tool manufacturer for the material and required thread.

Tap Drill Size Calculator

Estimate an inch cutting-tap pilot, compare a supported drill candidate and calculate its theoretical thread-height percentage.

Page last changed 2026-10-19 · based on n/a — undated chart, page 2

Quick Answer

D − pitch is a starting estimate. For 2-4½ UNC it gives 1.7778 in; the nearest supported drill is 1 25/32 in (1.78125 in). Selected numbered sizes use the stated supplier lookup; the other sizes use the geometric method. Confirm the selected tap’s hole range.

Tap Drill Calculator

Major diameter (in)

Threads per inch

The Formulas Used

Pitch = 1/TPI; geometric pilot = D − pitch. Theoretical cut-thread height (%) = (D − selected hole) × TPI / 0.01299, using inch diameters. This is not axial engagement length or joint strength.

Selection methods and coverage

Rows marked ICS wire-gauge lookup use the corresponding column of the ICS tap chart. Other rows select the nearest size to D − pitch from number drills #80–#1, letters A–Z and a 1/64-inch grid through 2 inches; metric candidates use 0.1 mm steps from 1–20 mm and 0.5 mm steps to 80 mm. Actual stock availability must be confirmed. An out-of-range pilot must not be clamped to the smallest or largest drill.

Thread height is not axial engagement

D − pitch is a geometric shortcut, not an exact 75% rule. The Guhring chart, page 2 gives theoretical cut-thread percentage as (major − hole) × TPI / 0.01299 for inch threads. This describes a thread-height convention, not axial engagement length, joint strength or an acceptance limit. Cutting taps only. The selected diameter is a starting reference, not a tolerance limit. Confirm the tap maker’s hole range, material, required thread and inspection method. Forming taps require different holes.

Frequently Asked Questions

What drill is listed for 2-4½ UNC?: 1 25/32 in (1.78125 in), also listed in the ICS chart. The geometric D − pitch estimate is 1.7778 in; a 1 inch drill is not a suitable rounding of that estimate.
Why is #2-56 listed with a #50 drill?: This row uses the ICS wire-gauge lookup, #50 at 0.0700 in, rather than the nearest fractional drill. Alternative columns in a supplier chart are not exact equivalents.
Is a tap drill the same as a clearance drill?: No. A tap drill leaves material for an internal thread. A clearance hole lets the fastener pass without threading. Select clearance dimensions from the drawing and applicable fit requirements.
Does this chart apply to forming taps?: No. These are cutting-tap starting sizes. Forming taps displace material and require a different hole range specified by the tool manufacturer for the material and required thread.

Value Sources

References and calculation methods used for this page are listed below. Numbered markers connect table columns where present. A reference listing does not itself establish that every value has been independently checked.

#	Source	Type	Revision / method
[1]	Geometric pilot and thread-height estimate	derived	The formulas displayed above, with the declared drill series and supplier lookups.
[2]	ICS Cutting Tools — tap drill chart	reference	n/a — undated supplier chart, selected rows checked 2026-10-19 — source
[3]	Guhring tap-drill chart	reference	n/a — undated chart, page 2 — source

Data Sources

Standard	Revision	What it covers on this page
Guhring cut-thread percentage convention	n/a — undated chart, page 2	the theoretical thread-height formula; does not define thread acceptance limits

Verification scope

2026-10-19 — Numbered-thread lookups and 2-4½ UNC example compared with ICS; numeric input and range checks are separate software tests. Evidence

Derived values — the following values on this page are calculated, not taken directly from the standard:

Value	How it is derived
Pilot, candidate units and theoretical percentage	Computed locally from the displayed formulas; supplier lookup entries are stated separately.

Cutting taps only. The selected diameter is a starting reference, not a tolerance limit. Confirm the tap maker’s hole range, material, required thread and inspection method. Forming taps require different holes.

Accuracy and use. Use the source notes and recorded verification scope above to assess each value. Calculated values follow the stated derivations; compilation values may lack a confirmed original document or product condition. No warranty, express or implied, is made as to the accuracy or completeness of this information, and no liability is accepted for any loss or damage arising from its use. Engineering reference data is provided for guidance in preliminary work — before a value is used for design, fabrication or acceptance testing, verify it against the current revision of the governing standard and against your own inspection. The user assumes all risk and responsibility in connection with the use of this information.

Cite This Page

Read the source notes, calculation methods and verification scope above before reusing a value. To cite this page:

Plain text

Engineering Reference. "Tap Drill Size Calculator." https://refmanual.com/calculators/tap-drill/ (accessed 2026-10-19). Data from Guhring cut-thread percentage convention, n/a — undated chart, page 2.

BibTeX

@misc{tap-drill,
  title        = {Tap Drill Size Calculator},
  author       = {{Engineering Reference}},
  howpublished = {\url{https://refmanual.com/calculators/tap-drill/}},
  note         = {Accessed: 2026-10-19},
  source       = {Guhring cut-thread percentage convention, n/a — undated chart, page 2}
}

RIS (EndNote, Zotero, Mendeley)

TY  - ELEC
TI  - Tap Drill Size Calculator
AU  - Engineering Reference
PB  - Engineering Reference
UR  - https://refmanual.com/calculators/tap-drill/
PY  - 2026
DA  - 2026-10-19
N1  - Source: Guhring cut-thread percentage convention, n/a — undated chart, page 2
ER  -