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Screw Size Chart

Look up numbered wood screw diameters, convert inches and millimeters, or identify the closest traditional screw size from a measurement.

Find a Screw Size

Reference table

Screw Size Chart: Number to Diameter

Nominal numbered sizes with exact inch values, calculated metric equivalents, approximate 1/64-in references, and historical TPI.

Search by screw number, decimal diameter, metric diameter, fraction, or TPI. The finder highlights the matching row.

Scroll horizontally to view additional columns and vertically to view additional rows. Column headers remain visible while scrolling.

Traditional numbered inch-series wood-screw nominal sizes. The 1/64-in column is rounded for measurement convenience and is not the governing screw designation. Modern specialty screws can use different geometry.
Nominal diameter Nearest 1/64 in
(approx.)
#0 0.060 1.52 1/16 32
#1 0.073 1.85 5/64 28
#2 0.086 2.18 3/32 26
#3 0.099 2.51 3/32 24
#4 0.112 2.84 7/64 22
#5 0.125 3.17 1/8 20
#6 0.138 3.51 9/64 18
#7 0.151 3.84 5/32 16
#8 0.164 4.17 5/32 15
#9 0.177 4.50 11/64 14
#10 0.190 4.83 3/16 13
#11 0.203 5.16 13/64 12
#12 0.216 5.49 7/32 11
#14 0.242 6.15 15/64 10
#16 0.268 6.81 17/64 9
#18 0.294 7.47 19/64 8
#20 0.320 8.13 5/16 8
#24 0.372 9.45 3/8 7

Scope: traditional inch-series wood-screw numbering. The TPI column is historical reference data for this series—not a universal thread specification for modern screws. Machine, tapping, drywall/deck, structural, self-drilling, and manufacturer-specific screws require their own governing data.

Table actions

CSV export contains the independently assembled public-domain NBS-derived nominal-size dataset plus Turn2Engineering calculations. It does not reproduce proprietary ASME dimensional tables.

Which Screw Size Chart Do You Need?

A numbered size such as #8 identifies a nominal diameter convention, but it does not by itself identify the thread form, pitch, or screw family. Choose the reference that matches the designation on the fastener.

Numbered wood screw

Use the finder and table on this page for traditional numbered wood-screw diameters such as #8 or #10.

Machine screw, such as #8-32

The second number is threads per inch. Use the UNC Thread Chart when you need Unified coarse thread geometry.

Dimension reference

Shank Diameter vs Thread-Root Diameter

The size chart uses nominal body/shank diameter D. Pilot-hole guidance depends on the smaller thread-root diameter dᵣ.

Wood screw diameter dimensions Side view of a traditional wood screw showing nominal shank diameter D, smaller thread-root diameter d r, and screw length L. The selected nominal D value is repeated beside the diagram. D dᵣ L
Conceptual dimension reference; not a manufacturing drawing or scale representation.
D — selected nominal diameter
0.164 in (4.17 mm)
dᵣ — thread-root diameter
Measure actual screw / product-specific
L — screw length
Separate from gauge number

Pilot Holes: Measure the Root, Not the Outside Diameter

USDA Forest Products Laboratory guidance accompanying Equation (8–10) states that lead holes are about 70% of the thread-root diameter in softwoods and about 90% in hardwoods. The equation has stated screw gauge and length limits; these percentages are not percentages of nominal outside diameter and are not a universal product prescription.

Scope warning: modern structural, deck, self-drilling, coated, and proprietary screws can require manufacturer-specific pilot holes. Use this calculator only as a reference to the cited FPL condition.

Optional calculation

Root-Diameter Pilot Reference

Enter a measured thread-root diameter; do not substitute the outside diameter.

Physical check uses the selected finder size #8 (0.164 in nominal outside diameter).

After determining a target diameter, use the Drill Bit Size Chart to choose a nearby available bit.

How to Read and Identify Screw Sizes

#8 × 1-1/2 in.
#8 identifies the traditional numbered diameter; 1-1/2 in. is nominal length.
#8-32 × 1 in.
#8-32 denotes a numbered machine-thread size with 32 threads per inch; diameter alone does not define the thread.
M4 × 0.7
M4 is a 4.00 mm metric nominal thread diameter and 0.7 mm is pitch. It is not the same designation as #8.

Screw gauge to diameter

The traditional numbered series increases by 0.013 in per consecutive gauge, beginning at #0 = 0.060 in. The historical table omits #13, #15, #17, #19, #21, #22, and #23, so this page does not synthesize those designations.

#4 checkpoint

#4 is 0.112 in nominal. Exact conversion gives 2.8448 mm; this page displays 2.84 mm.

#10 checkpoint

#10 is 0.190 in nominal. Exact conversion gives 4.826 mm; this page displays 4.83 mm.

How to identify an unknown screw

  1. Measure the outside/body diameter with calipers in an unthreaded shank area when available, or carefully across the major thread diameter.
  2. Enter the measured value in the finder and choose inches or millimeters.
  3. Use Nearest for identification; use At least or No larger than only when you are applying a dimensional threshold.
  4. Verify the thread family separately. Diameter alone cannot prove that a screw is wood, Unified machine thread, tapping, or metric.
  5. Check the product specification before using the result for structural loading, tapped-hole fit, or installation requirements.

Worked Screw Size Examples

Example 1 — #8 lookup

Input: #8. Result: 0.164 in nominal, 4.17 mm displayed, 15 traditional TPI. The nearest 1/64-in measuring reference is 5/32 in, but 5/32 in is not the governing #8 diameter.

Example 2 — 0.180 in measurement

Nearest: #9 at 0.177 in, a −0.003 in difference. At least this diameter: #10 at 0.190 in. This demonstrates why matching rule matters.

Example 3 — 4.20 mm measurement

Nearest traditional numbered size: #8 (4.1656 mm canonical). That does not make the fastener an M4 screw; metric thread designation must be checked separately.

Standards, Dataset Scope, and Source Notes

Dataset: T2E-TRADITIONAL-WOOD-SCREW-SIZES-v1.0 · Source checked September 4, 2026.

The published table is an independently assembled reference based on the public historical U.S. Bureau of Standards screw-number table, with overlapping modern engineering handbook values cross-checked against USDA Forest Products Laboratory. Current ASME B18.6.1 defines the traditional inch-series wood-screw scope but its complete normative dimensional tables are not reproduced here.

Dataset and Source-Check Details

Publisher
Turn2Engineering
Dataset label
T2E-TRADITIONAL-WOOD-SCREW-SIZES-v1.0
Canonical unit
Inches
Metric conversion
Exact 25.4 mm per inch; display rounded to 2 decimals
Listed range
#0 through #24 with historical omissions preserved
Full CSV rights
Allowed for this independently assembled public-domain-derived nominal dataset and calculated fields; proprietary ASME tables excluded
Known exclusions
Machine, tapping, drywall, deck, structural, self-drilling, and manufacturer-specific screw geometries
Source-check label
Research sources checked September 4, 2026; not a claim of ASME approval or project-specific engineering review

Browse all Engineering Charts & Tables.

Screw Size Chart FAQ

A #8 traditional wood screw has a nominal body/shank diameter of 0.164 in, which converts exactly to 4.1656 mm and is displayed as 4.17 mm.

Use the Diameter to Identify the Size, Then Verify the Thread Family

For a traditional numbered wood screw, use the finder or table to map the gauge to nominal diameter. When identifying an unknown screw, measure the outside diameter first, then verify whether its thread family is wood, Unified machine thread, metric, tapping, or a manufacturer-specific design before specifying a mating hole or installation requirement.

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