Screw Size Chart
Screw gauge numbers converted to real diameters, wood and machine.
Compiled by DIY Repair Guide · reviewed 2026-09-23 ·how we research these
A screw gauge number is an index into a standardized diameter table rather than a direct measurement in inches or millimeters. In the United States, common fastener sizes run from tiny number 0 fasteners up through number 14, with numbers 6, 8, and 10 doing the bulk of residential construction, woodworking, and repair work. Once you know the specific decimal equivalent assigned to each gauge number, you can verify fastener dimensions with a caliper, select the correct drill bit, and match existing hardware without guessing.
At DIY Repair Guide, we compiled this screw size chart using standard fastener dimensions published under ASME/ANSI B18.6.3 by the American Society of Mechanical Engineers (ASME) and the American National Standards Institute (ANSI). Understanding how these gauge numbers translate into real-world diameters helps you choose the right fastener for wood framing, drywall hanging, or metal fabrication.
What the Number on a Screw Actually Means
The primary number assigned to an imperial screw designates its nominal major diameter across the outside of the threads. A higher gauge number indicates a wider screw shank and thread diameter, meaning a number 10 screw is physically thicker and holds more weight than a number 8 or number 6 screw. The screw number size system established under ASME/ANSI B18.6.3 provides manufacturers and tradespeople with a consistent numbering sequence across wood screws, drywall screws, tapping screws, and machine screws.
This sizing progression works in the direct opposite direction of wire sizing. In electrical work, the American Wire Gauge (AWG) standard assigns smaller numbers to thicker wires, as detailed on our wire gauge chart. Fastener sizing does not follow that reverse order. For screws, larger numbers always represent larger diameters, which eliminates confusion when moving from light household tasks up to heavy structural fastening.
A screw gauge number alone tells you the outer diameter, but a full fastener designation also includes length. For wood and drywall screws, length is stated as a simple fractional measurement in inches, measured from the surface of the work piece to the tip of the screw. A flathead screw includes the head in its total length measurement because the entire fastener sits flush, while a roundhead, panhead, or hex-head screw measures length strictly from beneath the head down to the point.
Screw Gauge to Diameter
Translating a screw gauge number into a decimal measurement allows you to check unknown fasteners with a dial or digital caliper. Fasteners labeled from number 4 through number 12 cover the vast majority of consumer hardware, with 1/4-inch fasteners serving as the transition point where fractional labels replace gauge numbers.
The major diameter represents the widest point across the outside edges of the crest threads. While nominal values remain constant in standards tables, real-world manufacturing tolerances mean an actual fastener can measure a few thousandths of an inch below these figures to ensure clearance in pre-drilled holes. You can check how these decimal values map to common tape-measure divisions on our fraction to decimal chart.
| Screw Size Number | Major Diameter (Inches) | Metric Equivalent (mm) |
|---|---|---|
| #4 | 0.112 in | 2.8 mm |
| #6 | 0.138 in | 3.5 mm |
| #8 | 0.164 in | 4.2 mm |
| #10 | 0.190 in | 4.8 mm |
| #12 | 0.216 in | 5.5 mm |
| 1/4 in (#14) | 0.250 in | 6.4 mm |
Wood Screws vs Machine Screws
Wood screws and machine screws serve fundamentally different mechanical functions and feature distinct thread profiles. Wood screws feature coarse, aggressive threads designed to slice directly into wood fibers, pulling two timber pieces together and anchoring firmly in solid grain. Most wood screws feature an unthreaded smooth shank section near the head, which allows the top board to pull tight against the lower substrate without thread binding across the joint.
Standard deck screws and general wood screws typically range from number 6 through number 10 gauge. Number 8 wood screws serve as the standard general-purpose fastener for interior cabinetry, shelving, and furniture assembly. Number 9 and number 10 screws are standard for exterior decking and structural framing brackets where higher shear resistance prevents fastener breakage under load.
Drywall screws, by contrast, are thin fasteners designed specifically for securing gypsum panels to wooden or light-gauge steel studs. They are most commonly manufactured in number 6 and number 8 gauge, utilizing a trumpet-shaped bugle head that depresses the drywall paper facing without tearing through the core. Their brittle, hardened steel composition provides great holding power under tension but snaps easily under the lateral shear loads common in structural woodworking.
Machine screws carry uniform cylindrical shanks with fine, machine-cut threads running their entire length. Rather than carving into raw material, a machine screw fastens into a pre-threaded tapped hole or clamps parts together using a matching threaded nut. Because machine screws depend on precision metal-to-metal threading, they require both a diameter designation and a thread pitch rating to function.
Picking the Right Pilot Hole
Drilling a pilot hole prevents wood from splitting along the grain and ensures the screw drives straight without snapping under torque. For wood screws, the pilot hole diameter must be slightly smaller than the outer thread diameter of the screw. The hole should match the root diameter of the screw shank so the core slips through while the threads carve deeply into the surrounding wood walls.
Hardwoods such as oak, maple, and walnut require pilot holes drilled to the exact root diameter of the screw shank, because dense wood fibers cannot compress outward to accommodate excess metal. Driving a screw into hardwood without an adequate pilot hole frequently shears the steel head clean off the shank. Softer woods like pine and cedar allow for slightly smaller pilot holes because the fibrous cell walls compress easily around entering threads.
For machine screws, sizing the pilot hole depends on whether the fastener will receive a nut on the back or drive into a newly tapped thread. When tapping internal threads into steel, aluminum, or brass, the initial hole must leave sufficient metal stock for the tap teeth to form complete internal thread walls. You can find exact matching bit sizes for every common fastener gauge on our drill bit size chart.
Metric Screw Sizes vs Standard Gauge Numbers
Metric fasteners do not use arbitrary gauge numbers. Instead, metric machine screws are designated directly by nominal outside diameter in millimeters, marked with the capital letter M followed by the dimension. An M4 screw has an outer diameter of exactly 4 millimeters, which equals approximately 0.157 inches.
The standard US gauge system and the metric system operate on independent measurement baselines that do not interchange directly. While an M4 screw measures close in diameter to a US number 8 screw (4.0 mm compared to 4.2 mm), their thread spacing is entirely different. An M4 screw carries a standard thread pitch of 0.70 millimeters between ridges, whereas a standard coarse US number 8 machine screw carries 32 threads per inch.
Attempting to force a metric machine screw into a standard US nut or tapped hole ruins the internal threading within two turns. Thread pitch gauges allow you to verify whether a mystery fastener uses metric millimeter spacing or imperial threads per inch before you attempt assembly.
Reading a Screw Box or Package Label
Package labels on fastener boxes convey critical sizing dimensions in a compact notation. For standard wood, drywall, and sheet-metal screws, the label lists the gauge number first, followed by the overall length. A label reading '#8 x 1-1/4 in' indicates a number 8 gauge fastener measuring 0.164 inches across the threads, with a usable length of 1.25 inches.
Machine screws display a three-part designation that includes thread pitch alongside diameter and length. A label marked '#8-32 x 1 in' defines a number 8 gauge screw with 32 threads per inch along a 1-inch threaded shank. Coarser threads like 24 threads per inch on a '#10-24' screw drive faster and handle dirt or cast metals well, while fine threads like '#10-32' offer greater vibration resistance in sheet metal assemblies.
Confirming both the gauge number and thread pitch on the packaging protects against cross-threading. If a project requires replacing lost fasteners from an unknown assembly, taking the existing hardware to a thread-matching board at a hardware store identifies the exact gauge and thread count in seconds.
Common questions
What size is a number 8 screw?
A number 8 screw has a nominal major diameter of 0.164 inches, which is approximately 4.2 millimeters. It is the most common screw size used for household repairs, cabinet assembly, and light woodworking.
What is the difference between wood screw and machine screw sizes?
Wood screws are designated by gauge number and length (such as #8 x 1-1/2 in) and feature sharp, wide threads designed to bite directly into timber. Machine screws include a thread pitch rating (such as #8-32 x 1-1/2 in) and have flat, uniform threads designed to thread into matching nuts or tapped holes.
How do I know what size drill bit to use for a screw pilot hole?
Select a drill bit that matches the inner root diameter of the screw shank, excluding the protruding threads. When held behind the drill bit against a light source, the bit should conceal the central solid shank of the screw while leaving the thread teeth exposed on both sides.
What does #8-32 mean on a screw?
The '#8' represents the gauge number, indicating a major diameter of 0.164 inches. The '32' indicates thread pitch, meaning there are 32 threads per inch along the length of the machine screw shank.
Is a bigger screw number a bigger screw?
Yes, higher screw gauge numbers always indicate larger diameters. A number 10 screw is thicker and stronger than a number 8 screw, which is the direct opposite of wire gauge sizing where larger numbers designate thinner wires.
Can I use drywall screws instead of wood screws for building projects?
Drywall screws are made from hardened, brittle steel that handles downward tension well but breaks easily under lateral shear forces. Using them for structural framing, furniture building, or exterior decking frequently leads to snapped heads as the wood shifts or expands.