A #8 screw has a major diameter of 0.164 inch, which is 4.17 mm. The US gauge formula is major = 0.060 + 0.013 times the number, so #8 gives 0.060 + 0.104 = 0.164 inch. That puts it between a metric M4 (4.0 mm) and M4.5, which is why most charts list a #8 as the rough equivalent of an M4 wood screw. It is the workhorse size for cabinet carcasses and general framing.

What is the formula for US screw number to diameter?

Major diameter in inches equals 0.060 plus 0.013 times the gauge number. So #0 is 0.060 inch, #6 is 0.060 + 0.078 = 0.138 inch, and #12 is 0.060 + 0.156 = 0.216 inch. Multiply by 25.4 to get millimetres. The relationship is perfectly linear, which is why a one-step jump in gauge number always adds 0.013 inch (about 0.33 mm) of diameter.

How do metric M screw sizes work?

In the metric system the M number is the nominal major diameter in millimetres by definition. An M6 screw is 6.0 mm across the threads, an M3 is 3.0 mm, an M10 is 10.0 mm. That makes metric far easier to reason about than US gauge numbers, since there is no formula to remember. Pitch (thread spacing) is a separate spec, for example M6 coarse is 1.0 mm pitch, but the M number alone fixes the diameter.

What size pilot hole does a screw need?

A pilot hole is the small hole the threads bite into. In softwood like pine, drill about 70 percent of the major diameter, so a 4.17 mm #8 wants a roughly 2.9 mm pilot. In dense hardwood like oak, go larger, about 85 percent, near 3.5 mm, because hardwood splits if the threads have to force too much fibre aside. Always step to the nearest real drill bit and test on scrap before committing to the workpiece.

What is the difference between a pilot hole and a clearance hole?

A pilot hole is drilled into the bottom piece so the threads grip and pull the joint together. A clearance hole is drilled through the top piece so the shank slides through freely without biting, letting the screw clamp the two boards instead of jacking them apart. For a #8 the clearance hole is about 4.6 mm (roughly 1.1 times the major diameter) and the pilot is 2.9 to 3.5 mm depending on wood.

How do I convert an imperial screw to a metric one?

Look up the major diameter of the US number in millimetres, then pick the closest M size. A #6 is 3.5 mm so it maps to M3.5, a #8 is 4.17 mm so it maps to M4, a #10 is 4.83 mm so it sits between M4.5 and M5. The match is approximate because thread pitch and head style differ between systems, so use the cross column as a starting point, not a guarantee that the two screws are interchangeable in a tapped hole.

Match a wood screw to the bit you already own

The plan calls for #8 wood screws but your drill index is metric. Look up #8, read 4.17 mm, see the recommended 2.9 mm softwood pilot, and grab the 3 mm bit from your set. No more buying a fastener you cannot drill a clean hole for, or splitting a panel because you guessed the pilot size from memory.

Drill a pilot hole that pulls tight instead of splitting

You are screwing into oak face frames and the last two split on you. Switch the wood type to hardwood, read the larger pilot diameter, and drill to that instead of the softwood number. The thread still grips, but the fibre has room to move, so the joint clamps down clean rather than cracking along the grain.

Cross a metric fastener box to a US plan

A flat-pack came with M4 screws but your build sheet is written in gauge numbers. Switch to metric, enter M4, and the nearest-equivalent column tells you it lines up with a #8. Now you know the M4 box will stand in for the #8 the instructions wanted, at least for diameter.

Spec a fastener order without a caliper

Writing a parts list for a shop run and you only have the screw numbers, not the diameters. Copy the whole chart with one click, paste it into your build doc, and every size carries its inch, mm, pilot and clearance numbers so the person at the drill press never has to guess.

Screw Size Chart — Gauge to mm + Pilot Hole Sizes

US gauge #0-#14 and metric M2-M12 — major diameter in inch + mm, pilot and clearance hole sizes, one-click copy — browser-only

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Look up the major diameter of any wood or machine screw, in inches and millimetres, with recommended pilot and clearance hole sizes. Pick a US gauge number or a metric M size — the chart shows the nearest cross-system equivalent for both.

Screw numberStandard wood / machine screw numbers #0 to #14.

Result

Major diameter

0.164 in

4.17 mm

Nearest equivalent

Pilot hole (softwood)

2.92 mm

Pilot hole (hardwood)

3.54 mm

Clearance hole

4.59 mm

Drilling sizes are shop guidance, not a spec. Pilot holes scale with wood hardness — soft pine grips more, dense hardwood splits — so step to the nearest real drill bit and test on scrap first. For self-tapping into metal or plastic, follow the fastener datasheet, not these wood figures.

Screw size chart

US gauge screws

Size	Diameter (in)	Diameter (mm)	Equivalent	Pilot soft (mm)	Pilot hard (mm)	Clearance (mm)
#0	0.06	1.52	M2	1.06	1.29	1.67
#1	0.073	1.85	M2	1.3	1.57	2.04
#2	0.086	2.18	M2	1.53	1.85	2.4
#3	0.099	2.51	M2.5	1.76	2.13	2.76
#4	0.112	2.84	M3	1.99	2.41	3.12
#5	0.125	3.18	M3	2.23	2.7	3.5
#6	0.138	3.51	M3.5	2.46	2.98	3.86
#7	0.151	3.84	M4	2.69	3.26	4.22
#8	0.164	4.17	M4	2.92	3.54	4.59
#9	0.177	4.5	M4	3.15	3.83	4.95
#10	0.19	4.83	M5	3.38	4.11	5.31
#12	0.216	5.49	M5	3.84	4.67	6.04
#14	0.242	6.15	M6	4.31	5.23	6.77

Metric M screws

Size	Diameter (in)	Diameter (mm)	Equivalent	Pilot soft (mm)	Pilot hard (mm)	Clearance (mm)
M2	0.0787	2	#1	1.4	1.7	2.2
M2.5	0.0984	2.5	#3	1.75	2.13	2.75
M3	0.1181	3	#4	2.1	2.55	3.3
M3.5	0.1378	3.5	#6	2.45	2.98	3.85
M4	0.1575	4	#7	2.8	3.4	4.4
M5	0.1969	5	#10	3.5	4.25	5.5
M6	0.2362	6	#14	4.2	5.1	6.6
M8	0.315	8	#14	5.6	6.8	8.8
M10	0.3937	10	#14	7	8.5	11
M12	0.4724	12	#14	8.4	10.2	13.2

What this tool does

A free screw size chart and converter for DIY, woodworking and light machining. Pick a US gauge number from #0 to #14 or a metric M size from M2 to M12 and read the major (outside) diameter in both inches and millimetres, with the nearest cross-system equivalent so you can match a #8 wood screw to its rough M4 metric cousin. For each size the tool also gives the recommended pilot hole for softwood and hardwood and the clearance hole for the top board, the two numbers that decide whether your joint pulls tight or splits the grain. The US numbers follow the standard gauge formula major = 0.060 + 0.013 times the number, so the math is transparent and reproducible. Everything runs in your browser with a copy button for one size, a copy button for the whole chart, and a shareable link that reopens the exact size you looked up. 100 percent client-side, no upload, no tracking.

Tool details

Input: Numbers; The page exposes text boxes, numeric controls, file pickers, or structured inputs depending on the tool.
Output: Live result + Copy + Preview; The result area focuses on usable output, with copy, download, or preview actions when supported.
Privacy: Browser-side processing; The main tool logic does not call an external API, so inputs normally stay in the current tab.
Save / share: Shareable URL state; Key settings are encoded in the URL so another person can reopen the same setup.
Performance budget: Initial JS <= 10 KB; No WASM budget is declared, keeping the tool quick to open on mobile.
Best fit: Format Converter · Operations; Category and role tags drive related tools, internal links, and quick fit checks.

How to use

1. Input

Paste or drop your content into the tool panel.
2. Process

Click the button. All processing is local in your browser.
3. Copy / Download

Copy the result or download to disk in one click.

How Screw Size Chart fits into your work

Use it when the main problem is getting content from one practical format into another.

Conversion jobs

Turning pasted content or local files into a handoff-friendly format.
Previewing a conversion before you use it in a larger workflow.
Cleaning small format mismatches without opening a full editor.

Conversion checks

Try a small sample first when the source format is messy.
Check character encoding, separators, and line endings after conversion.
Keep the source until the converted output has been reviewed.

Good next steps

These links move the current task into a more complete workflow.

Real-world use cases

Match a wood screw to the bit you already own
The plan calls for #8 wood screws but your drill index is metric. Look up #8, read 4.17 mm, see the recommended 2.9 mm softwood pilot, and grab the 3 mm bit from your set. No more buying a fastener you cannot drill a clean hole for, or splitting a panel because you guessed the pilot size from memory.
Drill a pilot hole that pulls tight instead of splitting
You are screwing into oak face frames and the last two split on you. Switch the wood type to hardwood, read the larger pilot diameter, and drill to that instead of the softwood number. The thread still grips, but the fibre has room to move, so the joint clamps down clean rather than cracking along the grain.
Cross a metric fastener box to a US plan
A flat-pack came with M4 screws but your build sheet is written in gauge numbers. Switch to metric, enter M4, and the nearest-equivalent column tells you it lines up with a #8. Now you know the M4 box will stand in for the #8 the instructions wanted, at least for diameter.
Spec a fastener order without a caliper
Writing a parts list for a shop run and you only have the screw numbers, not the diameters. Copy the whole chart with one click, paste it into your build doc, and every size carries its inch, mm, pilot and clearance numbers so the person at the drill press never has to guess.

Common pitfalls

Treating a US gauge number as a millimetre size. A
Using the softwood pilot diameter in hardwood. Oak, maple and other dense species split when the thread forces too much fibre aside. Drill the larger hardwood pilot (about 85 percent of the major) for those, and save the smaller softwood pilot for pine and spruce.
Skipping the clearance hole in the top board. Without it the threads grab both pieces and jack them apart instead of clamping. The top board needs a clearance hole slightly wider than the major diameter so the shank slides through and the screw pulls the joint tight.

Privacy

Every number here, the gauge formula, the inch and mm diameters, the pilot and clearance holes, is plain JavaScript that runs in your browser tab. Nothing about what you looked up is sent anywhere, and there is no logging of your sizes. The one caveat: the shareable link encodes the system and size you picked in the URL query string, so a link pasted into chat will record that size in the recipient server access log. That is harmless for a screw size, but worth knowing the link carries your selection.

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Screw Size Chart — Gauge to mm + Pilot Hole Sizes

What this tool does

Tool details

How to use

1. Input

2. Process

3. Copy / Download

How Screw Size Chart fits into your work

Conversion jobs

Conversion checks

Good next steps

Real-world use cases

Match a wood screw to the bit you already own

Drill a pilot hole that pulls tight instead of splitting

Cross a metric fastener box to a US plan

Spec a fastener order without a caliper

Common pitfalls

Privacy

FAQ

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