Wood Screw Size Chart: Gauge, Diameter and Length

A wood screw size has two separate parts: its gauge gives the nominal diameter, and its length gives the distance to the tip from the correct head reference. A #8 wood screw is nominally 0.164 in (0.417 cm) across, but that number alone does not tell you which length or pilot hole your joint needs.

The useful rule is simple: identify the screw with a chart, then choose it from the joint. A larger gauge can still be the wrong choice if its head does not fit the hardware, its tip breaks through, or it splits a narrow rail.

Wood screw size chart: gauge, inches and metric

Use this wood screw diameter chart to identify ordinary numbered wood screws. These are nominal basic diameter values, not measured samples, pilot-hole sizes, or guaranteed dimensions for every modern screw product. The supplier's screw dimension chart identifies these values as basic shank diameters for traditional wood screws; modern reduced-shank and specialist designs need their own data.

Screw gauge	Nominal diameter, in	Converted diameter, mm	Converted diameter, cm
#2	0.086	2.18	0.218
#3	0.099	2.51	0.251
#4	0.112	2.84	0.284
#5	0.125	3.18	0.318
#6	0.138	3.51	0.351
#7	0.151	3.84	0.384
#8	0.164	4.17	0.417
#9	0.177	4.50	0.450
#10	0.190	4.83	0.483
#12	0.216	5.49	0.549
#14	0.242	6.15	0.615

Metric values are conversions, rounded for reference using 1 in = 25.4 mm = 2.54 cm. They are not metric screw substitutions. A nominal 4 mm wood screw is not automatically interchangeable with a #8, especially when a hinge hole or countersink must match.

For this numbered series, the basic diameter follows 0.060 + 0.013 times the gauge number, in inches. You can use that relationship as a cross-check, not as a drilling instruction. Do not apply it to metric sizes or to a manufacturer's proprietary naming system.

What the number on the box does not say

A label such as #8 x 1-1/4 in describes gauge and nominal length. It does not completely specify the head shape, drive recess, coating, material, threaded length, or suitability for a load. Write those details separately when you order replacements.

For furniture hardware, I would start with the hardware maker's specified screw. Guessing a larger gauge is often worse than buying the right replacement: a head that sits proud can stop a hinge closing even when the screw goes into the wood.

Compare eight screw diameters at the same scale

These code-drawn diagrams change only the nominal diameter. The circle is a diameter envelope, not a literal thread section, head size, drill bit, or a life-size print template. Compare the relative sizes, then use the labeled numbers for identification. Screen zoom does not preserve physical size.

The familiar #6, #8 and #10 sizes have nominal diameters of 0.138, 0.164 and 0.190 in (0.351, 0.417 and 0.483 cm). Each step between those three adds 0.026 in, or about 0.066 cm. The small difference is hard to judge from a photograph, so use a caliper and the original packet when you can.

How do you measure a wood screw correctly?

Measure diameter across the widest thread crests, away from the tapered tip, and compare it with the product's data. Measure length from the top of a flat countersunk head to the tip; for a non-countersunk head, start underneath the head. Do not measure the drive recess and call that the screw gauge.

The fastener measurement guide explains these different starting points. An oval countersunk head has a raised top, so check the maker's convention instead of including that dome automatically.

For an unidentified screw, clean off paint and debris first. Close the caliper gently across the widest threaded part, rotate the screw and repeat. A damaged thread, heavy coating, or unusual thread form can make one reading misleading. Check the smooth shank separately if one exists: on some modern screws it is thinner than the outer thread diameter.

Next, lay the screw against a ruler with the correct head reference on zero. Record the head shape and drive type before putting it back in a mixed box. If two candidates are close, compare them with a known labeled sample. Do not force a near match into valuable hardware just to see whether it fits.

Marked screw length	Exact inch value	Converted length, cm
1/2 in	0.500	1.270
3/4 in	0.750	1.905
1 in	1.000	2.540
1-1/4 in	1.250	3.175
1-1/2 in	1.500	3.810
2 in	2.000	5.080

These are standard nominal length labels with exact conversions. Availability depends on the screw range. The table does not recommend a length for a particular board.

Choose the length from the joint, not the gauge

Measure the material the screw passes through, the material receiving the threads, and the position of the far face. Then sketch the screw path with the actual head seating position. For a perpendicular, flush flat-head screw, subtract the first board's thickness from its nominal length to find penetration into the second board, including the pointed tip.

In the illustrated example, two boards are each 0.75 in (1.905 cm) thick. A 1.25 in (3.175 cm) flat-head screw seated flush passes through the first board and penetrates 0.50 in (1.27 cm) into the second. That leaves 0.25 in (0.635 cm) before the far face. Effective thread engagement is less than that penetration because the pointed end is not a full-diameter thread.

This is arithmetic, not a joint recommendation. It does not establish adequate holding power for your furniture. Recessing the head farther moves the tip deeper. Angled driving changes the geometry. Washers, brackets, gaps, and real board thickness change the available space too.

Measure sheet goods rather than trusting their store label. Our plywood thickness reference explains why a nominal panel name is not a reliable input for a screw-length calculation.

Match the situation before choosing the screw

Situation	My starting choice	Detail to check before drilling
Replacing a hinge screw	Match the hardware specification	Head seat, gauge, remaining wood and damaged holes
Fastening a thin cabinet back	Measure both materials and use a suitable fastening detail	Tip clearance and whether the back can split
Joining solid-wood furniture parts	Choose the joint first, then its specified screw	Grain direction, movement and thread engagement
Pocket-hole assembly	Use the jig system's specified screw	Head form, thread type, stock thickness and jig setting
Heavily loaded bracket or wall fixing	Follow the rated hardware system	Approved fastener, substrate and installation instructions

A generic wood screw chart is not a structural fastening schedule. For pocket joints, use the separate pocket-hole screw selection chart. A countersunk screw and a pocket-hole screw can share a gauge without sharing the head shape or intended use.

Pilot holes, mistakes and a clear workshop brief

Choose a pilot hole from the screw maker's guidance for that screw and wood, then test it in matching scrap. The chart at the top gives nominal screw diameters. Drilling the receiving pilot hole to that same diameter can remove the wood the threads need to grip.

A traditional wood screw can need a clearance hole through the first member, a smaller pilot in the receiving member, and a countersink for a flat head. Those are different operations. A self-drilling screw may have different instructions. Hardwood, end distance, screw material and thread design all affect the installation, so a single universal pilot-hole table would hide too much.

Mistakes that cost a part

• Choosing by length alone: the wrong head can fail to seat in hardware even when tip clearance is adequate.
• Driving without a trial hole: a narrow rail may split before the screw is fully seated. Test in the same species with similar edge distances.
• Over-tightening into sheet goods: stripped material cannot recover its grip by turning the driver harder. Stop and repair the fastening detail.
• Counting the tapered point as full engagement: the tip occupies space without providing the same grip as the full thread.
• Replacing every screw with a larger one: the larger diameter may damage hardware or leave too little wood beside the hole.

Record the fastener, not just the furniture photo

Make a simple section sketch. Mark measured board thicknesses, grain direction, the head seat, screw angle, receiving depth and far-face clearance. Add the exact screw product, coating, quantity and supplier's pilot-hole instructions. Keep unknowns marked as unknowns, then test a sample joint before preparing all the parts.

A photograph cannot reveal a concealed tip or prove the depth of an existing pilot hole. That is the point where visual references stop being enough. Use a furniture technical drawing checklist to separate visible details from dimensions you have actually checked.

CarpDraw is focused on furniture drawing and specification planning. Its current drawing generation is not a substitute for this measured fastener schedule, and we do not claim it can choose or validate your screws. Keep the verified schedule with the workshop brief; explore the site to see the intended workflow and current availability.