Free photo tool
What size screw is this?
Take a photo of the screw or bolt and Mechy works out what it is: diameter and thread, length, head and drive, and what to ask for at the store. Then it shows you how to measure it to be sure.
Lay it on paper next to a coin or a ruler, with the threads in focus.
Free, no sign-up. Your photos open in the Mechy web app, which makes your guide. Privacy
What you get back
Everything you need to buy the right one the first time.
Size and thread
Gauge or metric size and thread pitch: "#10-32" or "M5 × 0.8".
Length
Measured the right way for the head type.
Head and drive
Pan, flat or hex head; Phillips, Torx, square or slotted.
Material
Zinc, stainless or hardened, and the grade marks on bolt heads.
Pilot hole
The drill size for a pilot hole or a tap.
Buy-as text
"#10-32 × 3/4 in pan-head Phillips, stainless": paste it into a store search.
How to measure a screw by hand
- Diameter. Measure across the outside of the threads with calipers, or check which hole of a bolt gauge it fits.
- Thread pitch. Inch screws: count the threads in one inch (threads per inch). Metric: measure across 10 threads and divide by 10.
- Length. From under the head to the tip. Flat (countersunk) heads are measured overall, top of the head to the tip.
- Head and drive. Note the head shape and the recess. A worn recess often means it was the wrong bit.
Common screw sizes
Gauge numbers are inch sizes: the bigger the number, the thicker the screw.
| Size | Diameter (in) | Diameter (mm) | Coarse thread | Fine thread |
|---|---|---|---|---|
| #4 | 0.112 | 2.84 | 4-40 | 4-48 |
| #6 | 0.138 | 3.51 | 6-32 | 6-40 |
| #8 | 0.164 | 4.17 | 8-32 | 8-36 |
| #10 | 0.190 | 4.83 | 10-24 | 10-32 |
| #12 | 0.216 | 5.49 | 12-24 | 12-28 |
| 1/4 in | 0.250 | 6.35 | 1/4-20 | 1/4-28 |
| Metric | M3 | M4 | M5 | M6 | M8 | M10 | M12 |
|---|---|---|---|---|---|---|---|
| Coarse pitch (mm) | 0.5 | 0.7 | 0.8 | 1.0 | 1.25 | 1.5 | 1.75 |
Questions
Is there an app that can identify screw sizes?
Yes. Take a photo here (or in the Mechy app) with a coin or ruler beside the screw, and Mechy gives you the likely size, thread and length, plus how to measure to confirm it before you buy.
How do I measure the length of a screw?
For most screws, measure from under the head to the tip. Flat (countersunk) heads are the exception: they are measured overall, from the top of the head to the tip, because the head sits flush.
What is the difference between a #8 and a #10 screw?
Diameter. A #8 is 0.164 in (about 4.2 mm) across the threads and a #10 is 0.190 in (about 4.8 mm). The higher the gauge number, the thicker the screw.
How do I find the thread pitch?
Use a thread gauge if you have one. Otherwise count the threads in one inch for inch screws (threads per inch), or measure across 10 threads and divide by 10 for the pitch of a metric screw in millimeters.
Is it a wood screw or a machine screw?
Wood and sheet-metal screws taper to a sharp point and have coarse, widely spaced threads. Machine screws are the same diameter all the way down, have a blunt end and fine, even threads, and go into a nut or a tapped hole.
Stripped, stuck or snapped?
Snap it and say what happened. Mechy makes a step-by-step guide to get it out and fit the right one.
First guide free. No sign-up, no card.
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