Quick Fix
Check the stamped size on the wrench first. If it's faded or gone, grab some digital calipers and measure the flat-to-flat distance of the hex socket or wrench shank. Round that measurement to the closest standard size, then match it up with the chart below.
What’s Happening
You're trying to figure out which Allen key fits your bolt or screw.
An Allen key—also known as a hex key or hex wrench—is basically an L- or T-shaped tool designed to tighten or loosen bolts and screws with a hexagonal socket. The easiest way to identify its size? Look for the marking stamped right on the handle. Unfortunately, those markings tend to wear off over time. When that happens, you’ll need to measure the hex socket or the wrench itself and match it to the closest standard size.
How do I find the size?
Start by checking for a stamped size, then measure if needed.
Here’s the step-by-step process:
- Give the wrench a once-over. Run your eyes along the long leg of the key. Do you see any numbers or letters like “4 mm” or “5/32 in”? If the marking’s still clear, jot it down.
- Measure the socket. Slide a digital caliper between two parallel faces of the hex socket. Record the flat-to-flat distance in millimeters or inches.
- Measure the wrench shank. If the socket’s out of reach, measure the straight part of the hex key shank instead—the same faces you’d use on the bolt.
- Round to the nearest standard size.
Actual Measurement Closest Metric Size Closest SAE Size 2.9 mm – 3.1 mm 3 mm 1/8 in (0.125 in) 3.8 mm – 4.2 mm 4 mm 5/32 in (0.156 in) 4.7 mm – 5.3 mm 5 mm 3/16 in (0.188 in) 6.2 mm – 6.5 mm 6 mm 1/4 in (0.250 in) - Test the fit. Slip the wrench into the socket. It should sit flush with just a little resistance. If it wobbles or feels too loose, grab the next size up.
What if the stamped size is worn off and I can’t measure it?
Try these quick workarounds to figure out the size.
- Grab a mini socket set. A 1/4-inch drive socket set with 3 mm and 5 mm sockets often slides right over the Allen head—perfect for using with a ratchet.
- Improvise in a pinch. For tiny sizes like 2 mm or less, the tips of a pair of flat-tipped tweezers or even a paperclip might do the trick if you turn them slowly and carefully.
- Head to the hardware store. Bring the bolt or screw with you. Most stores will let you borrow a matching wrench while you shop—no guessing required.
How can I keep my Allen keys organized?
Label them, mark them, and store them smartly.
- Keep your keys in a labeled roll or magnetic tray. That way, sizes stay separate and the markings stay visible.
- Right after you buy a set, use a permanent marker to trace the stamped size onto the handle. It’s a simple trick that saves time later.
- For IKEA furniture or similar flat-pack items, tape the supplied 3 mm and 5 mm keys to the underside of a drawer or inside the cabinet frame. They’ll always be handy when you need them.
- Consider investing in a 10-piece L-shaped set that meets ISO 2936 and 2026 standards. The longer arm gives you better torque, and color-coded handles make size identification a breeze.
