A quick answer: no. A 22mm socket is not exactly the same as a 7/8‑inch socket. The imperial size 7/8" converts to 22.225 mm, so a 22 mm socket sits about 0.225 mm smaller. That tiny gap can matter under load, potentially slipping or damaging the fastener or socket if forced to fit.
In practice, metric and imperial sizing use different measurement scales, and sockets are manufactured to precise tolerances. While the two sizes are numerically close, they are not interchangeable for reliable, high-torque work. This article explains the relationship between 22 mm and 7/8", why the difference matters, and how to choose the right socket for your fastener.
Understanding metric vs. imperial sizing
Sockets measure the distance across the flats of the hex head (the width across the six sides). Metric sizes are labeled in millimeters (e.g., 22 mm), while imperial (or SAE) sizes are labeled in inches (e.g., 7/8"). The drive size that accepts the ratchet (such as 3/8" or 1/2") is a separate dimension and does not change the hex size.
Is 22 mm the same as 7/8 inch?
Direct comparison: 7/8" equals 22.225 mm, so 22 mm is slightly smaller by about 0.225 mm (roughly 0.009 inches). In practical terms, a 22 mm socket will rarely seat perfectly on a 7/8" fastener, and attempting to force it can round corners or slip under torque. For precise work or new fasteners, use the exact size specified by the fastener—22 mm for metric 22 mm fasteners, and 7/8" (22.225 mm) for imperial fasteners.
That said, there are rare situations where a near-match feels like it fits due to tolerance in the fastener head or the socket, but such fits are unreliable and not recommended for critical or high-torque applications.
Size relationships you should know
Before the list, here is a quick reference to how metric and imperial sizes relate in common socket use.
- 1 inch equals 25.4 millimeters.
- 7/8 inch equals 22.225 millimeters.
- 22 millimeters equals about 0.866 inches.
- The difference between 22 mm and 7/8" is about 0.009 inches (roughly 0.23 mm).
- Socket labeling reflects the hex size across flats; the drive size (the square hole for the ratchet) is a separate specification.
In short, while 22 mm and 7/8" are close, they are not identical. Use the exact size specified by the fastener to avoid damage and ensure proper torque.
Practical guidance for choosing the right socket
To help prevent sizing errors, consider these practical steps when selecting a socket for a fastener:
- Inspect the fastener head or consult the project specs to determine the correct size in either metric or imperial units.
- If the size isn’t labeled, measure across the flats with calipers or a small hex gauge to determine the exact dimension.
- Test-fit with a socket that matches the suspected size (start with the metric size if you’re unsure, then try the imperial size if needed) and ensure a snug, square engagement without slipping.
- Verify drive compatibility (e.g., 3/8" drive for mid-range work, 1/2" drive for heavy torque) to ensure the socket fits your ratchet and torque requirements.
- Keep both metric and imperial socket sets in your toolkit to handle mixed fastener standards confidently.
By measuring carefully and confirming fit before applying torque, you minimize the risk of rounding corners, stripping threads, or damaging tools.
Summary
22 mm and 7/8" are close but not identical. 7/8" equals 22.225 mm, making a 22 mm socket slightly undersized for a 7/8" head. In precise work, use the exact size labeled for the fastener. When in doubt, measure the fastener head and verify with matching metric or imperial sockets, and maintain a mixed toolbox to cover both measurement standards.