Today I needed a nut and washer to hold my bike tire. I had one nut already which I was able to lock on there with a 15mm socket & ratchet. So I figured this meant that the nut must be a 15mm and a 15mm washer, if that makes sense? I only had one nut and washer so I took a trip to the hardware store today and I go hunting for that nut & washer. Soon I am overwhelmed with new terminology there were metrics and hex and some other stuff I figured I needed a 15mm so the metric section must be it. I go hunting there but nothing is labeled with mm it's all +1.50 pitch or M8 I don't know what any of that means or how it relates to the 15mm diameter nut & washer. Essentially I couldn't find the right one, an employee showed me what he said was the 15mm nut & washer was but it was way too big so I knew I'd have to go back to do more research. Please explain what pitch means for mm I am very confused here. And if a 15mm socket can lock the nut onto the bike does that mean the nut would have to also be a 15mm nut & the washer?

Thank you

  • pitch when discussing threaded fasteners pertains to the thread angel in relation to the bolt shaft.
    – ojait
    Commented Jan 11, 2016 at 20:44
  • Angle, not angel. (Typo, I presume. )
    – keshlam
    Commented Jan 14, 2016 at 9:11

4 Answers 4


Nuts and bolts start with two options, US or metric. US units tend to be in inches, metric units are typically millimeters. The size of the socket is completely separate from the size of the bolt and nut. The size of the nut/bolt is measured outside of the threads on the bolt, not the nut. So a 15mm bolt measured from the diameter from outside the threads, which would be written M15, could easily require a 30mm socket to install.

The pitch is how steep the threads are. On US nuts and bolts, they are measured according to the number of threads per inch. So a pitch of 20 would have 20 threads per inch. For metric nuts and bolts, it's measured in mm per thread, so a 1.50 pitch will be a thread every 1.5 mm.

The best option, if you can, is to take the existing nut and bolt to the store to find the replacement and ensure you get a proper fit.

  • 1
    Great answer! I think I understand and I just want to clarify that I do. Nuts & bolts come in two ways to measure, US and metric, US is inches, metric is in millimeters. The nut is measured not by its entire diameter across the entire nut but by it's hole in the center, like this diagram shows (d) canleyclassics.com/images/diagrams/… It's where the threads are that matters. And a bolt is measured from its circular part at the bottom and not the hex part at the top is. So then again, where the threads are. Is this right? Commented Jan 12, 2016 at 16:53
  • That's correct, you measure at the threads (the d in the diagram) when sizing a nut and bolt to work together and this is how they are typically sold and labeled. The only time you measure the hex part is to find the socket to tighten/loosen them. The thread pitch will be measured in the direction of T in the linked diagram, either mm per thread or threads per inch depending on the country.
    – BMitch
    Commented Jan 12, 2016 at 16:58
  • Do the pitch have to be exact for the nut and bolt to work together? or could they have a little variation? Commented Jan 12, 2016 at 17:05
  • 1
    You want an exact match. If you vary them by more than 1/2 of a thread across the entire nut, I'd imagine it would lock up while attempting to thread on the nut. This is why it's a good idea to take the part you are matching to the store to test the fit.
    – BMitch
    Commented Jan 12, 2016 at 17:09

15mm is the size of the opening in the wrench that fits across the outside flat surfaces of the nut. However nuts and bolts are designated by the diameter of the hole in the nut, not the size across the outside flat faces. The hole might be only 5mm or 6mm in dia. (or maybe not metric at all). The other necessary measure is the pitch which identifies the number of threads per inch. Neither diameter nor pitch can be easily determined by just looking at the nut. Can you remove the nut and take it into the store? They will be able to tell what it is.

  • I will be doing that for sure! thank you so much for the help! so everything is in terms of where the threading is when we measure? the measurement of the hole (diameter) of the nut determines its size not the outer diameter of the ENTIRE nut itself. And pitch is just the number of threads per inch for US units and per mm for metric units. And even though I could get a socket of 15mm on there that says nothing about the actually size of where the threading actually is. So the hex size and circular thread part of the bolt have no correlation in their sizes. Commented Jan 12, 2016 at 17:02

It's not the diameter of the nut but the diameter of the bolt thread at its largest diameter. The inside diameter of the nut only equals the narrow part of the bolt thread.

This page explains it perfectly for you; https://www.boltdepot.com/fastener-information/Measuring/Measure-Diameter.aspx

  • Oh great link! I think I get it now. The diameter I am concerned with is "T" in that diagram. So when looking for nuts & bolts they both need to be measured by the diameter. The diameter for a nut being the hole with the threading not the entire bolt itself. And for a bolt it's the bottom part diameter with the threading not the hex head, going by the hex head size can output wrong results. One thing that confuses me is if M15=15mm bolt size for metric then how is that labeled for inches? & if I wanted a specific head hex & bolt size how would both be labeled?Like Hex head:M30 & Bolt size:M15? Commented Jan 12, 2016 at 20:09

There are conventions. Bolts are referred by the size of the hole the bolt (not head) will fit.

A metric 10mm bolt's threads are commonly 1.5mm, 1.25mm or 1.0mm between threads. By contrast, an English 3/8" bolt is standardized at either 16 or 24 threads per inch. Note the different way to measure threads. I suspect your bolt is one of these.

I'd go to a real hardware store and try them all. They won't mind. (Home Depot and Lowes are not hardware stores.) The thread should spin all the way on effortlessly, unless the bolt is chewed up. If the thread binds halfway on, it's the wrong pitch.

They make special tools for measuring pitch, or you can simply press your bolt against another bolt from the drawer and see if they interlock fully. If you have a nut, try spinning on bolts from the drawer.

Ignore the hex head size - bolts are never identified by that. But it can give hints. Metric (ISO) has no standards for head size, a 10mm bolt can have a 14, 15, 16 or 17mm head. The English (UFS) system does, and a 9/16" head is surely a 3/8" bolt. A 15mm wrench will turn a 9/16" head.

By the way, the old Whitworth system works the way you'd expect, bolt hexes are a standard size (for each bolt size) and wrenches are identified by the bolt size they fit. Very posh!

https://en.wikipedia.org/wiki/ISO_metric_screw_thread https://en.wikipedia.org/wiki/Unified_Thread_Standard https://en.wikipedia.org/wiki/British_Standard_Whitworth

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.