Free Resource: The Bolt Sizing Calculator

I used to break expensive stuff at work almost every day. Most of the things I broke were at least intended to be broken. As an engineer working for a mechanical testing laboratory I regularly performed physically destructive tests on client specimens. This could be anything from toothpicks to small sections of airplane wings.

Many of these diverse tests were performed on the same piece of lab equipment, a 50,000 lb static load testing machine.  No matter what the specimen was, my job was to find a way to fit it onto this machine and apply the loads exactly as directed. 

50 kip test machine

A 50,000 lb static load test machine is not a good place to store sporting goods

 The catch is that big blue over here only has one set of machined threads built in. Any custom fixturing I created required the use of adapters to adjust to the correct thread size. Doing so created a risk that the custom fasteners would fail before the specimen, simultaneously ruining the test results and my prospects for a promotion.

Of course, any engineer worth his pocket protector knows that you can approximate the maximum tensile force of a fastener by multiplying it’s rated tensile strength by cross sectional area. But the real trick is that the actual effective area of the fastener is not merely PI*R^2 but some number less than that due to the threads. The calculation for this is straight forward, but it is not something normal people can remember offhand (equation found here). The point is that I needed to know the load ratings of fasteners before deciding to take calculated risks, so I created an excel document to do just that.

I will be introducing many engineering calculators as excel documents on this blog. All of them will be posted in the “Resources” tab for easy access. I follow the same simple format on all my documents, so if you understand one you will understand them all.  Below is a screen shot of the bolt sizing calculator. The numbers in red bold are intended for you to change and everything else is automatically calculated. That’s all there is to it!

screen shot bolt calc

Screenshot of Bolt Sizing Calculator

 I have found this calculator to be immensely helpful.  It will quickly and accurately calculate the maximum bolt tensile load, shear load, torsion load, and the recommended tightening torque among other things for any size fastener (US customary units only). Also take notice of the other tabs on the bottom of the document including the tap drill chart and clearance hole size chart. You’re welcome to save this document to your desktop for your personal reference.

Take note: this document does not include safety factors, it simply presents the loads at which the fastener is no longer trustworthy (yield loads). It is up to you to ensure that adequate safety factors are put to use; I cannot claim responsibility for any accidents caused by the use or misuse of this resource. Having said that, I hope this calculator becomes as valuable to you as it has been to me!



  1. I really appreciate this sheet (pun unintended). I haven’t used yet, but on past weeks I had a hard time calculating different bolts and fasteners for different projects. Thank you very much.

    Liked by 1 person

  2. Great article and calculator. Very helpful! Frank Bacon Machinery Sales Co is a seller of used and reconditioned tensile and fatigue testing equipment. The picture of the 50,000 lbf Static machine in this article is actually a machine they reconditioned and retrofitted to computer controls.


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