Bolt torque seems like it should be straightforward. Pick a bolt size. Look up a torque. Tighten it. Move on. Unfortunately, that is where the fun starts.

The torque you apply with a wrench is not the thing you actually care about. What you usually care about is bolt tension, or preload. Torque is just the indirect way you try to get there.

So we built a Bolt Torque Calculator to make the first pass easier. Check it out Here.

The basic idea is this:

When you tighten a bolt, the bolt stretches slightly. That stretch creates tension in the bolt, which creates clamp force between the parts. A properly tightened bolt is basically acting like a very stiff spring, pulling the assembly together.

The tricky part is that torque does not convert cleanly into bolt tension.

A lot of the torque you apply gets used up by friction in the threads and under the bolt head or nut. Only a portion of it actually goes into stretching the bolt. That means two bolts tightened to the same torque can end up with very different preload if the surface condition changes.

Dry threads, zinc plating, wax, oil, anti-seize, rough bearing surfaces, smooth washers. All of it matters more than people want it to.

That’s why bolt torque calculations usually come down to a few practical inputs:

A common first-pass relationship is:

T = K × D × P

Where T is tightening torque, K is the nut factor, D is nominal bolt diameter, and P is the desired bolt preload.

That formula is useful, but the important part is not pretending it is more precise than it is. The nut factor is an estimate, and that estimate can move a lot depending on lubrication, plating, thread condition, and bearing surface. That is why torque charts are a starting point, not a universal answer.

The classic mistake is changing the surface condition without changing the torque.

For example, a torque value that worked for a plain dry bolt may be too high once the bolt is lubricated. Same wrench setting, very different preload. That is how “we made assembly easier” turns into “why are the bolts yielding?”

We made a calculator that helps you run that first-pass check without rebuilding the math every time.

It’s useful when you’re estimating an initial tightening torque, comparing bolt sizes, checking preload targets, or sanity-checking how sensitive the torque is to friction assumptions.

It’s not a replacement for test data, tension measurement, or a real engineering review when the joint is critical. But it is a faster way to move from “use the standard torque” to “here’s what that torque is actually assuming.”

We’re also building out more tools for the Drafter Toolbox.

If there’s a calculator, checker, or small engineering utility you wish existed, submit or vote on your favorite idea here. We’ll use those suggestions to decide what to build next.

Drafter helps hardware teams move faster from 3D model to manufacturing-ready output. Start with clean 2D engineering drawings in minutes, complete with proper GD&T, title blocks, and drawing notes without the manual clicking and rework.

But drawings are only the beginning. Drafter also supports assembly workflows, MBD, and the broader documentation teams need to get parts quoted, reviewed, and built with confidence.

We’re already working with teams in aerospace, robotics, automotive, medical devices, and energy. Basically anywhere engineering teams are trying to spend less time formatting drawings and more time building great products.

We’d love your feedback. If there are specific resources, workflows, or tools you’d like Drafter to create, please reach out anytime. Your input directly shapes what we build next.

Happy engineering,

The Drafter Team