How to Identify and Convert a Thread Size

You have a bolt in your hand and no idea what it is. It measures 8 mm across the threads, which makes it an M8 — except the box of M8 bolts on the shelf will not thread into it, because the one in your hand is 5/16 inch and the two are not the same thing.

Thread identification goes wrong in a specific way: diameters are close enough to feel interchangeable, and pitches never are.

Diameter is not enough

A thread has two dimensions that matter. Diameter is the easy one — calipers across the crests, and you have it to a tenth of a millimetre.

Pitch is the one that decides whether two parts mate. Metric threads state it as the distance between crests in millimetres. Imperial threads state it as threads per inch, counting how many crests fall in one inch.

The two systems describe the same physical property with reciprocal units:

TPI = 25.4 ÷ pitch in mm

An M8×1.25 has a 1.25 mm pitch, which is about 20.3 threads per inch. The nearest imperial fastener, 5/16-18 UNC, has 18. Diameters differ by about a fifth of a millimetre and pitches by roughly two threads per inch.

Forced together, they will start, bind after two turns, and strip. That is the failure mode — not a clean refusal, but a partial engagement that feels almost right.

Coarse and fine are different threads

Within each system, the same diameter comes in more than one pitch.

Coarse threads have fewer, deeper threads. They tolerate dirt and damage better, run down faster, and are less prone to cross-threading. Most general fastening uses them.

Fine threads have more threads per unit length. They give slightly higher tensile strength for the same diameter, resist loosening under vibration better, and allow finer adjustment. They are also easier to cross-thread and slower to assemble.

M8×1.25 and M8×1.0 are both M8. They are not interchangeable, and the difference is invisible without measuring pitch.

Four fasteners, two apparent diameters, four incompatible threads.

How to identify an unknown fastener

Measure the diameter across the thread crests with calipers. Round to the nearest standard size rather than treating the reading as exact — a used bolt has worn crests.

Measure the pitch with a thread gauge. This is the step people skip, and it is the one that gives the answer. A gauge is a set of leaves with known pitches; the one that seats without rocking is your pitch.

Without a gauge, measure across ten crests and divide. Counting one thread at a time accumulates error fast.

Check the thread form if it still does not match. Metric and unified threads both use a 60 degree flank angle, but Whitworth is 55 degrees and BSP pipe threads are tapered. A thread that measures right and still will not seat may be a different form entirely.

Common mistakes to avoid

• Identifying by diameter and assuming the pitch.
• Forcing a near-match and stripping the female thread, which turns a wrong-bolt problem into a repair.
• Measuring pitch by counting a single thread rather than averaging across ten.
• Assuming a fastener from an older machine is metric because it measures close to a round metric number.
• Treating a nearest-equivalent conversion as a substitution — it is an identification aid, not a compatibility statement.

How to do it with Screw Thread Size Converter

The Screw Thread Size Converter converts designations between systems and shows the pitch difference.

1. Enter the designation as it appears, or the measurements you have taken.
2. Select the thread system — coarse and fine share a diameter and differ in pitch.
3. Read the nearest equivalent in the other system, and the pitch gap alongside it.
4. Treat the equivalent as a way to name what you are holding, not as a part you can fit.

The ISO metric screw thread standard defines the metric series if you need the authoritative dimensions. Other conversion tools are in the tools directory.

Frequently asked questions

Can I substitute a metric bolt for an imperial one?

Almost never. Even where diameters are within a couple of tenths of a millimetre, the pitch differs, so the threads cross and strip. The nearest equivalent identifies an unknown fastener; it does not authorise a swap.

How do I measure pitch without a gauge?

Measure across ten crests and divide by ten. Counting a single thread compounds measurement error, and on a worn fastener a single-thread reading is frequently a whole size out.

What is the difference between coarse and fine threads?

Pitch only. Fine threads give more tensile strength and better vibration resistance; coarse threads assemble faster and tolerate damage and dirt. The same nominal diameter exists in both, and they do not mate.

Final thought

Measure the pitch, not just the diameter. Every stripped thread in a workshop started with someone who measured one of the two numbers and assumed the other.

Try the free Screw Thread Size Converter