How to Find Thread Size: A Complete Guide for Bolts, Screws, and Fittings

Whether you're replacing a bolt on a piece of hardware, swapping out a fitting, or trying to match a screw to an existing hole, knowing how to find thread size is a fundamental skill. Thread sizing isn't always printed on the fastener itself, and guessing wrong wastes time and can damage components. Here's how to measure and identify thread size accurately.

What "Thread Size" Actually Means

Thread size refers to two distinct measurements that work together:

• Thread diameter — the width of the bolt or screw shaft (or the inner diameter of a nut/fitting)
• Thread pitch — how closely spaced the threads are, measured as either threads per inch (TPI) or millimeters between thread crests

A complete thread specification always includes both. For example, M8 x 1.25 means an 8mm diameter with 1.25mm pitch. 1/4-20 means a 1/4-inch diameter with 20 threads per inch. Neither measurement alone tells the full story.

Tools You'll Need 🔧

Before measuring, gather the right tools:

A thread pitch gauge is the single most useful tool here — it's inexpensive and removes all guesswork from pitch identification.

Step 1: Determine Whether the Thread Is Metric or Imperial

This is the first fork in the road. Metric threads (used throughout most of the world and in most modern electronics and machinery) are measured in millimeters. Imperial (Unified National) threads (common in the US, older hardware, and some specialized equipment) are measured in fractions of an inch.

A few general clues:

• Metric fasteners often have a smooth rounded thread profile and measurements that fall neatly into millimeter increments
• Imperial fasteners commonly appear on American-made machinery, plumbing hardware, and older vehicles
• Electronics and computers almost universally use metric threads (M2, M3, M4 are very common in PC assembly)

If you're working on tech hardware — mounting drives, installing standoffs, attaching heatsinks — expect metric.

Step 2: Measure the Thread Diameter

Use calipers for this. Measure across the outer diameter of a bolt (called the major diameter) from crest to crest.

For a nut or threaded hole, measure the inner diameter instead.

Tips for accuracy:

• Take the measurement at least twice and average the results
• Measure undamaged threads — worn or cross-threaded areas give false readings
• For bolts, measure the widest point across the thread peaks

Once you have the measurement, compare it to a standard size chart. A reading of approximately 4.8mm to 5.0mm likely corresponds to M5. A reading near 6.35mm likely corresponds to 1/4 inch.

Step 3: Measure the Thread Pitch

With a thread pitch gauge, fan out the blades and press each one against the thread until one sits flush with no gaps or rocking. That blade's labeled value is your pitch.

Without a pitch gauge, you can estimate using the "count and measure" method:

1. Place the threaded fastener against a ruler
2. Count the number of complete thread crests within exactly 1 inch — that's your TPI for imperial threads
3. For metric, count the crests across 10mm and divide — that gives you the pitch in mm

This method works adequately for rough identification but a physical gauge is far more reliable for precise matching.

Step 4: Confirm With a Test Fit

Once you've identified a likely thread size, test it before committing. Thread a matching nut or fitting on by hand:

• It should engage smoothly with no wobbling or resistance from the first turn
• Cross-threading feels rough and binds quickly — stop immediately if that happens
• A correct match threads fully by hand with consistent, even resistance

Test fitting is especially important in tech hardware, where aluminum standoffs and plastic-threaded holes strip easily.

Common Thread Sizes in Tech Hardware 💡

These are general reference points — always verify against your specific hardware documentation when available.

When Markings Are Already on the Fastener

Many bolts and screws include head markings that indicate grade and sometimes standard. Check:

• The bolt head face for stamped numbers or letters
• Packaging if original hardware is still available
• Manufacturer documentation, parts manuals, or service guides — these almost always specify thread sizes for replaceable hardware

For PC components specifically, motherboard and chassis manuals frequently list the required screw types in their specifications section.

Variables That Affect Which Thread You're Working With

Thread identification isn't always straightforward because several variables shift the answer:

• Country of origin — determines whether metric or imperial is likely
• Age of the hardware — older equipment often uses non-standard or legacy thread specifications
• Industry or application — plumbing, electronics, automotive, and industrial machinery each favor different standards
• Manufacturer preference — some brands use proprietary or less-common thread sizes for specific assemblies

A bolt that measures close to a standard size might actually be a different standard from a different regional specification. Two threads can look identical visually but differ enough in pitch to cause binding or failure under load.

The only way to know with certainty what thread you have is to measure both diameter and pitch, cross-reference with standard size charts, and confirm with a physical test fit. What that means in practice depends entirely on what you're working with and what tools you have on hand.