How to Measure Bolts and Screws: Diameter, Thread Pitch & Length

Replacing a lost bolt or ordering a box of screws goes wrong in the same three places: the diameter, the thread pitch, or the length. Get those three right and you can identify almost any standard fastener. You need a pair of calipers (a ruler works in a pinch) and ideally a thread pitch gauge.

A complete size is written in this order:

• Inch: diameter – threads per inch × length, for example 1/4"-20 × 1-1/2"
• Metric: M + diameter × pitch × length, for example M8 × 1.25 × 30

Step 1: Measure the diameter

Measure across the outside of the threads, crest to crest. This is the major diameter, and it is what the size refers to. Don't measure the root of the thread or the head.

Expect a real bolt to read slightly under its nominal size, because threads are cut with a little clearance. A 1/4" bolt usually measures about 0.24"–0.249". Round to the nearest standard size.

Screws under 1/4" use number sizes instead of fractions:

For metric fasteners, the measurement in millimetres is the size: a bolt that measures about 7.8–8.0 mm is an M8.

Step 2: Find the thread pitch

Two fasteners with the same diameter can have different threads, so this step matters. A nut or tapped hole will only accept the matching pitch.

• With a pitch gauge: try leaves until one sits in the threads with no light showing under it. The leaf is stamped with the TPI (inch) or the pitch in mm (metric).
• Without a gauge (inch): mark a 1" span along the threads and count the crests in it. That count is the TPI.
• Without a gauge (metric): measure across 10 crests and divide by 10 to get the pitch in mm.

Most inch fasteners are either coarse (UNC) or fine (UNF). A 1/4" bolt with 20 threads per inch is 1/4-20 UNC; with 28 it is 1/4-28 UNF. The full list is in the thread pitch chart, and coarse vs. fine thread explains how to choose between them.

Step 3: Measure the length

Length is measured from the surface the head sits on, which depends on the head style:

• Heads that sit on top of the surface (hex, socket cap, button, pan, truss, flange, carriage): measure from directly under the head to the tip.
• Flat (countersunk) heads, which sit flush: measure the overall length, top of the head to the tip.
• Oval heads, which are countersunk with a domed top: measure from the widest edge of the head (where it meets the surface) to the tip.
• Set screws have no head: measure the overall length.

Standard inch lengths step in 1/8" or 1/4" increments for most sizes. Metric lengths follow a preferred series (10, 12, 16, 20, 25, 30 mm, then mostly 5 or 10 mm steps). If your measurement falls between two standard lengths, the bolt may be worn or cut; go with the nearest standard length.

Fully threaded or partially threaded?

Longer bolts are often only threaded part of the way, leaving a smooth shank under the head. If the threaded length matters (for example, so a nut can tighten against a thick stack), measure it separately from the tip to the last full thread.

How do I measure head size and drive?

You don't need the head size to order the right bolt, but it tells you which tool you need. Standard sizes for common inch fasteners:

More sizes, including head heights, are on the bolt size chart.

How do I measure a nut or washer?

• Nuts are sized by the bolt they fit, not their outside dimensions. A 3/8"-16 nut fits a 3/8"-16 bolt. Check the thread on a known bolt or with a pitch gauge.
• Washers are also named by bolt size, but if you need the exact fit, measure the inside diameter (ID), the outside diameter (OD) and the thickness. Flat washers and wider fender washers for the same bolt have the same ID but very different ODs.

What if it's metric or inch?

If a bolt measures close to a fraction of an inch (0.25", 0.3125", 0.375") and its threads match a TPI on the chart, it is inch. If it measures a round number of millimetres (5, 6, 8, 10 mm) and the pitch matches a metric value (0.8, 1.0, 1.25, 1.5 mm), it is metric. M6 and 1/4" are close enough to fool the eye, but their threads are not interchangeable. Forcing one into the other will strip the threads.

Quick checklist before you order

1. Diameter (crest to crest, rounded to the nearest standard size)
2. Thread pitch (TPI or mm)
3. Length (measured from the correct point for the head type)
4. Head style and drive
5. Material, such as 18-8 or 316 stainless. See 18-8 stainless vs. other grades if you are unsure.

With those five details you can find the matching part in the catalog, whether it is a hex head bolt, a machine screw or a socket head cap screw.