Metric vs Imperial Threads: How to Read and Tell Them Apart

Quick answer: Inch threads are called out as diameter-TPI (5/16-18 = 5/16 in diameter, 18 threads per inch). Metric threads are called out as M diameter x pitch (M8 x 1.25 = 8 mm diameter, 1.25 mm between crests). Several sizes look almost the same but do not interchange: M6 vs 1/4-20, M8 vs 5/16-18, M10 vs 3/8-16. To identify an unknown thread, measure the diameter with calipers, then match the pitch with a gauge, checking both metric and inch leaves.

Mixed inch and metric hardware is a fact of life in US shops. Imported machines, bikes, cars and 3D printers are mostly metric, while hardware stores and older equipment are mostly inch. Mixing them up wrecks threads. Here is how to read both and tell them apart.

Reading inch (Unified) callouts

An inch thread like 5/16-18 UNC breaks down as:

• 5/16 = nominal major diameter in inches (0.3125 in)
• 18 = threads per inch (TPI)
• UNC = the series: Unified Coarse. UNF is fine, UNEF is extra fine.

Small sizes use numbers instead of fractions. #10-32 means a #10 size (0.190 in major diameter) with 32 TPI. You may also see a class of fit on the end, such as 2A (external) or 2B (internal).

The tap drill for 5/16-18 is an F (0.257 in), giving about 76.9% thread.

Reading metric callouts

A metric thread like M8 x 1.25 breaks down as:

• M = ISO metric thread
• 8 = nominal major diameter in millimeters
• 1.25 = pitch, the distance from one crest to the next, in millimeters

Coarse pitch is the default, and it is common to see the coarse pitch left off: “M8” usually means M8 x 1.25. Fine pitches are always written out, as in M8 x 1. When in doubt, measure.

The tap drill for M8 x 1.25 is 6.8 mm, which gives about 73.9% thread.

Converting between pitch and TPI

The two systems describe the same thing in opposite ways:

• TPI = 25.4 ÷ pitch (mm)
• Pitch (mm) = 25.4 ÷ TPI

So a 1.25 mm pitch is about 20.3 TPI, and 18 TPI is about 1.41 mm. That small difference is exactly what causes trouble.

Near-equivalents that are NOT interchangeable

M6 vs 1/4-20

The 1/4-20 is 0.014 in larger and has a much coarser pitch. An M6 screw in a 1/4-20 hole feels sloppy and engages poorly. A 1/4-20 screw will not go into an M6 hole at all. Tap drills: 5.0 mm for M6 (77.0%) versus a #7, 0.201 in (5.1 mm), for 1/4-20 (75.4%).

M8 vs 5/16-18

This is the dangerous pair. The diameters are only 0.0025 in apart, so an M8 bolt will often start in a 5/16-18 hole and then bind as the pitch mismatch builds up. Forcing it damages both parts. Tap drills: 6.8 mm for M8 versus an F, 0.257 in (6.53 mm), for 5/16-18.

M10 vs 3/8-16

M10 is about 0.019 in larger than 3/8, and the pitches differ by roughly one thread per inch. A 3/8 bolt in an M10 hole is loose; an M10 bolt will not enter a 3/8-16 hole. Tap drills: 8.5 mm for M10 versus 5/16”, 0.3125 in (7.94 mm), for 3/8-16.

Two sneaky near-twins

Some pairs are close in both diameter and pitch, which makes them easy to confuse even with a gauge:

• M5 x 0.8 vs #10-32: 5.0 mm vs 4.83 mm, and 0.8 mm (31.75 TPI) vs 32 TPI.
• M4 x 0.7 vs #8-36: 4.0 mm vs 4.17 mm, and 0.7 mm (36.3 TPI) vs 36 TPI.

A pitch gauge alone can fool you here. Diameter is the tiebreaker.

How to identify an unknown thread

You need a pair of calipers and a thread pitch gauge with both metric and inch leaves. This follows the step-by-step procedure in Swagelok’s thread identification guide, simplified for machine screws and bolts.

1. Check that it is straight, not tapered

Measure the crest-to-crest diameter on the first, fourth and last full thread. If the diameter changes, it is a tapered pipe thread such as NPT, not a machine screw thread. Pipe threads have their own tap drills and need a different chart.

2. Measure the major diameter

• External thread (bolt): measure across the crests with the outside jaws.
• Internal thread (nut or hole): you can only measure the minor diameter directly with the inside jaws. Often it is easier to find a bolt that fits and measure that.

Expect a used bolt to measure a little under nominal. Swagelok notes measured diameters rarely match the nominal size exactly because of manufacturing tolerances. A 1/4 in bolt reading a few thousandths under 0.250 in is normal.

Compare the number to both systems. A reading near 0.236 in (6.0 mm) points to M6; near 0.250 in (6.35 mm) points to 1/4.

3. Match the pitch

Lay gauge leaves along the thread until one seats perfectly, with no rocking and no light showing between the teeth. Try both the metric and inch leaves near your diameter. Check along as much thread length as you can: a near-miss leaf may seem to fit over two or three threads but will rock over ten.

No gauge? Count threads over a known length with calipers or a ruler. Ten threads covering 12.5 mm is a 1.25 mm pitch. Twenty threads covering 1 in is 20 TPI.

4. Look it up, then test-fit

Combine diameter and pitch and look it up on the UNC chart, UNF chart or metric chart. Then confirm with a known nut or bolt. It should turn freely by hand for several full turns. If it tightens after a turn or two, stop: you probably have one of the near-equivalent pairs above.

Practical tips

• Keep inch and metric hardware in separate bins. Most mix-ups happen at the parts drawer.
• Mark tapped holes on fixtures and jigs (“M8” or “5/16-18”) with a paint pen.
• Drills cross over; taps do not. It is fine to drill an inch thread with a metric drill if the percent of thread works out. For 1/4-20, a 5.1 mm drill gives 75.7%, almost identical to the #7. See the tap drill formula guide.
• Coarse vs fine matters too. Even within one system, 1/4-20 and 1/4-28 do not mix. Our UNC vs UNF guide explains the difference.
• Clearance holes: see the clearance hole chart for both inch and metric screws.