SAE TPI vs Metric Thread Pitch

 

Quick Answer: Metric fasteners specify thread pitch — the distance between thread peaks measured in millimeters. US/SAE fasteners specify threads per inch (TPI) — how many threads occur along one inch. Both systems offer coarse and fine versions of the same nominal size, which can be confusing when you’re mixing measurement systems.

Need look-ups? See our Metric Thread Pitch Table and US Threads Per Inch (TPI) Table.

Metric Threads — Pitch (mm between threads)

In metric notation, the pitch is the distance (in millimeters) from one thread crest to the next. A smaller pitch number means a finer thread.

Diagram showing metric thread pitch as the millimeter distance between adjacent thread peaks
Metric thread pitch: measure the mm distance between adjacent thread peaks (e.g., 1.25 mm).

Example: M8-1.25 × 40 mm

  • M8 — nominal diameter 8 mm
  • 1.25 — thread pitch (1.25 mm between thread peaks)
  • 40 mm — length of the screw

Measuring pitch: use a thread gauge, or measure across several threads with calipers for accuracy (e.g., measure across 10 crests and divide by 10).

Quick reference: Metric Thread Pitch Table

US / SAE Threads — TPI (Threads Per Inch)

In SAE/US notation, the second number is TPI, the count of threads along one inch. A higher TPI means a finer thread. You’ll often see UNC (coarse) and UNF (fine) variants for the same size.

Diagram illustrating Threads Per Inch (TPI) by counting thread crests along a one-inch length
Threads Per Inch (TPI): count the number of crests within a one-inch span.

Example: #10-32 × 6″

  • #10 — nominal size (≈0.19″ major diameter)
  • 32 — TPI (UNF = fine; #10-24 would be UNC = coarse)
  • 6″ — length of the screw

Measuring TPI on short screws? Count threads over a shorter fraction (e.g., 1/2″) and multiply to one inch.

Quick references: US TPI TableUS Screw Sizes in Inches

Metric vs SAE — Key Differences at a Glance

Feature Metric (ISO) SAE / US (Inch)
Measurement Unit Millimeters (mm) Inches
Thread Spec Pitch (mm between peaks) TPI (threads per inch)
Typical Notation M8-1.25 × 40 mm #10-32 × 6″
Coarse vs Fine Smaller pitch = finer (e.g., 1.0 vs 1.25) Higher TPI = finer (e.g., 32 vs 24)
Common Usage Automotive, industrial, global standard US manufacturing, hardware
ID Tip Measure crest-to-crest spacing (mm) Count threads per inch (or scale up from a fraction)
Pro Tip: If the screw uses a metric hex key size (e.g., 3 mm) and the pitch measures cleanly in mm, it’s likely metric. If the TPI lines up with a UNC/UNF chart (e.g., #10-24 or #10-32), it’s SAE/US.

Frequently Asked Questions

What is thread pitch?

In metric threads, pitch is the distance in millimeters between adjacent thread peaks (e.g., 1.25 mm).

What is TPI in screws?

Threads Per Inch (TPI) is how many thread peaks fit in one inch along the screw. Higher TPI = finer thread.

How can I tell if a screw is metric or SAE?

Try a thread gauge or compare to charts. If spacing measures cleanly in mm, it’s metric. If threads per inch match UNC/UNF values, it’s SAE/US.

What’s the difference between coarse and fine threads?

Coarse threads (larger pitch or lower TPI) install quickly and are more tolerant of debris. Fine threads (smaller pitch or higher TPI) offer finer adjustment and more shear area but require better alignment.

 

Tpi vs thread pitch

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