Thread Size Chart: Metric & SAE Thread Standards Compared
Compare metric ISO 724, UNC, UNF, and Whitworth thread diameters and pitch side by side.
What Is a Thread Size Chart?
A thread size chart lists every common screw thread standard with its diameter, pitch, and cross-reference equivalents. Modern fasteners use four thread families in practice: metric (ISO 724), UNC (Unified National Coarse), UNF (Unified National Fine), and the legacy Whitworth standard still found on classic British vehicles and pipe fittings.
Reading a thread designation correctly matters more than reading a bolt's head size, since two threads can look nearly identical at a glance and still be completely incompatible. This page covers all four standards, the pitch and tensile-area numbers behind them, thread class and fit, and how to identify an unmarked thread with basic shop tools.
Metric Thread Sizes (M3 to M24)
| Thread | Diameter (mm) | Coarse Pitch | Fine Pitch | Tensile Area (mm²) |
|---|---|---|---|---|
| M3 | 3.0 | 0.5 | - | 5.03 |
| M4 | 4.0 | 0.7 | - | 8.78 |
| M5 | 5.0 | 0.8 | - | 14.2 |
| M6 | 6.0 | 1.0 | - | 20.1 |
| M8 | 8.0 | 1.25 | 1.0 | 36.6 |
| M10 | 10.0 | 1.5 | 1.25 | 58.0 |
| M12 | 12.0 | 1.75 | 1.25 | 84.3 |
| M14 | 14.0 | 2.0 | 1.5 | 115 |
| M16 | 16.0 | 2.0 | 1.5 | 157 |
| M18 | 18.0 | 2.5 | 1.5 | 192 |
| M20 | 20.0 | 2.5 | 1.5 | 245 |
| M22 | 22.0 | 2.5 | 1.5 | 303 |
| M24 | 24.0 | 3.0 | 2.0 | 353 |
UNC Thread Chart
| Designation | Diameter | TPI | Pitch (in) |
|---|---|---|---|
| #6-32 | 0.138" | 32 | 0.0313 |
| #8-32 | 0.164" | 32 | 0.0313 |
| #10-24 | 0.190" | 24 | 0.0417 |
| 1/4"-20 | 0.250" | 20 | 0.0500 |
| 5/16"-18 | 0.3125" | 18 | 0.0556 |
| 3/8"-16 | 0.375" | 16 | 0.0625 |
| 7/16"-14 | 0.4375" | 14 | 0.0714 |
| 1/2"-13 | 0.500" | 13 | 0.0769 |
| 5/8"-11 | 0.625" | 11 | 0.0909 |
| 3/4"-10 | 0.750" | 10 | 0.1000 |
| 1"-8 | 1.000" | 8 | 0.1250 |
UNF Thread Chart
| Designation | Diameter | TPI | Pitch (in) |
|---|---|---|---|
| #10-32 | 0.190" | 32 | 0.0313 |
| 1/4"-28 | 0.250" | 28 | 0.0357 |
| 5/16"-24 | 0.3125" | 24 | 0.0417 |
| 3/8"-24 | 0.375" | 24 | 0.0417 |
| 7/16"-20 | 0.4375" | 20 | 0.0500 |
| 1/2"-20 | 0.500" | 20 | 0.0500 |
| 5/8"-18 | 0.625" | 18 | 0.0556 |
| 3/4"-16 | 0.750" | 16 | 0.0625 |
| 1"-12 | 1.000" | 12 | 0.0833 |
Whitworth Thread Reference
Whitworth (BSW) threads were the world's first standardized thread system, devised by Joseph Whitworth in 1841. They use a 55-degree thread angle, compared to 60 degrees for metric and Unified threads, and rounded crests instead of flat ones. Whitworth threads still turn up on classic British motorcycles (Triumph, BSA, Norton), older agricultural equipment, and BSP (British Standard Pipe) plumbing fittings. Common sizes include 1/4 inch BSW at 20 TPI, 3/8 inch BSW at 16 TPI, and 1/2 inch BSW at 12 TPI, each requiring Whitworth-specific taps and dies since no other standard shares its thread angle.
ISO 724 Metric Thread Standard
ISO 724 defines the metric thread profile used worldwide, with a 60-degree thread angle, a flat crest, and a rounded root. The designation M10 × 1.5 means a 10mm nominal diameter with 1.5mm between thread crests. Fine-pitch versions, such as M10 × 1.25 or M10 × 1.0, pack thread crests closer together for higher tensile area and better resistance to loosening under vibration, at the cost of a slower, more precise installation.
Thread Pitch Explained
Pitch is the distance between adjacent thread crests, measured in millimeters for metric threads or expressed as threads per inch (TPI) for SAE threads. A 2mm pitch means a full rotation of the bolt advances it 2mm into the material. Coarse pitches install faster and resist cross-threading in dirty or damaged holes. Fine pitches resist loosening from vibration and provide slightly higher clamping force for the same bolt diameter. Always match pitch exactly when replacing a fastener, since a coarse-pitch bolt will not thread properly into a fine-pitch hole even if the diameter looks identical.
Thread Class and Fit
Thread class describes how tightly a male and female thread engage, independent of diameter and pitch. Metric threads use a tolerance grade and position, most commonly 6g for external (bolt) threads and 6H for internal (nut or tapped hole) threads, which suits general-purpose fastening. SAE threads use classes 1A/1B for a loose fit, 2A/2B for the standard commercial fit found on most hardware, and 3A/3B for a tight, precision fit used in aerospace and other close-tolerance work. The letter A always marks an external thread and B an internal one.
Metric vs SAE Thread Compatibility
Never mix metric and SAE threads. M6 × 1.0 is not interchangeable with a similar-diameter SAE thread like #12-24 or 1/4 inch-20, even though the diameters look close on a ruler. The pitch differs enough that forcing the two together cross-threads the fastener and the hole, ruining both. Use a thread pitch gauge whenever a thread standard isn't marked, and check the correct tap drill size before cutting new threads of either standard.
Thread Size Chart FAQ
What does M10 x 1.5 mean?
M10 × 1.5 means a metric thread with a 10mm nominal outer diameter and 1.5mm of distance between adjacent thread crests, known as the pitch. The M prefix signals a metric ISO 724 thread rather than a UNC/UNF or Whitworth thread.
Can I mix metric and SAE threads?
No. M6 × 1.0 is not interchangeable with a similar-diameter SAE thread like 1/4 inch-20, even though the diameters are close, since the pitch differs enough that forcing the two together cross-threads and destroys both the fastener and the hole.
What is the difference between UNC and UNF?
UNC (Unified National Coarse) uses fewer, deeper threads per inch and is the general-purpose default for most SAE fasteners. UNF (Unified National Fine) packs more threads into the same length, giving higher tensile strength and better resistance to vibration, at the cost of installing slightly slower.
Where are Whitworth threads still used?
Whitworth (BSW) threads survive on classic British motorcycles from Triumph, BSA, and Norton, older agricultural equipment, and BSP (British Standard Pipe) plumbing fittings. New consumer products no longer use Whitworth, but vintage vehicle restoration still requires Whitworth-specific taps, dies, and spanners.
How do I measure a thread I don't recognize?
Measure the outside diameter across the threads with calipers, then use a thread pitch gauge to check the distance between crests in millimeters, or count threads per inch with a ruler for TPI. Match both numbers against the chart to identify the standard.
What is thread class or fit?
Thread class describes how tightly a bolt and nut engage. Metric threads use a tolerance grade and position, such as 6g for an external thread or 6H for an internal thread, where a middle-of-the-range number like 6 suits general use. SAE threads use classes 1A/1B (loose), 2A/2B (standard commercial fit), and 3A/3B (tight, precision fit), with A always referring to external threads and B to internal.