UNC & UNF Thread Chart — Diameter, TPI & Pitch

UNC and UNF thread chart

Use the lookup table to find a common Unified inch coarse or fine thread by nominal size, series, major diameter, and TPI. Search for a complete designation such as 1/4-20, 1/4-28, #10-24, or #10-32. You can also search for a nominal size such as 1/4 or #10, then use the UNC/UNF filter to compare the available series.

This chart covers common UNC and UNF entries from ASME B1.1-2024. It is a thread-designation reference, not a substitute for a complete class, tolerance, limit, fit, or fastener specification.

How to read the designation

UNC versus UNF

UNC and UNF use the same Unified inch thread form, but they select different thread series. UNC is the coarse series and normally has fewer threads per inch for a given diameter. UNF is the fine series and normally has more threads per inch. A fine series therefore has a smaller axial pitch than the coarse series at the same nominal diameter.

The series name is part of the identification. Do not call every 1/4-inch thread “1/4 inch” and assume it will match. A 1/4-20 UNC internal thread and a 1/4-28 UNF external thread have the same nominal major diameter but different pitch. The pitch difference prevents them from being a correct mating pair.

What does 1/4-20 mean?

1/4-20 UNC means a nominal 1/4-inch Unified thread with 20 threads per inch in the coarse series. The table lists its basic nominal major diameter as 0.250 in, which converts to 6.350 mm. Its pitch is 1/20 in, or 0.050000 in. Converting that inch distance to millimeters gives 1.270000 mm.

1/4-28 UNF keeps the 0.250 in nominal diameter but uses 28 threads per inch in the fine series. Its pitch is 1/28 in, or approximately 0.035714 in and 0.907143 mm. The designation must include the TPI and series when you order, model, inspect, or match the fastener.

Number screw sizes

Numbered screw sizes use a number instead of a fractional diameter. The number is an identifier, not a decimal diameter that you should estimate. For example, #10-24 UNC has a 0.190 in basic nominal major diameter and 24 TPI. #10-32 UNF has the same nominal diameter and 32 TPI. The chart includes common numbered sizes from #0 through #12 where the UNC or UNF combination is commonly used.

Numbered sizes can have different coarse and fine TPI values. That is why a tap, screw, nut, or inspection gauge must be selected from the full designation. A #10 label alone does not identify the thread pitch.

Convert TPI to pitch

TPI is the native designation used by the Unified inch system. Pitch is the axial distance from one thread crest to the next. The chart calculates convenience values with these named formulas:

• pitch_in = 1 / TPI
• pitch_mm = 25.4 / TPI
• major_diameter_mm = major_diameter_in × 25.4

Keep the TPI value when identifying the thread. Rounded pitch values help with comparison and metric documentation, but they do not change the inch thread designation. The chart keeps the formula results separate from the ASME-sourced diameter and TPI fields.

Use the chart with tap-drill information

This chart identifies the thread. It does not choose a tap drill. Tap-drill recommendations depend on the tap type, target thread percentage, material, hole depth, coating, and the tap manufacturer’s guidance. Use the linked tap drill size chart as a separate lookup, and confirm the recommendation for the exact tap and workpiece.

Do not use this chart for NPT or another pipe-thread system. Pipe threads have different identification and sealing requirements. The related NPT thread in SOLIDWORKS guide is a modeling workflow for a different thread family.

Related references

• Metric Thread Pitch Chart — metric diameter and pitch lookup.
• Drill Bit Size Chart — fractional, number, letter, and metric drill designations.
• How to Use Hole Wizard in SOLIDWORKS — a CAD workflow for standard tapped holes.

Source note

The thread-series scope and common UNC/UNF rows are based on ASME B1.1-2024, Unified Inch Screw Threads. TPI remains the native value; the inch and millimeter pitch values are calculated conversions. Verify the current standard, class, tolerance, fit, and fastener specification for production-critical work.

Frequently asked questions

What is the pitch of 1/4-20 UNC?

1/4-20 UNC has 20 TPI. Its calculated pitch is 0.050000 in, or 1.270000 mm.

What is the pitch of 1/4-28 UNF?

1/4-28 UNF has 28 TPI. Its calculated pitch is approximately 0.035714 in, or 0.907143 mm.

What is the difference between #10-24 and #10-32?

Both have a 0.190 in nominal major diameter. #10-24 is UNC at 24 TPI; #10-32 is UNF at 32 TPI. They do not have the same pitch and should not be treated as interchangeable.

Does the chart include NPT?

No. The chart is limited to Unified inch UNC and UNF straight-thread data. NPT and other pipe-thread data are outside its scope.