← AskMachinist · Guides · Calculator · 🔎 Search

UNC and UNF Thread Size Chart

Guide · Updated October 28, 2026

Unified inch threads (UNC coarse, UNF fine and UNEF extra fine) all share the same 60 degree profile as ISO metric threads. Only the diameters and pitches differ. A thread is called out by size and threads per inch, then the series and class, such as 1/4-20 UNC-2A. The chart below gives the standard combinations from ASME B1.1, the basic diameters you can work out from them, and the tap drills commonly used for about 75% thread.

Need an exact reference size?

Build any gage block stack with the free Gage Block Calculator. Free, no login.

Step by step

Measure the major diameter: Mic the outside of a screw or the thread crests. It reads a little under nominal on a class 2A screw because of the allowance.
Count threads per inch: Use a thread pitch gage or count crests over one inch with a scale.
Find the series: Match the size and TPI to the chart to see if it is UNC, UNF or UNEF.
Work out basic diameters: Pitch P = 1 ÷ TPI. Basic pitch diameter = D − 0.6495P. Basic minor diameter = D − 1.0825P.
Pick the tap drill: Use the chart value, or D − 1/TPI for about 75% thread on most sizes.
Check the class: 2A/2B for general work, 3A/3B for closer pitch diameter limits; check with thread plug or ring gages.

Machine screw sizes (numbered)

#0 (0.0600): UNF 80 TPI, tap drill 3/64
#1 (0.0730): UNC 64, UNF 72, tap #53
#2 (0.0860): UNC 56, UNF 64, tap #50
#3 (0.0990): UNC 48 (#47), UNF 56 (#45)
#4 (0.1120): UNC 40 (#43), UNF 48 (#42)
#5 (0.1250): UNC 40 (#38), UNF 44 (#37)
#6 (0.1380): UNC 32 (#36), UNF 40 (#33)
#8 (0.1640): UNC 32 (#29), UNF 36 (#29)
#10 (0.1900): UNC 24 (#25), UNF 32 (#21)
#12 (0.2160): UNC 24 (#16), UNF 28 (#14), UNEF 32

Numbered sizes follow the rule D = 0.060 + 0.013 × number, so #10 = 0.060 + 0.130 = 0.190 in.

Fractional sizes

1/4: UNC 20 (#7, 0.201), UNF 28 (#3, 0.213), UNEF 32
5/16: UNC 18 (F, 0.257), UNF 24 (I, 0.272), UNEF 32
3/8: UNC 16 (5/16), UNF 24 (Q, 0.332), UNEF 32
7/16: UNC 14 (U, 0.368), UNF 20 (25/64), UNEF 28
1/2: UNC 13 (27/64), UNF 20 (29/64), UNEF 28
9/16: UNC 12 (31/64), UNF 18 (1/2), UNEF 24
5/8: UNC 11 (17/32), UNF 18 (9/16), UNEF 24
3/4: UNC 10 (21/32), UNF 16 (11/16), UNEF 20
7/8: UNC 9 (49/64), UNF 14 (13/16), UNEF 20
1: UNC 8 (7/8), UNF 12 (59/64), UNEF 20
1-1/8 and 1-1/4: UNC 7, UNF 12
1-3/8 and 1-1/2: UNC 6, UNF 12
1-3/4: UNC 5; 2: UNC 4-1/2

Note that 1-14 is widely sold as the common fine thread for 1 inch even though the UNF series lists 1-12; 1-14 is a UNS size. Check which one the print calls for.

The basic thread formulas

Pitch: P = 1 ÷ TPI
Fundamental triangle height: H = 0.866025 × P
Basic pitch diameter: E = D − 0.649519 × P
Basic minor diameter (internal): D − 1.082532 × P
Approximate tap drill for about 75% thread: D − 1/TPI

These are basic dimensions. Actual limits for each class come from the ASME B1.1 tables; class 2A external threads have an allowance below basic, while class 3A external threads have none.

Worked example: 1/4-20 UNC

P = 1 ÷ 20 = 0.0500 in. Basic pitch diameter = 0.2500 − 0.6495 × 0.0500 = 0.2175 in. Basic minor diameter = 0.2500 − 1.0825 × 0.0500 = 0.1959 in. A #7 drill (0.201 in) leaves about 75% thread, which is the chart value. Three-wire best wire size is 0.57735 × P = 0.0289 in.

Worked example: 1/2-13 UNC and 1/2-20 UNF

1/2-13: P = 0.07692 in, pitch diameter = 0.5000 − 0.04996 = 0.4500 in, basic minor = 0.5000 − 0.08327 = 0.4167 in, tap drill 27/64 (0.4219).

1/2-20: P = 0.0500 in, pitch diameter = 0.5000 − 0.03248 = 0.4675 in, basic minor = 0.4459 in, tap drill 29/64 (0.4531). The fine thread has a larger minor diameter and more stress area, so it is a little stronger in tension and easier to adjust finely, but it strips more easily in soft material and is more likely to gall in stainless.

Coarse, fine or extra fine?

Use UNC for most tapped holes in steel, cast iron and aluminum, where a coarse thread is faster to tap and more forgiving. Use UNF for adjusting screws, thin-wall parts and where higher bolt preload in a given size matters. UNEF is for very thin walls and fittings. Whichever series you choose, give enough length of engagement: about 1 × D in steel, 1.5 × D in cast iron and 2 × D in aluminum is a common rule of thumb.

Common problems

Mixing up 5/16-18 and 5/16-24 or 3/8-16 and 3/8-24 because they look alike, reading a worn screw as undersize, and forgetting that a class 2A screw is under basic. Check pitch with a pitch gage and pitch diameter with thread wires or a thread mic.

FAQ

What is the TPI of a 1/4 bolt?

1/4-20 is UNC (coarse) and 1/4-28 is UNF (fine).

What is the pitch diameter of 3/8-16?

Basic pitch diameter = 0.375 − 0.6495 × 0.0625 = 0.3344 in.

What is UNEF?

The Unified extra fine series, used on thin walls and fittings, such as 1/4-32 or 1/2-28.

How do I check a thread wire setup?

Check the gage itself before trusting a measurement: gage blocks set mics, calipers and indicators; build any stack free at gageblockcalc.com.

Related guides and calculators

Build your stack in seconds

The free Gage Block Calculator finds the fewest-block combination for inch and metric sets, with wear blocks. No login.

Questions? Ask a machinist.

General shop information only — follow your drawing, tool maker's data and your quality system.