Unified Thread Chart — UNC & UNF TPI, Major, Minor & Pitch Diameters
ASME B1.1 Unified thread dimensions #4 to 1″: UNC and UNF threads per inch with basic major, pitch, and external minor diameters to four decimal places.
Updated August 18, 2026
The Unified thread system of ASME B1.1 describes every inch thread by three basic diameters. The major diameter D is the nominal size — for numbered machine screws it follows D = 0.060 + 0.013 × the gauge number, so a #10 is 0.190 in. The pitch diameter E, where the thread flank carries load and where go/no-go gauging happens, sits 0.649519 × P below the major (P = 1/TPI, and 0.6495 is ¾ of the 0.866·P fundamental triangle height). The external minor diameter tabulated here is the classic flat-root reference, 1.226869 × P below the major.
Coarse (UNC) and fine (UNF) are the two standard series for each size: a 1/4″ bolt is 1/4-20 UNC or 1/4-28 UNF, and above 7/8″ note that UNF for 1″ is 12 TPI, not the legacy 1-14 that some charts mislabel. All diameters below are basic (nominal) dimensions — actual product threads carry class allowances and tolerances (2A/3A external) below basic. Modern B1.1 reproductions of Table 6 print a slightly larger external minor, the UNR rounded-root design value; this chart keeps the flat-root column that the classic basic-dimension tables publish.
UNC (Coarse) — Basic Dimensions, #4 to 1″
| Size | TPI | Major Ø D (in) | Pitch Ø E (in) | Minor Ø ext. Ks (in) |
|---|---|---|---|---|
| #4 | 40 | 0.1120 | 0.0958 | 0.0813 |
| #5 | 40 | 0.1250 | 0.1088 | 0.0943 |
| #6 | 32 | 0.1380 | 0.1177 | 0.0997 |
| #8 | 32 | 0.1640 | 0.1437 | 0.1257 |
| #10 | 24 | 0.1900 | 0.1629 | 0.1389 |
| #12 | 24 | 0.2160 | 0.1889 | 0.1649 |
| 1/4″ | 20 | 0.2500 | 0.2175 | 0.1887 |
| 5/16″ | 18 | 0.3125 | 0.2764 | 0.2443 |
| 3/8″ | 16 | 0.3750 | 0.3344 | 0.2983 |
| 7/16″ | 14 | 0.4375 | 0.3911 | 0.3499 |
| 1/2″ | 13 | 0.5000 | 0.4500 | 0.4056 |
| 9/16″ | 12 | 0.5625 | 0.5084 | 0.4603 |
| 5/8″ | 11 | 0.6250 | 0.5660 | 0.5135 |
| 3/4″ | 10 | 0.7500 | 0.6850 | 0.6273 |
| 7/8″ | 9 | 0.8750 | 0.8028 | 0.7387 |
| 1″ | 8 | 1.0000 | 0.9188 | 0.8466 |
Basic dimensions computed from ASME B1.1 geometry with P = 1/TPI: pitch Ø E = D − 0.649519·P, external minor Ø (flat-root reference) Ks = D − 1.226869·P; spot-verified digit-for-digit against published tables. B1.1-2019 Table 6 reproductions instead list the UNR design minor D − 1.19078·P — a different published column, not an error.
UNF (Fine) — Basic Dimensions, #4 to 1″
| Size | TPI | Major Ø D (in) | Pitch Ø E (in) | Minor Ø ext. Ks (in) |
|---|---|---|---|---|
| #4 | 48 | 0.1120 | 0.0985 | 0.0864 |
| #5 | 44 | 0.1250 | 0.1102 | 0.0971 |
| #6 | 40 | 0.1380 | 0.1218 | 0.1073 |
| #8 | 36 | 0.1640 | 0.1460 | 0.1299 |
| #10 | 32 | 0.1900 | 0.1697 | 0.1517 |
| #12 | 28 | 0.2160 | 0.1928 | 0.1722 |
| 1/4″ | 28 | 0.2500 | 0.2268 | 0.2062 |
| 5/16″ | 24 | 0.3125 | 0.2854 | 0.2614 |
| 3/8″ | 24 | 0.3750 | 0.3479 | 0.3239 |
| 7/16″ | 20 | 0.4375 | 0.4050 | 0.3762 |
| 1/2″ | 20 | 0.5000 | 0.4675 | 0.4387 |
| 9/16″ | 18 | 0.5625 | 0.5264 | 0.4943 |
| 5/8″ | 18 | 0.6250 | 0.5889 | 0.5568 |
| 3/4″ | 16 | 0.7500 | 0.7094 | 0.6733 |
| 7/8″ | 14 | 0.8750 | 0.8286 | 0.7874 |
| 1″ | 12 | 1.0000 | 0.9459 | 0.8978 |
Same B1.1 basic geometry as the UNC table. UNF for 1″ is 12 TPI per B1.1 — charts listing 1″-14 as UNF are reproducing the legacy UNS/NF series.
Sources & Further Reading
- ASME B1.1 — Unified Inch Screw Threads (UN and UNR thread form): basic-dimension formulas E = D − 0.649519·P and Ks = D − 1.226869·P, as reproduced in Machinery’s Handbook-style tables
- Published-table verification: Carbide Depot UNC/UNF tap chart, Electronic Fasteners Basic Thread Dimensions PDF, shender4 thread/drill chart, Engineers Edge ANSI thread chart (2008) — every computed cell matches digit-for-digit; Machining Doctor’s truncated-major pitch diameters and Carbide Depot’s three typos outvoted
- Numbered-size majors: D = 0.060 + 0.013 × gauge (confirmed across Wikipedia, Electronic Fasteners, Carbide Depot, Machining Doctor)
Frequently Asked Questions
What is the pitch diameter actually used for?
It is the diameter where the thread groove and ridge are equally wide — the surface where flanks engage and loads transfer. Thread gauging, class fits (2A/2B, 3A/3B), and tap-drill engagement math all work at the pitch diameter, which is why B1.1 tabulates it to more places than anything else.
Why do published charts disagree on the external minor diameter?
Two legitimate columns exist. Classic basic-dimension tables print the flat-root reference Ks = D − 1.226869·P (this chart). Reproductions of B1.1-2019 Table 6 print the UNR design minor D − 1.19078·P, which reflects the rounded root modern external threads are made with — about 0.002–0.005 in larger. Neither is a typo; check which column a chart cites before comparing.
How do I get the major diameter of a numbered screw size?
D = 0.060 + 0.013 × the gauge number, in inches: #4 → 0.112, #8 → 0.164, #10 → 0.190, #12 → 0.216. The series continues past #12 in principle (#14 → 0.242), but above #12 the fractional sizes take over — 1/4″ (0.250) is the next standard size.
Is the minor diameter here the same as the tap drill size?
No. The external minor is the bolt’s root diameter. A tap drill sizes the hole for an internal thread, which is larger than the external minor — the standard 75% engagement drill for 1/4-20 is #7 (0.201 in), well above the 0.1887 in external minor. Use the tap drill size calculator, not this column, to pick drills.
Try the Calculators
Unified Thread Chart — UNC & UNF TPI, Major, Minor & Pitch Diameters — reuven.tools/reference/unified-thread-dimensions — verified against: ASME B1.1 — Unified Inch Screw Threads (UN and UNR thread form): basic-dimension formulas E = D − 0.649519·P and Ks = D − 1.226869·P, as reproduced in Machinery’s Handbook-style tables; Published-table verification: Carbide Depot UNC/UNF tap chart, Electronic Fasteners Basic Thread Dimensions PDF, shender4 thread/drill chart, Engineers Edge ANSI thread chart (2008) — every computed cell matches digit-for-digit; Machining Doctor’s truncated-major pitch diameters and Carbide Depot’s three typos outvoted; Numbered-size majors: D = 0.060 + 0.013 × gauge (confirmed across Wikipedia, Electronic Fasteners, Carbide Depot, Machining Doctor)