Metric (ISO)
60°
flank angle
Measured by: Pitch in millimetres
Every modern vehicle worldwide, all European and Japanese machinery.
Thread Size Chart: Metric & SAE Thread Standards Compared
Three thread standards account for nearly everything you will meet: metric ISO threads measured by pitch in millimetres, Unified inch threads measured in threads per inch, and Whitworth threads which use a 55-degree flank angle instead of the 60 degrees the other two share. That flank angle is the detail that matters most, because it is invisible without a gauge and it is what makes Whitworth genuinely incompatible rather than merely differently sized. Two threads can share a diameter and a thread count and still destroy each other if the angles differ. This page sets the standards side by side so you can identify what you are holding before you cut, tap or force anything. Once the fastener is identified, sizing the tool that turns it is a separate question the socket conversion calculator answers from the head dimension.
The Three Standards at a Glance
UNC / UNF
60°
flank angle
Measured by: Threads per inch
US industrial hardware, pre-1980 domestic vehicles, aerospace.
Whitworth (BSW)
55°
flank angle
Measured by: Threads per inch
Pre-1970 British cars, motorcycles, machine tools and plumbing.
Metric and Unified threads share a 60-degree flank angle, which is why a metric bolt will sometimes start into an inch thread of similar size before it binds. The profiles are compatible and only the spacing is wrong. Whitworth's 55-degree angle and rounded crests and roots mean it will not start cleanly into anything else at all. In practice that difference is protective: a Whitworth fastener usually refuses immediately rather than damaging the thread it was forced into.
Metric Thread Sizes: M3 to M24
| Thread | Major dia | Pitch | Equivalent TPI | Major dia (in) |
|---|---|---|---|---|
| M3 | 3mm | 0.50mm | 50.8 | 0.1181 |
| M3.5 | 3.5mm | 0.60mm | 42.3 | 0.1378 |
| M4 | 4mm | 0.70mm | 36.3 | 0.1575 |
| M5 | 5mm | 0.80mm | 31.7 | 0.1969 |
| M6 | 6mm | 1.00mm | 25.4 | 0.2362 |
| M7 | 7mm | 1.00mm | 25.4 | 0.2756 |
| M8 | 8mm | 1.25mm | 20.3 | 0.3150 |
| M10 | 10mm | 1.50mm | 16.9 | 0.3937 |
| M12 | 12mm | 1.75mm | 14.5 | 0.4724 |
| M14 | 14mm | 2.00mm | 12.7 | 0.5512 |
| M16 | 16mm | 2.00mm | 12.7 | 0.6299 |
| M18 | 18mm | 2.50mm | 10.2 | 0.7087 |
| M20 | 20mm | 2.50mm | 10.2 | 0.7874 |
| M22 | 22mm | 2.50mm | 10.2 | 0.8661 |
| M24 | 24mm | 3.00mm | 8.5 | 0.9449 |
The Equivalent TPI column is there purely for comparison against the inch tables further down. Metric threads are never specified that way in practice, but converting pitch to threads per inch is the only way to see how the two systems line up. M6 × 1.0 works out to about 25 threads per inch, which sits between 1/4-20 UNC and 1/4-28 UNF without matching either. That near-miss is the whole story of thread standards in one row.
Thread Pitch Explained
Metric: distance between crests
Metric pitch is a length. The distance from one thread crest to the next, in millimetres. M10 × 1.5 has 1.5mm between crests. A smaller number means finer threads, which is the opposite of the inch convention and a persistent source of confusion. Coarse pitch is implied when the second number is left off, so a bolt listed as M10 is M10 × 1.5.
Inch: threads per inch
Inch pitch is a count, how many threads fall within one inch of length. 1/2-13 has 13 threads per inch. A larger number means finer threads. To convert to metric pitch, divide 25.4 by the thread count: 13 TPI is 25.4 ÷ 13 = 1.95mm between crests, which is close to but not the same as the 1.75mm of an M12.
Pitch determines the tap drill as directly as diameter does, which is why the tap drill size chart is organised by thread designation rather than by diameter alone. It also determines the socket you will eventually need only indirectly. The head size follows the fastener standard, and the bolt size chart maps thread to head for both systems.
UNC Thread Sizes
| Thread | Major dia (in) | Major dia (mm) | TPI | Metric pitch |
|---|---|---|---|---|
| #4-40 UNC | 0.1120 | 2.84mm | 40 TPI | 0.64mm |
| #6-32 UNC | 0.1380 | 3.51mm | 32 TPI | 0.79mm |
| #8-32 UNC | 0.1640 | 4.17mm | 32 TPI | 0.79mm |
| #10-24 UNC | 0.1900 | 4.83mm | 24 TPI | 1.06mm |
| #12-24 UNC | 0.2160 | 5.49mm | 24 TPI | 1.06mm |
| 1/4-20 UNC | 0.2500 | 6.35mm | 20 TPI | 1.27mm |
| 5/16-18 UNC | 0.3125 | 7.94mm | 18 TPI | 1.41mm |
| 3/8-16 UNC | 0.3750 | 9.52mm | 16 TPI | 1.59mm |
| 7/16-14 UNC | 0.4375 | 11.11mm | 14 TPI | 1.81mm |
| 1/2-13 UNC | 0.5000 | 12.70mm | 13 TPI | 1.95mm |
| 9/16-12 UNC | 0.5625 | 14.29mm | 12 TPI | 2.12mm |
| 5/8-11 UNC | 0.6250 | 15.88mm | 11 TPI | 2.31mm |
| 3/4-10 UNC | 0.7500 | 19.05mm | 10 TPI | 2.54mm |
| 7/8-9 UNC | 0.8750 | 22.22mm | 9 TPI | 2.82mm |
| 1"-8 UNC | 1.0000 | 25.40mm | 8 TPI | 3.17mm |
The Metric pitch column converts each thread count into the millimetre spacing a metric caliper would measure. It is the fastest way to see that no UNC thread coincides with a metric one: 1/4-20 works out to 1.27mm pitch, sitting between the 1.25mm of M8 and nothing else useful, while its 6.35mm diameter falls between M6 and M8. Diameter and pitch both miss, which is exactly why these threads cannot be interchanged.
UNF Thread Sizes
| Thread | Major dia (in) | Major dia (mm) | TPI | Metric pitch |
|---|---|---|---|---|
| #4-48 UNF | 0.1120 | 2.84mm | 48 TPI | 0.53mm |
| #6-40 UNF | 0.1380 | 3.51mm | 40 TPI | 0.64mm |
| #8-36 UNF | 0.1640 | 4.17mm | 36 TPI | 0.71mm |
| #10-32 UNF | 0.1900 | 4.83mm | 32 TPI | 0.79mm |
| #12-28 UNF | 0.2160 | 5.49mm | 28 TPI | 0.91mm |
| 1/4-28 UNF | 0.2500 | 6.35mm | 28 TPI | 0.91mm |
| 5/16-24 UNF | 0.3125 | 7.94mm | 24 TPI | 1.06mm |
| 3/8-24 UNF | 0.3750 | 9.52mm | 24 TPI | 1.06mm |
| 7/16-20 UNF | 0.4375 | 11.11mm | 20 TPI | 1.27mm |
| 1/2-20 UNF | 0.5000 | 12.70mm | 20 TPI | 1.27mm |
| 9/16-18 UNF | 0.5625 | 14.29mm | 18 TPI | 1.41mm |
| 5/8-18 UNF | 0.6250 | 15.88mm | 18 TPI | 1.41mm |
| 3/4-16 UNF | 0.7500 | 19.05mm | 16 TPI | 1.59mm |
| 7/8-14 UNF | 0.8750 | 22.22mm | 14 TPI | 1.81mm |
| 1"-12 UNF | 1.0000 | 25.40mm | 12 TPI | 2.12mm |
UNF shares every major diameter with UNC and changes only the thread count, so the two series are distinguishable by nothing except counting threads. That makes a thread pitch gauge the single most useful cheap tool in a fastener drawer. A 1/2-13 and a 1/2-20 look identical at arm's length, mate with completely different nuts, and take different tap drills.
Whitworth Reference
| BSW size | TPI | Hex A/F (in) | Hex A/F (mm) |
|---|---|---|---|
| 1/8" | 40 TPI | 0.338 | 8.6mm |
| 3/16" | 24 TPI | 0.383 | 9.7mm |
| 1/4" | 20 TPI | 0.445 | 11.3mm |
| 5/16" | 18 TPI | 0.525 | 13.3mm |
| 3/8" | 16 TPI | 0.600 | 15.2mm |
| 7/16" | 14 TPI | 0.710 | 18.0mm |
| 1/2" | 12 TPI | 0.820 | 20.8mm |
| 5/8" | 11 TPI | 0.920 | 23.4mm |
| 3/4" | 10 TPI | 1.100 | 27.9mm |
| 7/8" | 9 TPI | 1.300 | 33.0mm |
| 1" | 8 TPI | 1.480 | 37.6mm |
The two right-hand columns are the ones that catch people out. A Whitworth spanner is stamped with the bolt diameter, not the jaw size, so a spanner marked 1/4 W has a jaw of 0.445", nearly 7/16" AF. Reading the third column as though it were the spanner marking, or the first column as though it were the jaw, produces a rounded fastener either way. The wrench size chart covers the same trap from the spanner side, including how AF and Whitworth markings relate.
Never Mix Thread Standards
A fastener that starts and then stiffens is not tight. It is cutting a new thread through the old one. The most dangerous near-misses are the ones that engage far enough to feel plausible. M6 × 1.0 into a 1/4-20 hole will take two or three turns before it binds, by which point the original thread is destroyed. 3/8-16 UNC and M10 × 1.5 are close enough in both diameter and pitch to do the same. The rule is absolute: if a fastener does not run down its full depth under finger pressure alone, back it out and identify the thread properly rather than reaching for a wrench. Recovering from a cross-threaded hole means drilling it out and either tapping oversize or fitting a thread insert, which is a far longer afternoon than checking with a pitch gauge would have been. Both recovery paths start at the drill bit size chart and continue through the tap drill sizes for whichever thread you settle on.
Identification takes three measurements and about a minute: outside diameter with a caliper, thread count or spacing with a pitch gauge, and flank angle by eye against the gauge's 55 and 60 degree profiles. Those three numbers place any fastener uniquely on one of the tables above. From there the head size tells you the tool, and the metric to standard socket chart handles the case where the only socket you own is from the other system.
More Conversion Tools
The rest of the fastener, thread and drill references on this site, every one of them is one click away from here.
SAE to Metric Converter Convert standard fractional sizes back to millimetres Impact Socket Sizes Impact-rated socket sizing for 1/4 through 3/4 inch drives Deep Socket Sizes Deep well socket lengths for recessed fasteners Drive Size Adapters Stepping between 1/4, 3/8, 1/2 and 3/4 inch drive sizes mm to Inches General millimetre to fractional inch conversion Fraction to Decimal Wrench fraction to decimal inch reference Allen Wrench Sizes Metric and SAE hex key equivalents