REFERENCE · CONSTRUCTION · FASTENERS
A 1/2-inch bolt is really 0.500 inches across the shank (12.7 mm), cut with 13 threads per inch (1/2″-13 UNC), tapped with a 27/64-inch (0.4219″) drill, and turned with a 3/4-inch wrench. That one sentence is the whole chart in miniature: diameter, thread count, tap drill, wrench. Below it, the full SAE table (1/4″–3/4″, UNC and UNF) and metric table (M5–M20, coarse and fine) — every size printable, every size clickable in the lookup.
Pick a bolt size and thread series. The lookup returns the full designation, the nominal diameter in inches and millimeters, the thread spec (TPI for SAE, pitch for metric), the tap drill for a standard 75%-thread hole, and the wrench size across flats — plus what the size is typically used for. Try the presets: M10, 3/8″-16, or M8 fine.
Three conventions run the whole chart. Diameter is nominal: a 1/2″ bolt measures 0.500″ across the unthreaded shank (an M10 measures 10 mm) — the threads themselves cut slightly into that. SAE threads are counted per inch: 1/2″-13 means 13 threads in one inch (coarse UNC); 1/2″-20 is the fine UNF version of the same bolt. Metric pitch is millimeters between threads: M10 × 1.5 means 1.5 mm from one thread crest to the next (coarse); M10 × 1.25 is the fine version. Pick the wrong series and the nut will not start — SAE and metric never interchange, not even when the diameters look close (1/2″ is 12.7 mm, not 12 or 13).
Read it as “half-inch, thirteen, UNC.” Diameter: 0.500″ (12.7 mm) nominal. Thread: 13 threads per inch, Unified Coarse. Tap drill: 27/64″ (0.4219″) — the hole you drill before cutting the thread (about 75% thread engagement, the shop standard). Wrench: 3/4″ across flats. Typical uses: structural connections, heavy brackets, tow hardware — the most common structural bolt in North America. The fine sibling, 1/2″-20 UNF, takes a 29/64″ (0.4531″) tap drill and the same 3/4″ wrench.
| Size | UNC (coarse) | UNF (fine) | Tap drill, coarse | Tap drill, fine | Wrench | Typical uses |
|---|---|---|---|---|---|---|
| 1/4″ | 20 TPI | 28 TPI | #7 (0.201″) | #3 (0.213″) | 7/16″ | Electrical panels, light brackets, small machine guards |
| 5/16″ | 18 TPI | 24 TPI | F (0.257″) | I (0.272″) | 1/2″ | Lawn and garden equipment, medium brackets, machinery covers |
| 3/8″ | 16 TPI | 24 TPI | 5/16″ (0.3125″) | Q (0.332″) | 9/16″ | Motor mounts, trailer hitches, machinery bases |
| 7/16″ | 14 TPI | 20 TPI | U (0.368″) | W (0.386″) | 5/8″ | Automotive suspension parts, engine accessories |
| 1/2″ | 13 TPI | 20 TPI | 27/64″ (0.4219″) | 29/64″ (0.4531″) | 3/4″ | Structural connections, heavy brackets, tow hardware |
| 5/8″ | 11 TPI | 18 TPI | 17/32″ (0.5312″) | 37/64″ (0.578″) | 15/16″ | Heavy equipment frames, large trailer couplers |
| 3/4″ | 10 TPI | 16 TPI | 21/32″ (0.6562″) | 11/16″ (0.6875″) | 1-1/8″ | Foundation anchors, heavy structural steel, rigging |
| Size | Coarse pitch | Fine pitch | Tap drill, coarse | Tap drill, fine | Wrench | Typical uses |
|---|---|---|---|---|---|---|
| M5 | 0.8 mm | 0.5 mm | 4.2 mm | 4.5 mm | 8 mm | Electronics, small machine screws, hobby work |
| M6 | 1.0 mm | 0.75 mm | 5.0 mm | 5.2 mm | 10 mm | Brackets, machinery covers, sheet-metal assembly |
| M8 | 1.25 mm | 1.0 mm | 6.8 mm | 7.0 mm | 13 mm | Machinery assembly, automotive brackets and mounts |
| M10 | 1.5 mm | 1.25 mm | 8.5 mm | 8.8 mm | 16 mm | Auto suspension, structural joints, heavy brackets |
| M12 | 1.75 mm | 1.25 mm | 10.2 mm | 10.8 mm | 18 mm | Structural steel, engine mounts, heavy machinery |
| M16 | 2.0 mm | 1.5 mm | 14.0 mm | 14.5 mm | 24 mm | Heavy structural connections, large equipment frames |
| M20 | 2.5 mm | 1.5 mm | 17.5 mm | 18.5 mm | 30 mm | Heavy equipment, foundation anchors, rigging hardware |
| Output | Formula | What it means |
|---|---|---|
| Full designation | size + TPI/pitch + series | 1/2″-13 UNC or M10 × 1.5 — everything a parts counter needs to hand you the right bolt. |
| Nominal diameter | SAE: fraction of an inch · metric: millimeters | 0.500″ (12.7 mm) for 1/2″ — measured across the shank, before the threads are cut. |
| Thread spec | SAE: threads per inch · metric: mm between threads | 13 TPI (UNC coarse) vs 20 TPI (UNF fine) on the same 1/2″ bolt — the number after the dash is the whole story. |
| Tap drill | ≈ diameter − thread depth (75% engagement) | 27/64″ (0.4219″) for 1/2″-13 — drill this hole, then cut the thread with the matching tap. |
| Wrench size | across-flats of the hex head/nut | 3/4″ for a 1/2″ bolt — the head is always bigger than the shank, so never size the wrench by the bolt diameter. |
Tap drills are the 75%-thread standard used by nearly every shop chart — roll-form taps and soft materials want a larger hole; check your tap maker’s chart for critical threads. Wrench sizes are modern ISO / ASME finished-hex: older hardware may carry the superseded sizes (17 mm on old M10, 19 mm on old M12) and Japanese equipment often uses JIS heads (14 mm on M10). Heavy-hex structural bolts use larger heads than the chart. Grades (the radial lines on SAE heads, the 8.8/10.9 on metric) set strength, not size — this chart covers dimensions only. Always confirm critical fasteners against the manufacturer’s spec sheet.
Bright Side Kit. "Bolt Size Chart: SAE & Metric Thread Guide | Bright Side Kit." https://brightoolbox.com/bolt-size-chart/ Accessed October 24, 2026.Data sourcesAcross the shank, yes: 0.500″ (12.7 mm). “Nominal diameter” means the diameter of the bar the threads were cut into — the thread crests reach about 0.500″, the roots cut slightly under. The head is bigger (a 1/2″ bolt takes a 3/4″ wrench), and the length is measured separately. When a bolt “looks bigger than its size,” you’re looking at the head.
Fewer, deeper threads vs more, shallower ones. Coarse (1/2″-13) assembles faster, resists cross-threading, and holds better in soft materials — it’s the default for general work. Fine (1/2″-20) has more threads sharing the load, resists vibration loosening, and allows finer adjustment — common in automotive and aerospace. Same diameter, same wrench; different tap drill (27/64″ vs 29/64″).
16 mm — the modern ISO size. But check the hardware’s age and origin: older metric bolts used 17 mm, and Japanese (JIS) equipment commonly uses 14 mm heads on M10 threads. If the 16 feels sloppy, try 17 before you round the corners off; if it won’t start, try 14.
Different diameters and different thread shapes-per-length. A 1/2″ bolt is 12.70 mm — between M12 and M13, matching neither — and its 13-TPI thread never aligns with a 1.75 mm metric pitch. Forcing one into the other’s nut strips both. Close is not close enough with threads: always match system to system.
Threads per inch (TPI). 1/2″-13 has 13 thread crests in every inch of length. Bigger number = finer thread. In metric the equivalent number is the pitch — millimeters between crests — so M10 × 1.5 has a crest every 1.5 mm, or about 17 threads per inch.
Metric: subtract the pitch from the diameter — M10 × 1.5 wants an 8.5 mm hole. SAE: subtract 1/TPI from the diameter — 1/2″-13 wants 0.5 − 1/13 ≈ 0.423″, and the nearest standard drill is 27/64″ (0.4219″). Both give roughly 75% thread engagement, the strength sweet spot for cut threads.
You may be holding a specialty fastener. Carriage bolts, flange bolts, and socket-head cap screws use different head dimensions; heavy-hex structural bolts (A325) use larger heads than standard hex; and #6–#12 machine screws live below this chart’s 1/4″ floor (see our screw sizes chart). Measure the shank with calipers, count threads against a gauge, and match both — diameter alone is never enough.