Technically reviewed by Vincent, Founder at HEXVARO
Last reviewed
How each system is written
| Input | Metric example | Inch example |
|---|---|---|
| Designation | M12 × 1.75 × 50 (pitch often omitted for coarse) | 1/2-13 UNC × 2 in |
| Diameter | 12 mm | 1/2 in (12.7 mm) |
| Thread | Pitch in mm: 1.75 mm coarse, 1.25 or 1.5 fine | Threads per inch: 13 UNC coarse, 20 UNF fine |
| Thread fit | Tolerance class 6g (bolt) / 6H (nut) | Class 2A (bolt) / 2B (nut) |
| Length | mm, under the head | inches, under the head |
| Product standard | ISO 4014 / 4017 (DIN 931 / 933) | ASME B18.2.1 hex cap screw or hex bolt |
| Strength | Property class 8.8, 10.9… | SAE J429 Grade 2, 5, 8 or ASTM spec |
Near-size pairs that do not interchange
These pairs are close in diameter but have different thread pitches, so the nut will not run on. The mm and TPI equivalents are calculated.
| Metric | Diameter (mm) | Pitch (mm) | Inch | Diameter (mm) | Pitch (mm, calc.) |
|---|---|---|---|---|---|
| M6 | 6.0 | 1.0 | 1/4-20 UNC | 6.35 | 1.27 |
| M8 | 8.0 | 1.25 | 5/16-18 UNC | 7.94 | 1.41 |
| M10 | 10.0 | 1.5 | 3/8-16 UNC | 9.53 | 1.59 |
| M12 | 12.0 | 1.75 | 1/2-13 UNC | 12.7 | 1.95 |
| M16 | 16.0 | 2.0 | 5/8-11 UNC | 15.88 | 2.31 |
| M20 | 20.0 | 2.5 | 3/4-10 UNC | 19.05 | 2.54 |
| M24 | 24.0 | 3.0 | 1-8 UNC | 25.4 | 3.17 |
- M8 and 5/16 in are almost the same diameter; a 5/16 nut may start on an M8 bolt and then jam. This is a common mix-up in maintenance stores.
- Never mix a metric bolt with an inch nut, even if it seems to tighten; the thread engagement is poor and the joint can fail.
Spanner sizes compared
| Metric bolt | Spanner (ISO 4017) | Inch bolt | Spanner (hex cap screw) |
|---|---|---|---|
| M6 | 10 mm | 1/4 in | 7/16 in (11.1 mm) |
| M8 | 13 mm | 5/16 in | 1/2 in (12.7 mm) |
| M10 | 16 mm | 3/8 in | 9/16 in (14.3 mm) |
| M12 | 18 mm | 1/2 in | 3/4 in (19.1 mm) |
| M16 | 24 mm | 5/8 in | 15/16 in (23.8 mm) |
| M20 | 30 mm | 3/4 in | 1-1/8 in (28.6 mm) |
Grade equivalents (similar strength, not interchangeable)
Use these pairs only to understand a foreign drawing. Do not convert a grade by name on an approved design; get engineering approval against the governing specification.
| Metric class | Min. tensile | Closest inch grade | Min. tensile |
|---|---|---|---|
| 4.6 | 400 MPa | SAE Grade 2 / ASTM A307 | 60–74 ksi (414–510 MPa) |
| 8.8 | 800–830 MPa | SAE Grade 5 | 120 ksi (827 MPa) up to 1 in |
| 10.9 | 1,040 MPa | SAE Grade 8 | 150 ksi (1,034 MPa) |
How to choose the system
- Follow the mating part: tapped holes, existing nuts and equipment decide the thread system.
- US-designed equipment and many oil, gas and petrochemical projects use inch fasteners (ASME / ASTM), even in the Middle East and Asia.
- European and Asian machinery and most building projects use metric fasteners (ISO / DIN / EN).
- If a drawing mixes both, list each line separately with its own standard.
Prevent common purchasing errors
- Do not round a converted inch diameter to an M size and keep the original nut.
- Keep diameter, pitch or threads per inch, length and units in separate columns of your size list.
- Always write UNC or UNF for inch threads; 1/2-13 and 1/2-20 need different nuts.
- Check head size, spanner clearance and thread length as well as diameter.
RFQ checklist for an international hex bolt order
Use the metric or inch product link below to start a product-specific Request a Quote. HEXVARO supplies fasteners made in Hebei and Anhui; reference MOQ is 500 pcs per specification, subject to quotation, and we reply within 2 working hours.
- Product standard and edition (ISO 4017, DIN 933, ASME B18.2.1 hex cap screw…).
- Thread system and designation: M12 × 1.75 or 1/2-13 UNC, plus thread class if controlled.
- Length and units, full or partial thread.
- Grade: property class or SAE / ASTM grade; finish with a full designation.
- Matching nuts and washers in the same system, quantity per line, packaging and documents.
Frequently asked questions
Is M12 the same as 1/2 inch?
No. A 1/2 in bolt is 12.7 mm in diameter with 13 threads per inch (about 1.95 mm pitch); M12 is 12 mm with 1.75 mm pitch. The nuts are not interchangeable.
What is the metric equivalent of a 5/16 bolt?
M8 is the closest (8.0 mm vs 7.94 mm), but the threads differ: 1.25 mm pitch vs 18 threads per inch. Do not mix them.
What does UNC mean?
Unified National Coarse — the standard coarse inch thread series. UNF is the fine series. 1/2-13 UNC has 13 threads per inch; 1/2-20 UNF has 20.
Is grade 8.8 the same as Grade 5?
Their minimum tensile strengths are similar (800–830 MPa vs 120 ksi / 827 MPa), but one is metric and one is inch, so they are not interchangeable.
Do Middle East projects use metric or inch bolts?
Both. Oil, gas and petrochemical projects often follow ASME / ASTM inch standards, while building and infrastructure work commonly uses metric. Follow the project specification.