Oct 20, 2026Quality Control
Metric vs Inch Fasteners: A Quick Standards & Conversion Guide
Understand metric vs inch fasteners fast. DIN/ISO vs ANSI/IFI naming, thread pitch vs TPI, common equivalents and tips to avoid mismatch in assemblies.
International buyers often maintain equipment that mixes metric and inch hardware. Misunderstandings here create delays and scrap. This quick guide clarifies the essentials so orders are right the first time.
Naming basics
Metric uses millimeters: M6 × 1.0 × 20, meaning 6 mm diameter, 1.0 mm pitch, 20 mm length. Coarse pitch is assumed if no pitch is given. - Inch (Unified) uses inches and threads-per-inch (TPI): 1/4-20 × 3/4, meaning 0.25 in diameter, 20 TPI, 0.75 in length. Coarse (UNC) and fine (UNF) are specified explicitly.
Standards you’ll see
Metric: DIN and ISO (e.g., ISO 4762 for hex socket cap screws). - Inch: ANSI/ASME and IFI (e.g., ASME B18.3 for socket cap screws). Hextro can build to either or follow your drawing with a cross-reference list during quoting.
Common near-equivalents (approximate)
• M3 ≈ 4-40 UNC
• M4 ≈ 8-32 UNC
• M5 ≈ 10-24 UNC
• M6 ≈ 1/4-20 UNC
• M8 ≈ 5/16-18 UNC
• M10 ≈ 3/8-16 UNC These are not interchangeable; pitch and diameter differ slightly. Use only for selection guidance.
Avoiding mix-ups
Write units everywhere—on BOM, drawings and emails. 2. For metric, include pitch if you need fine pitch (e.g., M10×1.25). 3. For inch, specify UNC/UNF and any Class (2A/2B). 4. Heads and drives have different standards too; provide the standard number if you must match recess depth or head height. 5. When converting lengths, round to available standards (e.g., inch hardware typically steps by 1/8 in).
Conversions you’ll actually use
mm to inch: divide by 25.4 (10 mm ≈ 0.394 in). - inch to mm: multiply by 25.4 (3/4 in ≈ 19.05 mm). - Pitch ↔ TPI: TPI ≈ 25.4 ÷ pitch(mm). 1.25 mm pitch ≈ 20.32 TPI (closest UNF is 1/4-28).
Ordering with Hextro
Send either the standard code (e.g., ISO 4762) or your drawing. If you’re switching systems, we can propose closest equivalents and supply mixed kits labeled by system to simplify maintenance.
