Bolt & Nut Size Chart — ISO Metric with Torque

ISO metric bolt sizes (M3 to M36) with thread pitch, tap drill, hex AF and torque values.

About this calculator

A bolt and nut size reference covers the dimensions, thread pitch, tensile strength, and proof load of standard metric bolts (ISO 4014, ISO 4017) and imperial bolts (UNC, UNF). Specifying the correct bolt is critical in any mechanical or structural assembly — under-spec and the joint fails, over-spec and you waste material and add weight.

Bolt grades (8.8, 10.9, 12.9 metric; Grade 5, Grade 8 imperial) indicate the tensile strength. The first number × 100 = ultimate tensile strength in MPa; the decimal indicates yield ratio. An 8.8 bolt has UTS of 800 MPa and yield of 0.8 × 800 = 640 MPa — the most common structural grade.

Formula

T = K \times D \times F

  • T — Tightening torque (Nm)
  • K — Friction coefficient (≈ 0.2 unlubricated, 0.15 with thread sealant)
  • D — Nominal bolt diameter (m)
  • F — Preload force (N) — typically 75% of yield strength

Worked example

Inputs: M12 × 1.75 bolt, grade 8.8, dry threads

  1. Diameter d = 12 mm = 0.012 m
  2. Bolt area (stress) = 84.3 mm²
  3. Yield = 640 MPa → preload at 75% = 0.75 × 640 × 84.3 = 40,464 N
  4. Tightening torque T = 0.2 × 0.012 × 40,464 = 97 Nm
  5. Lubricated threads (K = 0.15): T = 73 Nm — always use a torque wrench

Frequently asked questions

What grade bolt do I need?

For general structural assembly: 8.8 (metric) or Grade 5 (imperial). For high-stress applications like engine components, connecting rods, cylinder head bolts: 10.9 or 12.9 (metric), Grade 8 (imperial). For corrosion-prone environments: stainless A2 or A4 (note that stainless bolts have lower strength — A4-80 has UTS of 800 MPa, equivalent to grade 8.0).

Coarse thread or fine thread?

Coarse (M10 × 1.5) is faster to assemble and more tolerant of dirt, damage, and misalignment. Fine (M10 × 1.25) provides higher tensile load capacity per unit length and resists vibration loosening better. Use coarse for general fastening; specify fine for precision instruments, automotive engine components, and high-vibration applications.

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