Screw Size Reference

How Are Metric Screw Dimensions Determined?

Metric screw dimensions follow ISO 261 and ISO 262 standards, which define thread geometry based on a 60° thread angle profile. The major diameter, pitch, and minor diameter are mathematically related through the basic thread profile equations. Coarse pitch is the default; fine pitch is used when vibration resistance or fine adjustment is needed.

Tap drill size is calculated from the minor diameter to achieve approximately 75% thread engagement — the industry standard that balances thread strength (achieving ~90% of full thread strength) with reasonable tapping torque. Clearance holes are specified in close (H12), medium (H13), and free (H14) fits per ISO 273.

Common sizes range from M1.6 to M64 for general engineering. Pitch values follow a standardized geometric progression: M6×1.0 (coarse), M8×1.25, M10×1.5, M12×1.75. Fine pitch versions (e.g., M10×1.0, M10×1.25) are used in automotive, aerospace, and precision instruments.

Thread engagement length is critical for tapped hole design: in steel, 1.0-1.5× nominal diameter provides full thread strength. In softer materials like aluminum or cast iron, use 1.5-2.0× diameter. Beyond these limits, adding more threads provides negligible additional strength because the bolt will fail in tension before the threads strip.

Formula: Minor Diameter = Major Diameter − 1.0825 × Pitch Pitch Diameter = Major Diameter − 0.6495 × Pitch Tap Drill = Major Diameter − Pitch (for ~75% engagement)

Example Calculation

For an M10×1.5 (coarse) thread: Minor diameter = 10 − 1.0825 × 1.5 = 8.376 mm. Pitch diameter = 10 − 0.6495 × 1.5 = 9.026 mm. Tap drill = 10 − 1.5 = 8.5 mm. Close fit clearance hole = 10.5 mm.

When to Use This Calculator

• Programming a CNC tapping cycle and needing the correct tap drill size for the specified thread
• Designing a bolted joint and selecting the appropriate clearance hole size for the assembly
• Replacing a damaged fastener in the field and needing to identify the thread specification from measurements
• Creating technical drawings that require complete thread callout with minor diameter and pitch diameter dimensions

Common Mistakes to Avoid

• Confusing coarse and fine pitch for the same nominal size — M10×1.5 (coarse) and M10×1.25 (fine) require different tap drills and are not interchangeable
• Using free-fit clearance holes where close-fit is needed — excess clearance allows bolt movement that can cause joint fatigue failure under cyclic loading
• Forgetting that tap drill size assumes 75% thread engagement — specifying 100% engagement dramatically increases tapping torque and tap breakage rate with negligible strength gain
• Not accounting for the difference between drill size and actual hole size — drills cut slightly oversize (typically +0.02-0.05 mm), which affects thread engagement percentage

How to Interpret Results

• The tap drill value targets approximately 75% thread engagement — the industry standard that delivers about 90% of full thread strength at manageable tapping torque; round to the nearest available standard drill, preferring the larger size
• Minor diameter is the thread root dimension — use it for tensile stress area and strip-out calculations, not the tap drill size
• Close clearance holes locate the fastener precisely; free clearance allows for positional tolerance stack-up across multi-hole patterns — choose based on the pattern's positional tolerance and assembly method
• Coarse and fine pitch share the same nominal diameter but every derived dimension (pitch, minor diameter, tap drill) differs — confirm the pitch type against the drawing callout before drilling
• All values are basic ISO 261/262 dimensions — production threads carry class tolerances (6H/6g typical), so use thread gauge limits for final acceptance decisions

Related Standards & References

• ISO 261 — ISO general purpose metric screw threads — general plan
• ISO 262 — ISO general purpose metric screw threads — selected sizes for general use
• ISO 273 — Fasteners — clearance holes for bolts and screws
• ISO 68-1 — ISO general purpose screw threads — basic profile

Metric Screw Size Reference — Coarse Pitch

Dimensions for every standard designation at coarse pitch, computed by this calculator. Figures are in millimetres.

Frequently Asked Questions

When should I use fine pitch instead of coarse pitch?

Use fine pitch when: (1) the joint is subject to vibration and must resist loosening (e.g., automotive axle nuts), (2) fine adjustment is needed (e.g., micrometers, precision fixtures), (3) wall thickness is limited and fewer deep threads are better, or (4) higher tensile strength per thread is required. Coarse pitch is preferred for general assembly due to faster installation and greater tolerance to damaged threads.

What is the recommended thread engagement length?

For steel-into-steel, 1.0× to 1.5× the nominal diameter provides full thread strength. For aluminum or cast iron tapped holes, use 1.5× to 2.0× diameter. Beyond 1.5D in steel, adding more threads provides negligible additional strength because the bolt shank will fail before the threads strip.

What is the minimum thread engagement to prevent stripping?

In steel-to-steel connections, 0.5× nominal diameter provides approximately 75% of the bolt's tensile strength in strip-out resistance. At 1.0× diameter, strip-out strength equals bolt tensile strength. The practical recommendation is 1.0-1.5× diameter for steel, 1.5-2.0× for aluminum, and 2.0-2.5× for plastics.