Engineering Precision Converter

Professional millimeter to inch conversion tool for machining, CAD, metrology, and manufacturing. High-precision calculations with tolerance support and engineering standards.

High-Precision Calculator

Millimeters (mm)

Inches (in)

Decimal Precision

Select precision based on application: General (4-5), Machining (5-6), Metrology (6-8)

Engineering Tolerance Guide

Standard ISO tolerance classes and their imperial equivalents for precision engineering and manufacturing.

ISO Tolerance	MM Range	Inch Equivalent	Typical Use
IT01	±0.0008	±0.00003"	Gauge blocks
IT6	±0.008	±0.0003"	Precision bearings
IT7	±0.012	±0.0005"	Close-fitting parts
IT8	±0.018	±0.0007"	General machining
IT10	±0.048	±0.0019"	General purpose
IT12	±0.12	±0.0047"	Rough machining

💡 Professional Tip

Always specify tolerance class in engineering drawings. Use IT7-IT8 for general machining, IT6 for precision work, and IT01-IT5 for metrology applications.

Fasteners & Thread Conversion

Metric and imperial thread comparison for engineering and manufacturing applications.

Metric vs Imperial Thread Comparison

Tap Drill Sizes

Machining Parameters

Feed rates, cutting speeds, and surface finish conversions for CNC machining and manufacturing.

Surface Finish (Roughness) Conversion

Feed Rate Conversion

mm/min ↔ in/min (IPM)

300 mm/min = 11.81 IPM

Formula: IPM = mm/min ÷ 25.4

Cutting Speed Conversion

m/min ↔ SFM

120 m/min = 393.7 SFM

Formula: SFM = m/min × 3.281

CAD & Technical Drawing Standards

Standard drawing scales, text heights, and line weights for converting between ISO and ASME standards.

Drawing Scales

Text Heights & Line Weights (ISO → ASME)

Tool Size Conversions

Drill bits, end mills, and cutting tool size conversions for machining and manufacturing.

Common Drill Bit Sizes

Engineering FAQ

Why is 25.4mm = 1 inch EXACTLY?

This exact definition comes from the 1959 International Yard and Pound Agreement:

1 inch = 25.4 millimeters (exact, by definition)
1 yard = 0.9144 meters (exact)
1 pound = 0.45359237 kilograms (exact)

Before 1959, slight variations existed between countries. Now it's internationally standardized for engineering and scientific applications.

How to handle tolerance stack-up in mixed-unit assemblies?

Best Practice Steps:

Convert all dimensions to ONE system (prefer mm for precision)
Calculate worst-case tolerance stack
Apply RSS (Root Sum Square) for statistical analysis
Add safety factor (typically 1.5-2.0)

Example:

Part A: 10.00 ±0.05 mm

Part B: 0.5000 ±0.002" → 12.700 ±0.051 mm

Total: 22.70 ±0.101 mm (worst case)

RSS: ±√(0.05² + 0.051²) = ±0.071mm (statistical)

Should I design in metric or imperial for global manufacturing?

Metric (ISO) - Recommended if:

✓ Exporting to Asia/Europe
✓ Using metric fasteners/bearings
✓ Following ISO standards
✓ Modern CNC/CAM software

Imperial (ASME) - Consider if:

✓ US domestic market only
✓ Legacy parts/tooling
✓ Aerospace (often mixed)
✓ Inch-based material stock

Best Practice: Design in ONE system, provide conversions as reference, never mix units in tolerances, and document clearly.

How precise do engineering conversions need to be?

Application	Precision	Example
General Engineering	4-5 decimals	25.4mm = 1.0000"
Precision Machining	5-6 decimals	12.345mm = 0.486024"
Metrology/Inspection	6-8 decimals	1.0000mm = 0.03937008"
Scientific	Full precision	inches = mm / 25.4

How do I convert engineering drawings from metric to imperial?

CAD Conversion Checklist:

1. Units Setup

Change base units: mm → inches
Scale factor: 1/25.4 or 0.0393701

2. Dimensions

Update dimension precision (0.001 → 0.0001)
Convert tolerances appropriately
Update GD&T values

3. Text & Annotations

Update text heights (3.5mm → 0.125")
Convert notes and callouts
Update title block

4. Standards Compliance

ISO → ASME Y14.5
Update symbols if needed
Change drawing templates

5. Verification

Check critical dimensions manually
Verify tolerance zones
Review with stakeholders