MM to Thou Converter
Convert millimetres to thou (thousandths of an inch) with precision. The thou, also known as a mil, equals one-thousandth of an inch (0.001 inches or 0.0254 millimetres) and serves as a fundamental unit in engineering, machining, and precision manufacturing across industries.
Quick Convert
Conversion Tables
Common MM to Thou Conversions
| Millimetres (mm) | Thou (Thousandths of an Inch) |
|---|---|
| 0.1 mm | 3.937 thou |
| 0.25 mm | 9.843 thou |
| 0.5 mm | 19.685 thou |
| 1 mm | 39.370 thou |
| 2 mm | 78.740 thou |
| 3 mm | 118.110 thou |
| 5 mm | 196.850 thou |
| 10 mm | 393.701 thou |
| 15 mm | 590.551 thou |
| 20 mm | 787.402 thou |
| 25 mm | 984.252 thou |
| 50 mm | 1,968.504 thou |
| 100 mm | 3,937.008 thou |
Precision Measurements
| Millimetres (mm) | Thou (Exact) | Common Application |
|---|---|---|
| 0.01 mm | 0.3937 thou | Micrometre precision |
| 0.05 mm | 1.9685 thou | Gauge block tolerance |
| 0.127 mm | 5.0000 thou | PCB trace width |
| 0.254 mm | 10.0000 thou | Sheet metal thickness |
| 0.508 mm | 20.0000 thou | Gasket material |
| 0.762 mm | 30.0000 thou | Plastic card thickness |
| 1.016 mm | 40.0000 thou | Wire diameter |
| 1.270 mm | 50.0000 thou | Shim stock |
Conversion Formula
The mathematical relationship between millimetres and thou is straightforward:
Thou = Millimetres × 39.3701
The conversion factor 39.3701 derives from the ratio between metric and imperial systems:
1 inch = 25.4 mm (exactly)
1 thou = 0.001 inches = 0.0254 mm
1 mm = 1 ÷ 0.0254 = 39.370078740157 thou
Step-by-Step Conversion
To convert 5 millimetres to thou:
Step 1: Identify the value in millimetres: 5 mm
Step 2: Multiply by the conversion factor: 5 × 39.3701
Step 3: Perform the multiplication: 5 × 39.3701 = 196.8505
Step 4: Round to desired precision: 196.85 thou (2 decimal places) or 196.8505 thou (4 decimal places)
Measurement Systems
Millimetre (mm)
System: Metric (SI)
Definition: One-thousandth of a metre (10⁻³ m)
Primary use: International standard for engineering, scientific measurements, and manufacturing worldwide
Precision: Base-10 system enables straightforward decimal calculations
Thou (Thousandth of an Inch)
System: Imperial/US Customary
Definition: One-thousandth of an inch (0.001″)
Primary use: Machining, engineering tolerances, manufacturing in non-metric countries (particularly UK and USA)
Precision: Ideal for fine mechanical tolerances and specifications
Historical Context: The thou was introduced around 1844 by Joseph Whitworth, who advocated for decimal divisions of the inch rather than traditional fractions. This revolutionised precision engineering by enabling machinists to work with measurements smaller than 1/64 inch, which was previously the practical limit for manual measurement.
Industry Applications
Engineering & Manufacturing
- Precision Machining: Tolerances for metal components often specified in thou, with high-precision work requiring accuracy within “a few tenths” (0.1 thou or 2.54 micrometres)
- Sheet Metal Work: Material thickness specifications range from 10 thou (0.254 mm) for thin sheets to 125 thou (3.175 mm) for structural applications
- Automotive Engineering: Head gasket thickness, piston ring gaps, and bearing clearances measured in thou
- Hydraulic Cylinders: Tolerance specifications critical for proper seal function and pressure containment
Electronics & Technology
- Printed Circuit Boards (PCBs): Track widths, via hole diameters, and spacing between conductors specified in thou
- Component Manufacturing: IC lead spacing and connector pin dimensions
- Wire & Cable: Conductor diameter and insulation thickness measurements
General Manufacturing
- Paper Industry: Stock thickness typically 2-4 thou for newspaper, 8-10 thou for standard office paper
- Film & Foil: Plastic films, aluminium foil, and protective coatings measured in thou
- Latex Products: Medical and industrial glove thickness specifications
- Plastic Cards: Standard ID cards approximately 30 thou (0.76 mm) thick
Precision Levels
| Precision Level | Tolerance (Thou) | Tolerance (MM) | Application |
|---|---|---|---|
| Standard | ±5-10 thou | ±0.127-0.254 mm | General fabrication |
| Fine | ±1-5 thou | ±0.0254-0.127 mm | Precision parts |
| High Precision | ±0.5-1 thou | ±0.0127-0.0254 mm | Aerospace components |
| Ultra Precision | ±0.1 thou (1 tenth) | ±0.00254 mm | Gauge blocks, calibration |
| Extreme Precision | <0.1 thou | <0.00254 mm | Research, nanotechnology |
Measurement Equipment
Accurate measurement in thou requires specialised instruments, as the unit is too small for reliable measurement with standard rulers:
- Micrometres (Screw Gauges): Provide direct readings to 0.001 inches (1 thou), with vernier scales enabling 0.0001 inch (0.1 thou) resolution
- Vernier Callipers: Typically read to 1 thou on imperial scales, suitable for quick checks and less critical measurements
- Dial Indicators: Measure relative displacement with 1 thou or 0.1 thou graduations, essential for setup and inspection
- Gauge Blocks: Precision reference standards manufactured to tolerances of ±0.000002 inches (±2 millionths) for calibration purposes
Frequently Asked Questions
What does “thou” mean in measurements?
“Thou” is an abbreviation of “thousandth” and refers to one-thousandth of an inch (0.001 inches). It’s also called a “mil” in some contexts, though this can cause confusion with millimetres. In British engineering and machining, “thou” is the preferred term.
How many thou are in 1 millimetre?
One millimetre equals exactly 39.370078740157 thou. For most practical purposes, this is rounded to 39.37 thou or simply 39.4 thou. The conversion factor derives from the definition that 1 inch = 25.4 mm (exactly).
Why do engineers still use thou instead of millimetres?
In countries with historical imperial measurement systems (particularly the UK and USA), thou remains prevalent in machining and engineering because existing equipment, tooling, and technical drawings were designed around imperial units. Many industries maintain thou specifications to ensure compatibility with legacy systems and international suppliers.
What is a “tenth” in machining?
A “tenth” refers to one-tenth of a thou, which equals 0.0001 inches or 2.54 micrometres. Machining “to within a few tenths” represents extremely high precision, approaching the practical limits of conventional machining equipment. This level of accuracy is essential for gauge blocks, precision bearings, and high-performance engine components.
Can I convert thou to millimetres by dividing by 39.37?
Yes, to convert thou to millimetres, divide the thou value by 39.3701 (or multiply by 0.0254). For example, 100 thou ÷ 39.3701 = 2.54 mm. This reverse conversion is equally important when working with international specifications or mixed measurement systems.
What tolerance level is acceptable for precision engineering?
Acceptable tolerance depends on the application. General manufacturing might accept ±5-10 thou (±0.127-0.254 mm), whilst precision engineering typically requires ±1-2 thou (±0.0254-0.051 mm). Aerospace, medical devices, and gauge manufacturing demand tolerances of ±0.5 thou or tighter. The specification should always match functional requirements whilst remaining economically feasible.
Is thou the same as mil?
In most engineering contexts, “thou” and “mil” both refer to one-thousandth of an inch (0.001 inches). However, “mil” can occasionally cause confusion because it’s also an abbreviation for millimetre in some European languages. British engineers predominantly use “thou” to avoid ambiguity, whilst “mil” is more common in American technical documentation.