Like all good stories this one starts with clarification of terminology and notation formats.

Inches can be written in a few ways:

1. Fractions, as X-Y/Z". X is whole inches. Y/Z is then a fraction of an inch in addition to the whole inches.
2. Decimals, as 0.00000...", with as many decimal places as you can accurately measure to.
3. Thou, as .000", which is similar to decimals but specifically refers to thousandths of an inch and is often written without the leading zero.

Here's how 'half an inch' would be written in each:

1. Fraction = 1/2"
2. Decimal = 0.5"
3. Thou = .500"

Millimetres (and other metric measurements) are more uniform, always being written as decimals. There is a minor difference in how the decimal is written between Europe and everywhere else:

• Europe uses a , as a decimal place
• Everywhere else uses a .

So three and a half millimetres would be written:

• In Europe as 3,5mm
• Everywhere else as 3.5mm

Here are the most common inch fractions converted to decimal inches and to mm. The smallest fraction we've given is 32nds - anything smaller than this tends to be represented as thou.

"Thou" is used for smaller measurements under one inch when fractions don't offer the size required - tolerances, sheet thicknesses, machining parameters, threads. All sorts of small stuff. It can of course be expressed as a fraction, X/1000, but is written as a decimal. The leading 0 is usually omitted, so the measurement starts with a decimal. Here's how thou convert to metric:

Larger sheet sizes are often quoted in inches. Below is a chart showing the conversion between common inch and metre sizes.

Despite individual ones being tiny, millimetres are used for measuring everything from the very small all the way up to large sheets. Here's how mm convert to cm, m, inches and feet.