CNC / machinist calculator
Fraction, Decimal and Millimetre Converter
Switch between decimal inches, millimetres and fractional inches without reaching for a chart. Enter a value in inches or millimetres and this converter gives the decimal inch, the millimetre value and the nearest fractional inch down to a sixty-fourth, along with how far that fraction is from the exact number. It is the quick lookup for reading a metric drawing on an inch machine or sizing the closest drill to a decimal dimension.
- Decimal inch
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- Millimetre
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- Nearest fraction
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How it works
Inches and millimetres are a fixed ratio: one inch is exactly 25.4 millimetres, so converting either way is a single multiply or divide. The harder everyday question is the nearest usable fraction, because drills, reamers and wrenches come in fractional sizes, most commonly in steps of a sixty-fourth of an inch.
This converter rounds the decimal to the nearest sixty-fourth, then reduces the fraction so a value like sixteen sixty-fourths reads as a quarter inch. It also reports the rounding error, the gap between your exact value and the fraction, so you can judge whether the nearest standard size is close enough or whether you need the exact decimal on the machine.
A common use is finding the closest drill to a metric hole: enter the millimetre size, read the decimal inch, and see the nearest fractional drill and how much it is over or under. For the hole to drill before tapping a thread, use the tap drill size calculator instead.
Worked example
A 10 mm hole is 0.3937 in, whose nearest fractional drill is 25/64 in (0.3906 in), about 3 thousandths under the metric size.
Frequently asked questions
How do I convert millimetres to inches?
Divide the millimetre value by 25.4, since one inch is exactly 25.4 millimetres. For example 10 mm is 10 divided by 25.4, about 0.3937 inch, whose nearest sixty-fourth is 25/64 at 0.3906 inch.
How do I convert a decimal inch to a fraction?
Multiply the decimal by 64, round to the nearest whole number, and put that over 64, then reduce. For example 0.25 inch times 64 is 16, which over 64 reduces to a quarter inch exactly.
Why show the rounding error?
Fractional sizes are discrete, so the nearest fraction is rarely exact. The error tells you how far the standard size is from your number, which decides whether a stock drill is close enough or you must hit the exact decimal.
What is the nearest drill to a metric size?
Enter the millimetre size and read the nearest fractional inch and its error. For odd sizes a number or letter drill may be closer; the drill size chart lists those decimal equivalents alongside the fractions.
Is one inch exactly 25.4 mm?
Yes. Since 1959 the international inch is defined as exactly 25.4 millimetres, so inch-to-millimetre conversion is exact and only the fractional rounding introduces any error.
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Sources
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These calculators are for planning and as a starting point. Recommended speeds and feeds are published starting values that vary with your specific tool, coating, machine rigidity, workholding and coolant. Always start conservative, listen to the cut, and follow your tool maker data sheet.