17/64 in = 0.2656 in = 6.7469 mm.
- Fraction, reduced
- 17/64 in
- Decimal inches
- 0.2656 inFull value 0.265625 in
- Millimeters (× 25.4, exact)
- 6.7469 mmFull value 6.746875 mm
What it computes
Fractional inches still appear on older drawings, in stock and tool sizes, and on shop rules and drill indexes, while current US drawing practice gives inch dimensions as decimals. Moving between the two by hand invites two errors: a wrong decimal for an awkward fraction such as 23/64, and a millimeter size that someone rounds before the tolerance has been considered. The converter gives the exact figure both ways.
Fraction to decimal returns the fraction in lowest terms, its decimal inch value and its millimeter value. Decimal to fraction returns the value in the other unit, the nearest fraction at the step you choose, that fraction in inches and in millimeters, and the residual.
Formulas and factors
- Decimal inches = whole + numerator / denominator.
- Millimeters = inches × 25.4. The factor is exact by definition: the inch has been 25.4 mm since 1959.
- Nearest fraction at a step of 1/D: n = round(value in inches × D), then reduced, so 8/64 shows as 1/8.
- Residual = value − fraction. A positive residual means the value is larger than the fraction.
Exact arithmetic
A fraction whose denominator is a power of two always ends as a decimal: 1/64 is 0.015625 in, and 17/64 is 0.265625 in. Multiplying by 25.4 keeps it finite, so 17/64 in is exactly 6.746875 mm. The converter computes these values exactly and rounds only the displayed figure, to the decimals you choose, with the full value shown beside it.
The one figure that cannot always end is a millimeter value turned into inches, because 1 mm is 5/127 in. That figure is shown with ≈. The nearest fraction is still found from the exact value, so a millimeter size exactly halfway between two steps is never pushed the wrong way by floating-point error. A value exactly halfway takes the larger fraction.
Worked example
01 Convert a fraction
17/64 in = 17 ÷ 64 = 0.265625 in, and 0.265625 × 25.4 = 6.746875 mm. A mixed size works the same way: 1 3/8 in = 1.375 in = 34.925 mm.
02 Find the nearest 64th to a metric size
6 mm ÷ 25.4 ≈ 0.23622 in. Times 64 that is about 15.118, so the nearest step is 15/64 in, which is 0.234375 in or 5.953125 mm.
03 Read the residual
6 − 5.953125 = 0.046875 mm, about 0.0018 in. The 15/64 size is smaller than 6 mm by that much.
04 Check a decimal inch size
0.201 in at the nearest 1/64 rounds to 13/64 in (0.203125 in); the residual is −0.002125 in, so the fraction is the larger of the two.
Limitations
- Binary fractions only: halves to sixty-fourths. A drawing that uses other fractions needs the decimal entered directly.
- The nearest fraction is a rounding, not a fit. Whether a 15/64 in drill can stand in for a 6 mm hole depends on the hole's tolerance, which the converter does not know.
- It converts sizes, not tolerances. Convert both limits of a toleranced size, and round them so the band does not grow.
- Values take a decimal point, not a comma, and at most 15 significant digits.
When to involve a qualified engineer
Ask the drawing's owner before you substitute a fractional tool or stock size for a metric one, or the other way round, on a toleranced feature. Ask, too, when a drawing mixes fractions, decimals and millimeters and does not say which one governs.
What the product reads
In the Technical Preview, TruePart reads drawings dimensioned in millimeters only.
Questions
Is 1 inch exactly 25.4 mm?
Yes, by definition since 1959. Every fractional inch with a power-of-two denominator therefore has an exact, finite millimeter value.
Why does 6 mm round to 15/64 in and not 1/4 in?
At a step of 1/64 the nearest fraction is 15/64, 0.047 mm below 6 mm. At a step of 1/4 the nearest is 1/4 in, 0.35 mm above. The step decides the answer.
What does a negative residual mean?
The fraction is larger than the value you entered. 0.201 in rounds up to 13/64 in, so the residual is −0.002125 in.
Sources
- NIST Special Publication 811, Guide for the Use of the International System of Units (SI) · NISTAppendix B: the inch is exactly 25.4 mm.
- ASME Y14.5-2018, Dimensioning and Tolerancing · ASMEDecimal inch and millimeter dimensioning on engineering drawings.
What TruePart does today
- LimitIn the Technical Preview, TruePart reads drawings dimensioned in millimeters only.