Convert a decimal inch to the nearest fraction (e.g. 0.375 = 3/8") and back — for woodworking, machining and construction.
Numbers • inches
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On a US tape measure, inches are divided into halves, quarters, eighths, sixteenths and thirty-seconds. Converting a decimal inch to the nearest fraction — 0.625" = 5/8" — is essential for woodworking, machining, carpentry and DIY.
This converter rounds any decimal inch to the closest common fraction and converts fractions back to decimals, so you can move between calculator/CAD numbers and the markings printed on your ruler.
US construction and machining work in binary fractions of an inch — halves, quarters, eighths, sixteenths and beyond — because they subdivide by repeated halving, which is what a folding rule and a tape measure physically do. Converting a decimal to the nearest fraction means multiplying by the desired denominator, rounding to a whole number, and reducing. The precision you choose should match the trade: carpentry typically works to 1/16 inch, cabinetmaking to 1/32, and machining abandons fractions for thousandths of an inch entirely.
Numerator = round(decimal × denominator)Fraction = numerator / denominator, reduced by the GCDError = |decimal − (numerator ÷ denominator)|where:
Assumptions: Rounds to the nearest available fraction, so some error is unavoidable — that error is at most half the chosen increment, which for sixteenths is 1/32 inch (0.031 in).
Convert a decimal measurement to sixteenths, then check the error at a coarser precision.
Result0.4375 in = 7/16 in exactly
Decimals that are exact in binary fractions — .0625, .125, .1875, .25 and so on — convert with no error because sixteenths are powers of two. A measurement of 0.40 in has no exact fraction and lands at 13/32 (0.40625), which is 0.006 in over.