Worked example
0.4 in × 64 = 25.6, which rounds to 26, so the nearest 64th is 26/64 = 13/32 (0.40625 in). It is 0.00625 in – 0.159 mm – too large. In 16ths the nearest is 6/16 = 3/8 (0.375 in), 0.025 in (0.64 mm) too small: a coarser fraction is easier to read but further from the real value.
When a fraction is good enough
- Stock sizes (bar, plate, drill sets) are specified in fractions, so the nearest standard size is usually what you want.
- Close-tolerance work is not: a bore of 0.4062 in is not "13/32" if the tolerance is a few thousandths. Check the error shown against the tolerance.
- For the full list of 64ths with their decimal and millimetre values see the inch to mm converter.
Frequently asked questions
How do I convert a decimal inch to a fraction?
Multiply the decimal by the denominator you want (64 is common), round to the nearest whole number and put it over that denominator, then reduce. 0.4 × 64 = 25.6, so 26/64 = 13/32.
What is 0.5 inch as a fraction?
1/2. Every 0.015625 (1/64 inch) step is a fraction; 0.5 = 32/64 = 1/2.
How do I convert mm to a fraction of an inch?
Divide the millimetres by 25.4 to get inches, then find the nearest fraction. 10 mm = 0.3937 in, nearest 64th 25/64 in, off by 0.078 mm.
Why is the nearest fraction not exact?
Most decimal sizes fall between two fractions. The calculator shows the error so you can judge whether the fraction is close enough for the tolerance.
More free machinist calculators
- Inch to mm – Convert inches to millimetres and back, fractions like 1 3/8 included. Exact factor 25.4, with a chart of every 1/64 inch.
- RPM and feed – Calculate spindle RPM from surface speed and diameter, surface speed from RPM, and feed rate from chip load – metric and imperial, with formulas.
- Taper – Taper ratio, half angle and taper per foot from the large and small diameter and the length, or the small diameter from the angle.
- Bolt circle – X and Y coordinates of evenly spaced holes on a bolt circle, with the hole spacing, start angle and centre offset. Copy the table into your program.
- Sine bar – Gauge block height for a sine bar angle, or the angle from a known stack, for 5 in, 10 in, 100 mm, 200 mm or any other bar length.
- Metal removal rate – Metal removal rate in cm³/min or in³/min from depth, width and feed – milling and turning, metric and imperial, with the formulas.
- Drill point depth – Length of a drill's point cone from diameter and point angle, and the total depth needed for a full-diameter hole to depth.
- Right triangle – Hypotenuse and angles from two legs, or the legs from the hypotenuse and an angle – for chamfers, dovetails, tapers and hole positions.
- Machining time – Cutting time from length, feed and spindle speed – turning with feed per revolution, milling with feed rate and number of passes.
- Degrees to DMS – Convert decimal degrees to degrees, minutes and seconds and back, with the angle in radians.
- SFM to m/min – Convert surface feet per minute to metres per minute and back, and inches per minute to mm per minute, with the exact factors.
- TPI to pitch – Convert a thread pitch in millimetres to threads per inch (TPI) and back, for matching metric and inch threads.
- Cpk calculator – Paste measurements from Excel, enter USL and LSL and get Cp, Cpk, Pp and Ppk. Says plainly when 25 values are too few for an index.
- Tolerance stack-up – Add up the tolerances of a chain of parts: closing gap, worst case and statistical RSS for up to eight dimensions. Free, in your browser.
- Fit clearance – Enter hole and shaft limits and get minimum and maximum clearance and the kind of fit – clearance, transition or interference.
- Chip thinning – Feed per tooth to program when the radial width of cut is below half the cutter diameter, so the real chip thickness stays right.
- Ball-nose stepover – Scallop height left by a ball-nose cutter from the stepover, or the stepover for a wanted scallop height, on a flat surface.
- Surface finish – Theoretical Rt and estimated Ra of a turned surface from the feed per revolution and the tool nose radius.
- Torque and power – Torque from power and speed in newton-metres or pound-feet, and power from torque and speed. The formulas with worked examples.
- Spur gear – Pitch and outside diameter, centre distance, ratio and circular pitch of a pair of external spur gears from the module and the tooth numbers.
Making parts for a customer who wants paperwork?
Automotive and aerospace customers ask for proof that the parts match the drawing. See first article inspection (FAI), PPAP explained and the true position calculator.