Convert decimal inches to fractional inches at any resolution (1/2 to 1/64). Shows rounding error, mm equivalent, ruler visual, and complete fraction reference table.
Fractional inches — 1/2, 1/4, 1/8, 1/16, 1/32, and 1/64 — are the language of American woodworking, plumbing, and machining. But digital calipers and CAD software output measurements in decimal inches, and metric tools give you millimeters. Bridging the gap between these formats is a daily task for anyone who works with their hands.
This calculator converts decimal inches (or millimeters) into the nearest fractional inch at your chosen resolution. Select from halves down to sixty-fourths, and see exactly how close the fraction is to the true value. The rounding error is displayed in millimeters so you can decide if a coarser fraction is accurate enough for your work.
A visual ruler segment shows where your measurement falls, and a complete fraction reference table for the selected resolution provides a printable quick-reference chart. Whether you are reading a digital caliper and need to mark a board with a tape measure, or translating a metric drill bit size to a fractional inch, this tool has you covered.
Digital measurements must be translated into tape-measure fractions for layout work. Doing this in your head introduces errors—especially at 16ths or 32nds. This calculator eliminates guesswork by showing the nearest fraction at every resolution, the exact rounding error, and the metric equivalent, all in one view for faster, safer, and more accurate layout decisions.
Fraction numerator = round(decimal_part × resolution) Simplified fraction: divide numerator and denominator by their GCD Rounding error (mm) = |decimal_value − fraction_value| × 25.4 Millimeters from inches: mm = inches × 25.4
Result: 3 3/8″
3.375 inches: the whole part is 3, the remainder is 0.375. At 1/16 resolution: 0.375 × 16 = 6 sixteenths = 6/16 = 3/8 (simplified). So 3.375″ = 3 3/8″ exactly, with zero rounding error.
The fractional inch system divides each inch into halves, quarters, eighths, sixteenths, thirty-seconds, and sixty-fourths. Each step doubles the number of divisions. A tape measure typically marks down to sixteenths, with longest tick marks at inches, shorter at halves, shorter still at quarters, and so on. Learning to read a tape measure fluently is a fundamental skill in any trade that uses imperial measurements.
Decimal inches (e.g., 1.500″) are preferred in engineering, CNC machining, and 3D printing because they are easier to add, subtract, and input into software. Fractional inches (e.g., 1-1/2″) dominate hand-tool trades — carpentry, plumbing, and HVAC — because tape measures and rulers are marked in fractions. The ability to convert fluently between both systems saves time and prevents costly mistakes.
Standard fractional drill bits come in 1/64″ increments from 1/64″ to 1″, then 1/32″ increments up to 1-1/2″. Number drills (1–80) and letter drills (A–Z) fill the gaps between fractional sizes. When a metric hole is needed, find the closest fractional drill or use a metric drill set. This calculator helps identify the nearest fraction and tells you exactly how much error the substitution introduces.
It means the result is rounded to the nearest sixteenth of an inch. An inch is divided into 16 equal parts, each 1.5875 mm. The finest resolution available here is 1/64 (0.397 mm per increment).
0.3125 × 16 = 5, so 0.3125″ = 5/16″. This is exact with no rounding error. At 1/8 resolution, it rounds to 3/8″ with a small error.
Halving is the simplest way to subdivide a unit physically (fold a tape in half repeatedly). Powers of 2 (2, 4, 8, 16, 32, 64) naturally emerge from this process, which is why rulers use them.
1/16 inch = 1.5875 mm. For quick reference: 1/8″ ≈ 3.2 mm, 1/4″ = 6.35 mm, 1/2″ = 12.7 mm.
Yes. Enter the decimal drill bit diameter and choose your resolution to find the nearest fractional size. Check the rounding error — if the hole tolerance is tight, you may need a number or letter drill instead of a fractional one.
Most quality tape measures are graduated to 1/16″. Economy tapes may only show 1/8″. For work requiring 1/32″ or 1/64″ accuracy, use a steel rule or calipers instead of a tape measure.