Most US construction, carpentry, and trade work is measured in fractional inches — 1/16, 1/8, 1/4 — while modern instruments output decimal inches. Converting between the two is one of the most common shop-floor tasks. This calculator takes a decimal value (e.g. 1.375 in) and gives you the nearest tape-measure fraction (1 3/8 in) at your chosen precision.
You can also see the same length in feet-inches, centimeters, and millimeters — useful when working with mixed metric/imperial drawings or international suppliers.
What is a fractional inch?
A fractional inch is a length expressed as a fraction whose denominator is a power of two — 1/2, 1/4, 1/8, 1/16, 1/32, 1/64. This binary system is what every standard US tape measure prints, and it's the native language of US construction. The smallest division typically marked is 1/16 (~1.59 mm); more precise tools (caliper-style measures) show 1/32 or 1/64.
Decimal inches, by contrast, are used by digital calipers, CAD software, CNC machines, and engineering drawings. The two notations describe the same physical length — 0.625 in and 5/8 in are identical — but they're written differently, and converting between them is a skill every tradesperson develops.
The decimal-to-fraction formula
Fraction = simplify(Numerator / Denominator)
Pick the denominator that matches your tool's precision (16 for a standard tape measure, 32 for finer work, 64 for machining). Multiply your decimal by that denominator and round to the nearest whole number. The result, over the denominator, is your fraction — then simplify by dividing numerator and denominator by their greatest common factor.
Worked example: 1.375 inches at 1/16 precision
- Choose denominator: 16.
- Multiply: 1.375 × 16 = 22.
- Round: 22 (no rounding needed — exact match).
- Fraction: 22/16.
- Simplify: GCD(22, 16) = 2, so 22/16 = 11/8 = 1 3/8 inches.
Result: 1 3/8 in, also known as 1.375 in, ~34.93 mm, or ~3.49 cm.
How to use this calculator
- Enter the decimal inch value — up to four decimal places. Negative values aren't supported; if you have one, drop the sign and add it back to the final answer.
- Choose your tape-measure precision — 1/16 for general carpentry, 1/32 for trim and cabinet work, 1/64 for machining.
- Read the fraction, the feet-inches form (for lengths over 12 inches), and the metric equivalents (cm, mm).
- Check the rounded-decimal output against your input to see how much rounding error your chosen precision introduced.
Precision comparison table
| Precision | Smallest division | Max rounding error | Typical use |
|---|---|---|---|
| 1/2 | 0.5 in (12.7 mm) | ±0.25 in (±6.35 mm) | Rough sketches, signage |
| 1/4 | 0.25 in (6.35 mm) | ±0.125 in (±3.18 mm) | Rough framing |
| 1/8 | 0.125 in (3.18 mm) | ±0.0625 in (±1.59 mm) | Light framing, decking |
| 1/16 | 0.0625 in (1.59 mm) | ±0.03125 in (±0.79 mm) | General carpentry — standard tape |
| 1/32 | 0.03125 in (0.79 mm) | ±0.0156 in (±0.40 mm) | Trim, cabinetry, finish work |
| 1/64 | 0.0156 in (0.40 mm) | ±0.0078 in (±0.20 mm) | Machining, metalwork, instruments |
Standard fraction-to-decimal chart
| Fraction | Decimal Inch | Millimeters |
|---|---|---|
| 1/64 | 0.0156 | 0.397 |
| 1/32 | 0.0313 | 0.794 |
| 1/16 | 0.0625 | 1.588 |
| 1/8 | 0.1250 | 3.175 |
| 3/16 | 0.1875 | 4.763 |
| 1/4 | 0.2500 | 6.350 |
| 5/16 | 0.3125 | 7.938 |
| 3/8 | 0.3750 | 9.525 |
| 7/16 | 0.4375 | 11.113 |
| 1/2 | 0.5000 | 12.700 |
| 9/16 | 0.5625 | 14.288 |
| 5/8 | 0.6250 | 15.875 |
| 11/16 | 0.6875 | 17.463 |
| 3/4 | 0.7500 | 19.050 |
| 13/16 | 0.8125 | 20.638 |
| 7/8 | 0.8750 | 22.225 |
| 15/16 | 0.9375 | 23.813 |
| 1 | 1.0000 | 25.400 |
When to use fractional vs decimal inches
- Tape measure & ruler work — fractions. If you're going to mark and cut wood or drywall, work in fractions. The tape itself shows the marks you'll align to.
- Digital calipers — decimals. Calipers display 3-decimal precision; converting back to a fraction reintroduces rounding error unless your tolerance allows it.
- CAD & engineering drawings — decimals. Software stores everything as floating-point. Fractions are a presentation choice for shop drawings.
- CNC machining — decimals. The control software speaks decimal natively.
- Metric drawings — mm. Convert to decimal inches, then to fractions only if your tool requires it.
- Material orders — usually fractions. US lumber, hardware, and trim are specified in fractional dimensions.
Common conversion mistakes
- Using too coarse a precision. Rounding 0.0625 (1/16) to 1/8 doubles your error — match the calculator's precision to your tool.
- Forgetting to simplify. 8/16 should always be reduced to 1/2 — most calculators do this automatically, but verify.
- Not converting feet-inches. 15.5 in = 1 ft 3 1/2 in. Forgetting to break out the foot value confuses anyone reading the result.
- Mixing metric and imperial mid-calculation. If a drawing specifies 38 mm, convert to inches (1.4961 in) BEFORE rounding to a fraction (1 1/2 in). Rounding metric to imperial in two steps amplifies error.
- Treating 0.5 as 1/2 then adding 0.25 as 1/4 without a common denominator. 1/2 + 1/4 = 3/4, not 1/6. Use decimals for the math and convert the result.
- Reading the wrong precision line on the tape. The middle marks on a tape are 1/2, then 1/4, then 1/8, then 1/16. Count divisions, don't eyeball.
Tips for working with fractional inches
- Memorise the eighths. 0.125, 0.25, 0.375, 0.5, 0.625, 0.75, 0.875 — these eight values appear constantly.
- Use decimal for math, fraction for marking. Calculate in decimals to avoid common-denominator headaches; convert to a fraction only at the moment of cutting or marking.
- Mark your tape with the precision. Some tradespeople put a tiny piece of tape at the 1/16 mark closest to a frequently-used dimension so they don't have to read the scale every time.
- For long runs, use decimal feet. 10 ft 3 1/2 in is awkward to add; 10.2917 ft is easy. Convert back to feet-inches at the end.
- Always double-check critical dimensions. The cost of cutting a board twice is a couple of minutes; the cost of cutting it short is a new board.
Related calculators
- Feet and Inches Calculator.
- Inches to Centimeters Converter.
- Inches to Millimeters Converter.
- Inches to Feet Converter.
- Height in Inches Calculator.
- Pounds and Ounces Calculator — same fraction idea for weight.
- Board Foot Calculator — for lumber estimating.
- All conversion calculators.
The bottom line
Decimal-to-fraction conversion is the bridge between digital tools (calipers, CAD, CNC) and analogue trade work (tape measures, hand tools, shop drawings). Pick a precision that matches your tool, accept the inevitable rounding error, and verify critical dimensions before cutting. With practice, the common fractions become second nature — and this calculator handles everything else.