Inch Fraction Calculator

Decimal Inches

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Convert decimal inches to fractions (1/16, 1/32, 1/64) and back. Add or subtract inch fractions instantly. Essential for woodworking, construction, and precision measurement.

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Sources & Methodology

✓ Formulas verified against authoritative sources listed below.

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NIST Handbook 44 — Specifications and Tolerances for Measuring Devices

NIST reference for measurement precision, tolerances, and fractional inch specifications

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ASME Y14.5 — Dimensioning and Tolerancing

ASME standard for engineering drawings using fractional inch dimensions

Methodology: Fractional inch = round(decimal x denominator) / denominator. Simplified by dividing numerator and denominator by their GCD. Whole number extracted from integer part of decimal. Metric: 1 inch = 25.4 mm exactly (NIST). Common denominators: 2, 4, 8, 16, 32, 64.

⏱ Last reviewed: April 2026

How to Convert Decimal Inches to Fractions

Working with fractional inches is essential in woodworking, construction, and mechanical trades where measurements are given in fractions like 3/8, 5/16, or 7/32 of an inch. Converting between decimal and fractional inches requires knowing the target precision (1/16, 1/32, etc.) and applying a simple rounding formula. This calculator handles the conversion, simplification, and metric equivalent instantly.

The Decimal to Fraction Conversion Formula

To convert decimal inches to a fraction: multiply the decimal portion by the denominator (e.g., 16 for sixteenths), round to the nearest whole number for the numerator, then simplify by dividing both by the greatest common divisor (GCD). Example: 0.375 inches to sixteenths: 0.375 x 16 = 6. Numerator = 6, denominator = 16. GCD(6,16) = 2. Simplified: 3/8 inch.

Common Fractional Inch Equivalents

The most common fractions used in construction and woodworking: 1/2 = 0.5000 in, 1/4 = 0.2500 in, 3/4 = 0.7500 in, 1/8 = 0.1250 in, 3/8 = 0.3750 in, 5/8 = 0.6250 in, 7/8 = 0.8750 in, 1/16 = 0.0625 in, 3/16 = 0.1875 in. Memorizing these anchor values speeds up mental conversion.

Precision Selection — When to Use 1/32 vs 1/64

General carpentry and rough framing: 1/8 or 1/16 inch precision is sufficient. Finish carpentry and joinery: 1/32 inch. Precision woodworking, lutherie, and machining: 1/64 inch or decimal. Architectural drawings use 1/16 inch as the standard minimum. Choose precision based on the application tolerance — over-specifying precision does not improve accuracy.

Fractional Inches vs. Metric

The United States uses fractional inches for most construction and woodworking. Metric (mm) is standard globally and in scientific/engineering contexts. 1 inch = 25.4 mm exactly (NIST). A 3/8 inch gap = 9.525 mm. Converting to mm by multiplying by 25.4 gives the exact metric equivalent without rounding.

Numerator = round(decimal x denominator) | Simplified = numerator/GCD over denominator/GCD

GCD = Greatest Common Divisor. Example: 0.625 to 1/8 precision: 0.625 x 8 = 5.0. Numerator = 5, denominator = 8. GCD(5,8) = 1. Result = 5/8 inch. Metric: decimal inches x 25.4 = millimeters.

Decimal to Fraction Inch Quick Reference

Decimal (in)	1/16 Fraction	1/32 Fraction	Millimeters
0.0625	1/16	2/32 (1/16)	1.588 mm
0.125	2/16 (1/8)	4/32 (1/8)	3.175 mm
0.1875	3/16	6/32 (3/16)	4.763 mm
0.250	4/16 (1/4)	8/32 (1/4)	6.350 mm
0.3125	5/16	10/32 (5/16)	7.938 mm
0.375	6/16 (3/8)	12/32 (3/8)	9.525 mm
0.4375	7/16	14/32 (7/16)	11.113 mm
0.500	8/16 (1/2)	16/32 (1/2)	12.700 mm
0.625	10/16 (5/8)	20/32 (5/8)	15.875 mm
0.750	12/16 (3/4)	24/32 (3/4)	19.050 mm
0.875	14/16 (7/8)	28/32 (7/8)	22.225 mm

💡 Woodworking Tip: When measuring, always read from the end of the tape, not from the hook. A 1/16 inch error compounded over multiple cuts becomes significant. Mark measurements with a sharp pencil point or marking knife rather than the wide side of the pencil. For critical joinery, transfer measurements directly rather than reading and re-marking.

Frequently Asked Questions

Multiply the decimal portion by the denominator you want (e.g., 16 for sixteenths). Round to the nearest whole number for the numerator. Simplify by dividing both by the GCD. Example: 0.375 to 16ths: 0.375 x 16 = 6. Result = 6/16 = 3/8 inch.

0.5 inches = 1/2 inch. Calculation: 0.5 x 2 = 1. Numerator = 1, denominator = 2. Result = 1/2 inch. In metric: 0.5 x 25.4 = 12.7 mm.

0.375 inches = 3/8 inch. Calculation: 0.375 x 8 = 3. Numerator = 3, denominator = 8. GCD(3,8) = 1. Result = 3/8 inch. In metric: 9.525 mm.

0.25 inches = 1/4 inch. 0.25 x 4 = 1. Numerator = 1, denominator = 4. Result = 1/4 inch = 6.35 mm.

Divide the numerator by the denominator: 3/8 = 3 divided by 8 = 0.375 inches. 5/16 = 5 divided by 16 = 0.3125 inches. 7/32 = 7 divided by 32 = 0.21875 inches.

1/16 = 0.0625 inches = 1.5875 mm. This is the minimum precision on most standard tape measures (the smallest graduation on a common US tape is 1/16 inch).

There are 16 sixteenths (16/16) in one inch. A tape measure with 1/16 inch markings shows 16 marks per inch.

3/4 inch = 0.75 inches = 19.05 mm. It is one of the most common fractional dimensions in construction (3/4 inch plywood, 3/4 inch pipe fittings).

Find a common denominator, add numerators, simplify. Example: 1/4 + 3/8 = 2/8 + 3/8 = 5/8 inch. Or convert to decimal: 0.25 + 0.375 = 0.625 = 5/8 inch. Decimal addition is often faster for complex calculations.

5/8 inch = 0.625 inches x 25.4 = 15.875 mm. This is exactly 15.875 mm — a common dimension in North American construction where inch fractions map to precise metric values.

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