# Inch to Decimal Calculator

Inch to decimal calculator. Convert fractional inches like 3/8, 5/16, or 1-1/2 into decimal inches, thousandths, and millimeters with a built-in reference chart.

## What this calculates

Convert fractional inch measurements into decimal form for CAD, machining, and digital calipers. Enter a whole inch value, a numerator, and pick a denominator, and the calculator returns the exact decimal, thousandths, and millimeters.

## Inputs

- **Whole Inches** (in) — min 0 — Whole inch part, e.g., the 1 in 1-5/16". Use 0 if there is no whole part.
- **Fraction Numerator** — min 0 — Top of the fraction, e.g., the 5 in 5/16.
- **Fraction Denominator** — options: Halves (/2), Quarters (/4), Eighths (/8), Sixteenths (/16), Thirty-seconds (/32), Sixty-fourths (/64), Hundredths (/100), Thousandths (/1000) — Bottom of the fraction. 1/16 and 1/32 are standard tape-measure and machining precisions.

## Outputs

- **Decimal Inches** (in) — Fractional value as a decimal (e.g., 3/8 = 0.375).
- **Thousandths** (thou) — Decimal inches × 1000 (e.g., 0.375 in = 375 thou). Common in machining.
- **Millimeters** (mm) — Equivalent length in millimeters (1 in = 25.4 mm).
- **Expression** — formatted as text — Pretty-printed version of the fraction you entered.

## Details

Tape measures mark inches in fractions (1/2, 1/4, 1/8, 1/16, 1/32), but CNC controllers, CAD software, and digital calipers want decimal inches. The inch to decimal calculator handles this conversion exactly, with no rounding unless you choose a coarse denominator.

## Formula

For a value written as W and n/d (whole inches + fraction), the decimal is:

**decimal = W + (n ÷ d)**

So 1-5/16" = 1 + (5 ÷ 16) = 1.3125". No extra steps.

## Standard fraction to decimal reference

This chart covers the standard tape-measure and machinist fractions. Every value is exact (not rounded).

| Fraction | Decimal Inches | Thousandths | Millimeters |
|----------|----------------|-------------|-------------|
| 1/64 | 0.015625 | 15.625 | 0.3969 |
| 1/32 | 0.03125 | 31.25 | 0.7938 |
| 1/16 | 0.0625 | 62.5 | 1.5875 |
| 3/32 | 0.09375 | 93.75 | 2.3813 |
| 1/8 | 0.125 | 125 | 3.175 |
| 5/32 | 0.15625 | 156.25 | 3.9688 |
| 3/16 | 0.1875 | 187.5 | 4.7625 |
| 7/32 | 0.21875 | 218.75 | 5.5563 |
| 1/4 | 0.25 | 250 | 6.35 |
| 5/16 | 0.3125 | 312.5 | 7.9375 |
| 3/8 | 0.375 | 375 | 9.525 |
| 7/16 | 0.4375 | 437.5 | 11.1125 |
| 1/2 | 0.5 | 500 | 12.7 |
| 9/16 | 0.5625 | 562.5 | 14.2875 |
| 5/8 | 0.625 | 625 | 15.875 |
| 11/16 | 0.6875 | 687.5 | 17.4625 |
| 3/4 | 0.75 | 750 | 19.05 |
| 13/16 | 0.8125 | 812.5 | 20.6375 |
| 7/8 | 0.875 | 875 | 22.225 |
| 15/16 | 0.9375 | 937.5 | 23.8125 |
| 1 | 1.0000 | 1000 | 25.4 |

## Why decimal inches matter

Machinists and engineers work almost entirely in decimal because it lets them add, subtract, and multiply without converting between denominators. A bill of materials listing 0.250, 0.375, and 0.125 is immediately addable (total = 0.750). The same list in fractions (1/4, 3/8, 1/8) requires finding a common denominator first.

Thousandths ("thou" or "mil") are the standard shop unit for tolerance specs. A tolerance of ±0.005" means ±5 thousandths, or about ±0.127 mm. The calculator outputs thousandths directly so you can read values off like a caliper.

## Converting back: decimal to fraction

If you already have a decimal and want the nearest fraction, this calculator pairs well with our inches to fraction tool, which takes a decimal like 0.392 and returns the closest 1/16 or 1/32 inch.

## Frequently Asked Questions

**Q: How do I convert a fraction of an inch to a decimal?**

A: Divide the numerator by the denominator. 3/8 becomes 3 ÷ 8 = 0.375. For a mixed number like 1-5/16, do 5 ÷ 16 = 0.3125 first, then add the whole part: 1 + 0.3125 = 1.3125 inches.

**Q: What is 3/8 of an inch as a decimal?**

A: 3/8 of an inch equals 0.375 inches (or 375 thousandths, or 9.525 mm). This is one of the most common tape-measure fractions and shows up on drill bits, bolts, and plumbing fittings.

**Q: What is 5/16 as a decimal inch?**

A: 5/16 inch equals 0.3125 inches. As thousandths that is 312.5 thou, and as metric it is 7.9375 mm. 5/16 is a standard drill bit and wrench size.

**Q: How do I enter a mixed number like 1-1/2 in the calculator?**

A: Put 1 in the whole inches field, 1 in the numerator field, and choose 2 (halves) as the denominator. The result is 1.5 decimal inches, which is 1500 thousandths or 38.1 mm.

**Q: What is the difference between decimal inches and thousandths?**

A: They represent the same length, just with a different decimal place. 0.375 decimal inches and 375 thousandths are identical. Machinists use thousandths because tolerances and feed rates are easier to read as whole numbers (±5 thou is cleaner than ±0.005 in).

**Q: How accurate is the inch to decimal conversion?**

A: It is exact for any fraction you enter, because simple division does not introduce error. The only potential rounding is in the displayed result, which this calculator shows to six decimal places (plenty for any shop work).

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Source: https://vastcalc.com/calculators/conversion/inch-decimal
Category: Conversion
Last updated: 2026-04-08