Imperial and metric measurements collide constantly in the workshop. This is especially true in guitar work and luthiery, where headstock specs, fretboard radii, and nut widths are still frequently quoted in inches and "thou" (thousandths of an inch).
Understanding the Terminology
Inch Notation
Inches are typically written in three ways:
-
Fractions (X-Y/Z"): e.g.,
(where X is the whole inch, and Y/Z is the fraction).2 83” -
Decimals (0.0000"): High-precision measurements.
-
Thou (.000): Thousandths of an inch. The leading zero is usually omitted (e.g., .500 instead of 0.500).
Example (Half an Inch):
-
Fraction: 1/2"
-
Decimal: 0.5"
-
Thou: .500
Metric Notation
Millimetres and other metric units are written as decimals. Note the regional difference in decimal separators:
-
Europe: Uses a comma (e.g., 3,5mm)
-
Everywhere else: Uses a point (e.g., 3.5mm)
Quick Conversions
Inches to Millimetres (mm)
Formula: Inches x 25.4 = Millimetres
Example: 1.5" x 25.4 = 38.1mm
For Fractions: Convert the fraction to a decimal first. For
,2 83”.3/8=0.375
2.375" x 25.4 = 60.325mm
Inches to Centimetres (cm)
Formula: Inches x 2.54 = Centimetres
Example: 15" x 2.54 = 38.1cm
Millimetres (mm) to Inches
Formula: Millimetres ÷ 25.4 = Inches
(Or: Millimetres x 0.03937 = Inches)
Example: 15mm ÷ 25.4 = 0.59" (to 2 decimal places)
Conversion Tables
1. Inch Fractions to Metric (mm/cm)
To keep this readable on mobile, the fraction columns are consolidated, and metric values are focused on millimetres (mm).
| Fraction | Decimal | Metric (mm) |
| 1/32" | 0.03125" | 0.79 mm |
| 1/16" | 0.06250" | 1.59 mm |
| 3/32" | 0.09375" | 2.38 mm |
| 1/8" | 0.12500" | 3.18 mm |
| 5/32" | 0.15625" | 3.97 mm |
| 3/16" | 0.18750" | 4.76 mm |
| 7/32" | 0.21875" | 5.56 mm |
| 1/4" | 0.25000" | 6.35 mm |
| 9/32" | 0.28125" | 7.14 mm |
| 5/16" | 0.31250" | 7.94 mm |
| 11/32" | 0.34375" | 8.73 mm |
| 3/8" | 0.37500" | 9.53 mm |
| 13/32" | 0.40625" | 10.32 mm |
| 7/16" | 0.43750" | 11.11 mm |
| 15/32" | 0.46875" | 11.91 mm |
| 1/2" | 0.50000" | 12.70 mm |
| 17/32" | 0.53125" | 13.49 mm |
| 9/16" | 0.56250" | 14.29 mm |
| 19/32" | 0.59375" | 15.08 mm |
| 5/8" | 0.62500" | 15.88 mm |
| 21/32" | 0.65625" | 16.67 mm |
| 11/16" | 0.68750" | 17.46 mm |
| 23/32" | 0.71875" | 18.26 mm |
| 3/4" | 0.75000" | 19.05 mm |
| 25/32" | 0.78125" | 19.84 mm |
| 13/16" | 0.81250" | 20.64 mm |
| 27/32" | 0.84375" | 21.43 mm |
| 7/8" | 0.87500" | 22.23 mm |
| 29/32" | 0.90625" | 23.02 mm |
| 15/16" | 0.93750" | 23.81 mm |
| 31/32" | 0.96875" | 24.61 mm |
| 1" | 1.00000" | 25.40 mm |
2. Thousandths of an Inch (Thou) to Metric
Used for fine tolerances, material thicknesses, and tool setup.
| Thou | Pronounced As | Metric (mm) |
| .001 | One thou | 0.03 mm |
| .002 | Two thou | 0.05 mm |
| .005 | Five thou | 0.13 mm |
| .010 | Ten thou | 0.25 mm |
| .015 | Fifteen thou | 0.38 mm |
| .020 | Twenty thou | 0.51 mm |
| .025 | Twenty-five thou | 0.64 mm |
| .030 | Thirty thou | 0.76 mm |
| .040 | Forty thou | 1.02 mm |
| .050 | Fifty thou | 1.27 mm |
| .060 | Sixty thou | 1.52 mm |
| .070 | Seventy thou | 1.78 mm |
| .080 | Eighty thou | 2.03 mm |
| .090 | Ninety thou | 2.29 mm |
| .100 | One hundred thou | 2.54 mm |
| .150 | One hundred fifty thou | 3.81 mm |
| .200 | Two hundred thou | 5.08 mm |
| .250 | Two hundred fifty thou | 6.35 mm |
| .500 | Five hundred thou | 12.70 mm |
| .750 | Seven hundred fifty | 19.05 mm |
3. Whole Inches to Metric
Commonly used when ordering raw materials like acrylic or plywood sheets.
| Inches (Feet) | Metric (mm) | Metric (m) |
| 1" | 25.4 mm | 0.025 m |
| 2" | 50.8 mm | 0.051 m |
| 3" | 76.2 mm | 0.076 m |
| 4" | 101.6 mm | 0.102 m |
| 6" (0.5') | 152.4 mm | 0.152 m |
| 12" (1') | 304.8 mm | 0.305 m |
| 24" (2') | 609.6 mm | 0.610 m |
| 36" (3') | 914.4 mm | 0.914 m |
| 39.37" | 1000.0 mm | 1.000 m |
| 48" (4') | 1219.2 mm | 1.219 m |
| 72" (6') | 1828.8 mm | 1.829 m |
| 96" (8') | 2438.4 mm | 2.438 m |
4. Metric (mm) to Imperial (Inches)
Use this table when converting European metric measurements back to standard imperial workshop sizes.
| Metric (mm) | Imperial Fraction | Decimal / Thou |
| 0.79 mm | 1/32" | .031 |
| 1.00 mm | - | .039 |
| 1.59 mm | 1/16" | .063 |
| 2.00 mm | - | .079 |
| 3.18 mm | 1/8" | .125 |
| 4.00 mm | - | .157 |
| 5.00 mm | - | .197 |
| 6.35 mm | 1/4" | .250 |
| 8.00 mm | - | .315 |
| 10.00 mm | - | .394 |
| 12.70 mm | 1/2" | .500 |
| 15.00 mm | - | .591 |
| 20.00 mm | - | .787 |
| 25.40 mm | 1" | 1.000 |
| 50.80 mm | 2" | 2.000 |
| 100.00 mm | - | 3.937 |
