Unit converter· 3 min read
Inches to Millimetres for DIY: Fractions Table and Why a 2×4 Isn't 2 × 4 Inches
Inch fractions in millimetres, feet to metres and the real sizes of 2×4, 2×6 and 4×4 lumber from US plans — every value converted exactly, ready to use.
A US plan for a bench, a shed build video or a drawing from the internet — inches, feet and fractions like 5/8" everywhere. The conversion is simple and exact: 1 inch = 25.4 mm and 1 foot = 12 inches = 304.8 mm. These values are defined exactly, not rounded.
Inch fractions in millimetres
US tape measures are divided into sixteenths of an inch, which is why plans use fractions. These are the ones you will see most:
| Inches | Millimetres |
|---|---|
| 1/16" | 1.5875 mm |
| 1/8" | 3.175 mm |
| 3/16" | 4.7625 mm |
| 1/4" | 6.35 mm |
| 5/16" | 7.9375 mm |
| 3/8" | 9.525 mm |
| 1/2" | 12.7 mm |
| 5/8" | 15.875 mm |
| 3/4" | 19.05 mm |
| 7/8" | 22.225 mm |
| 1" | 25.4 mm |
For a mixed number like 5 1/2" add the whole inches and the fraction: 5 × 25.4 + 12.7 = 139.7 mm.
Example: a cut list from a US plan
A shelving plan lists 4 shelves of 36" and 6 sides of 22 1/2". Converted: 36 × 25.4 = 914.4 mm and 22.5 × 25.4 = 571.5 mm. A size written in feet and inches, such as 12' 6 1/2", means 12 feet and 6.5 inches: 12 × 304.8 + 6.5 × 25.4 = 3,822.7 mm. You can't saw tenths of a millimetre, but don't round them until you have added up all the pieces — otherwise the errors pile up.
A 2×4 is not 2 × 4 inches
US lumber is named by its nominal size. A “2×4” (two-by-four), once dried and surfaced on all four sides, actually measures 1 1/2 × 3 1/2 inches. When a plan says 2×4, work with 38.1 × 88.9 mm.
| Name | Actual size (inches) | Actual size (mm) |
|---|---|---|
| 2×4 | 1 1/2 × 3 1/2" | 38.1 × 88.9 mm |
| 2×6 | 1 1/2 × 5 1/2" | 38.1 × 139.7 mm |
| 2×8 | 1 1/2 × 7 1/4" | 38.1 × 184.15 mm |
| 2×10 | 1 1/2 × 9 1/4" | 38.1 × 234.95 mm |
| 4×4 | 3 1/2 × 3 1/2" | 88.9 × 88.9 mm |
Outside North America timber is sold in other cross-sections. When you build a US plan from local timber, start from the actual size in millimetres and check every dimension that depends on thickness — such as the length of rails between two legs.
Feet to metres
| Feet | Millimetres |
|---|---|
| 8 ft | 2,438.4 mm |
| 10 ft | 3,048 mm |
| 12 ft | 3,657.6 mm |
Note: an “8 ft” board is shorter than 2.5 m. If a plan expects exactly four 2 ft pieces from it, a 3 m metric board leaves you a remainder — and at 2.4 m you are already 38.4 mm short.
And back: millimetres to inches
When you need a size for a US supplier or an inch tape, divide by 25.4. 18 mm plywood is 0.709", which rounds to 11/16" on a sixteenths scale. A 40 mm tube is 1.575", roughly 1 9/16". The sixteenth-rounded value is precise enough for a tape measure, but write the exact number on the drawing.
Unit converterOpen the converter with 3.5 inchesWorkshop tips
- Convert every dimension in the plan first and write it down in millimetres. Mixing inches and millimetres while cutting is a sure way to make mistakes.
- Don't round 25.4 to 25. Over 8 feet (96 inches) you would be off by 38.4 mm.
- All our calculators work in inches and feet too — type the plan's numbers in directly and switch the result to millimetres.