Inches to mm for Laser Cutting Files: Conversion Chart and Setup Guide
One inch is exactly 25.4 mm, so 1/8 in is 3.175 mm and 1/4 in is 6.35 mm. For laser files, pick one unit, check that the SVG or DXF imports at the right size by measuring a known square, and remember that nominal plywood thickness often differs from the real one, so measure with calipers.
You download a lovely 4 by 6 inch sign, drop it into your laser software, and the preview shows a stamp-sized speck. Or worse, a monster that will not fit the bed. Welcome to the inches to mm laser cutting files problem, the quiet source of half the "my SVG is the wrong size" posts on every forum. The good news is that the maths is simple and the habits that prevent mistakes take about five minutes to learn.
American hardware stores speak inches and fractions. Most laser files and many laser machines speak millimetres. You will translate between them for as long as you cut, so let us make it painless.
The one number to remember
1 inch = 25.4 mm, exactly. Multiply inches by 25.4 for millimetres, divide millimetres by 25.4 for inches. That is the whole conversion. Everything else is bookkeeping.
| Fraction | Decimal inches | Millimetres | Where you meet it |
|---|---|---|---|
| 1/16 | 0.0625 | 1.5875 | Thin veneer, paper stock |
| 1/8 | 0.125 | 3.175 | "Three-millimetre" plywood, acrylic |
| 3/16 | 0.1875 | 4.7625 | Thicker acrylic and boards |
| 1/4 | 0.25 | 6.35 | Thicker plywood and acrylic |
| 3/8 | 0.375 | 9.525 | Thick stock, multiple passes |
| 1/2 | 0.5 | 12.7 | Large stock, rarely cut at home |
| 1 | 1.0 | 25.4 | Reference |
Notice that 1/8 inch is 3.175 mm, not 3 mm. A 0.175 mm difference is bigger than a typical laser kerf (about 0.1 to 0.25 mm), so it matters for finger joints. We cover that in how to measure kerf.
Common sheet sizes in both units
| Sheet | Inches | Millimetres |
|---|---|---|
| Letter paper | 8.5 x 11 | 215.9 x 279.4 |
| Craft sheet | 12 x 12 | 304.8 x 304.8 |
| Common laser plywood cut | 12 x 20 | 304.8 x 508 |
| Larger cut | 12 x 24 | 304.8 x 609.6 |
| Large plywood | 24 x 48 | 609.6 x 1219.2 |
| DXF World example sheet | about 11.8 x 15.7 | 300 x 400 |
Our reindeer ornament is 90 x 124 mm, which is about 3.54 x 4.88 inches, and 9 of them fit a 300 x 400 mm sheet. Cut length is 0.47 m, or about 18.5 inches. If your bed is listed in inches, convert the sheet first and then count the pieces.
Why the size comes out wrong
Files do not carry a ruler; they carry numbers and a unit hint. When the hint is missing or misread, software guesses.
| Format | How size is stored | Typical trap |
|---|---|---|
| SVG | Width and height with a unit, plus a viewBox | No unit given, so the program assumes pixels at a fixed density |
| DXF | Unitless coordinates plus an optional units variable in the header | Missing units: 1 in a file may be read as 1 mm or 1 inch |
| Points (1/72 in) | Scaled to the page when imported | |
| PNG/JPG | Pixels plus optional DPI | Lasers care about DPI for engraving |
For SVG, density is the usual troublemaker. Inkscape today treats 96 pixels as one inch, older Inkscape versions used 90, and Adobe Illustrator uses 72 points to the inch. A file saved at one density and opened at another shifts by 6.7 to 33 percent. More on that in why an SVG is the wrong size in LightBurn and how to scale files without breaking them.
For DXF, the header can hold a units code; commonly 1 means inches and 4 means millimetres. If it is missing, your software will ask or guess. See what units a DXF should use. DXF World files state millimetres, so import them as mm.
Step-by-step: bring any file in at the right size
- Decide your working unit. Many US makers design in inches and let the software show mm, or the reverse. Pick one and set it in the software's preferences.
- Import the file. Look at the unit choice in the import dialog if there is one.
- Measure a known part. If the design says 4 in wide, select it and read the width.
- Fix the scale once. If it is off by a clean factor (25.4, 2.54, 0.9375 or 1.0667), you have a unit or density mismatch; apply that factor, not a guess.
- Resave in your working unit so the next import is correct.
- Cut a test square on scrap and measure it with calipers.
Common mistake: Scaling by eye until it "looks right". You end up with a part that is 97.3 percent of the intended size, and every future file needs the same fix. Find the factor, apply it once, note it.
Pro tip: Add a 100 mm square (or 4 in square) to the corner of your template. If the square measures right on the screen, everything else will too. Delete it before cutting.
Plywood: nominal versus actual
A board sold as 1/8 in is often a little different from 3.175 mm; many "1/4 in" plywood sheets are thinner than 6.35 mm. Nominal numbers are marketing labels, not guarantees. Measure the board with calipers in three places, take the average, and use that real number in your joint design, tab sizes or box generator. See the finger joint box generator, which lets you set thickness from 3 to 10 mm and kerf from 0 to 0.4 mm.
| Thing to measure | Tool | What to do with it |
|---|---|---|
| Sheet thickness | Calipers | Enter real value in your design |
| Cut width (kerf) | Test cut + calipers | Offset or kerf compensation |
| Finished part | Calipers or ruler | Compare with the design |
Kerf in inches
If your spec sheets are in inches, a typical kerf of 0.1 to 0.25 mm is about 0.004 to 0.010 in. That is thinner than a sheet of paper in some cases, but it adds up across a ten-finger joint.
Quick mental maths
You will not always reach for a calculator, so keep a few shortcuts in your head.
| Trick | Example |
|---|---|
| 1 in is about 25 mm | 4 in is about 100 mm (exactly 101.6) |
| 4 in is about 10 cm | A 4 in coaster is about 10 cm wide |
| 12 in is about 305 mm | A 12 in sheet edge is 304.8 mm |
| 0.04 in is about 1 mm | A 1 mm gap is about 0.04 in |
These approximations are fine for planning a layout, never for a kerf or a joint. For joints, use the exact numbers.
Keep a conversion card at the bench
A small laminated card with the table above beside your laser saves a lot of fumbling. Add your own kerf, your own plywood thickness and your own favourite sheet sizes, such as 12 x 20 in for ornaments.
Next step
Open one file you already own, measure it and write the correct import factor on a note taped to your monitor. When you are ready for a real project, grab a ready-sized ornament and test your cut against the stated 90 x 124 mm.
Questions and answers
How many millimetres is 1/8 inch?
Exactly 3.175 mm. Many sellers call it 3 mm plywood, but measure your sheet with calipers.
How do I convert inches to mm quickly?
Multiply by 25.4. For example 4 in is 101.6 mm and 12 in is 304.8 mm.
Why does my SVG import at the wrong size?
Usually a unit or pixel-density mismatch, such as 72, 90 or 96 pixels per inch. Find the clean factor and apply it once.
Should I design in inches or mm?
Either, as long as you set the same unit in design software and laser software. Many laser users prefer mm for kerf and joint work.
Is 1/4 inch plywood really 6.35 mm?
Not always. Many sheets sold as 1/4 inch measure thinner. Measure the actual sheet.