Why Is My SVG the Wrong Size in LightBurn?
An SVG opens at the wrong size in LightBurn mostly because the file has no real units and its pixels are converted at a different DPI than the program that made it. Illustrator uses 72 DPI, older Inkscape used 90, and 96 is the modern web standard, so the error is typically 75%, 93.75% or 133.3%.
You designed a 100 mm coaster, imported it, and LightBurn says it is 75 mm. Or 133 mm. The design looks perfect, the number is wrong, and you have a sheet of plywood waiting. Do not guess and do not scale by eye: the size is wrong for a reason, and the reason is usually a number you can calculate.
An SVG file is a drawing described in units. If the file says "width=90mm", every program agrees on the size. If it just says "width=340" with no unit, each program decides how many of those units make an inch, and that decision is the DPI (dots, or pixels, per inch).
The three DPI values that cause nearly every size error
| Created in | Pixels per inch | Appears in a 96 DPI program as | Fix by scaling |
|---|---|---|---|
| Illustrator (points) | 72 | 133.3% (too big) | 75% |
| Older Inkscape (0.91 and earlier) | 90 | 106.7% (too big) | 93.75% |
| Modern Inkscape, browsers, most tools | 96 | Correct | none |
The reverse applies if your laser program assumes 72 or 90 and the file was drawn at 96: the part appears too small. The maths is simple: correct size = shown size × (DPI the file used ÷ DPI the program assumes).
Other causes of wrong size
| Cause | Typical error | How to spot it | Fix |
|---|---|---|---|
| No width and height, only viewBox | Random size | Size shown in px, not mm | Add width="90mm" height="124mm" in the file or scale |
| Document set to px at the wrong scale | Off by 3.78 (px to mm) | Shape is about 3.78 times too large or small | Set document units to mm |
| Transform or scale applied to a group | Mismatch with what you drew | Group has odd width | Select all, read the real size, rescale |
| DXF imported in the wrong units | 25.4 times off | Size is 25.4 times too big or small | See what units a DXF should use |
| Rounded or cropped artboard | Small errors of 1 to 5 mm | Includes whitespace | Crop the artboard to the artwork |
How to fix it, step by step
- Draw a 50 mm square in your design before exporting.
- Import the SVG into LightBurn and select the square.
- Read its width in the Size box. Say it reads 66.7 mm.
- Scale factor = 50 ÷ 66.7 = 0.75. Select everything, set the width of the group to the correct number.
- Delete the square. Better still, fix the export so you do not need to repeat this each time: set the document to mm in Inkscape, or in Illustrator export with a scaled artboard that reads in mm.
Some LightBurn versions also have a preference for the default SVG import DPI under the application settings; if yours does, set it to match the program you design in. Check your version, because the option names move between releases.
Pro tip: when a customer sends an SVG, ask for its intended size. Then check it against the number in LightBurn before you do anything else.
Worked example
Our reindeer ornament is 90 × 124 mm. If you open an Illustrator export of it in a program that assumes 96 DPI, it reads 120 × 165.3 mm (a 133.3% error). Divide by 1.333, or multiply by 0.75, and you are back to 90 × 124 mm. A 4 mm hanging hole would otherwise show as 5.3 mm, and the cut time on a 40 W CO2 would jump from 44 s to about 59 s, which is a quick way to notice something is off.
Common mistake: fixing the size by dragging the corner handle until it "looks right". The file ends up 98 or 102 mm, nothing fits together, and joints do not close. Always type the exact value.
Quick checklist before every job
- Does the Size field match the dimension you intended, to within 0.1 mm?
- Is a known feature (the 50 mm square, a 4 mm hole) the expected size?
- Does the cut length shown by the software match the file's data? For the reindeer, 0.47 m.
If the file came from our library, sizes are real millimetres. If it still looks wrong, see how to scale laser files correctly and how to prepare an SVG.
Questions and answers
Why does LightBurn show my SVG at 75% size?
The file was likely made at 96 DPI but read at 72, or the reverse. Scaling by 133.3% or 75% usually fixes it. Check with a known square to be sure.
What DPI does Inkscape use?
Modern Inkscape (0.92 and later) uses 96 pixels per inch for user units. Versions up to 0.91 used 90.
What DPI does Illustrator use?
Illustrator works in points at 72 per inch, so SVGs from it often open about 133.3% too large in programs that assume 96.
How can I stop SVG size problems for good?
Set the document units to millimetres, draw at real size, export with explicit width and height in mm, and keep a 50 mm test square to check on import.
Is a DXF the wrong size for the same reason?
Not exactly. DXF problems come from unit flags such as millimetres versus inches, which gives a 25.4 times error, not DPI.