Fix
My STL file is too big to upload
Published · How we check our guides
Short answer
An STL's size comes from its triangle count: 50 bytes per triangle, so a million triangles is about 50 MB. Reduce the triangle count (decimate) to a level you can't see in a print, often a tenth of the original, and the file shrinks by the same factor.
Why STL files get so big
A binary STL stores every triangle separately: a direction (12 bytes), three corners (36 bytes) and 2 spare bytes. That’s 50 bytes per triangle, plus an 84-byte header. Nothing is shared or compressed, so the size follows the triangle count exactly:
| Triangles | Binary STL | Typical source |
|---|---|---|
| 10,000 | 0.5 MB | Simple mechanical part |
| 100,000 | 5 MB | Detailed CAD export, small figure |
| 1,000,000 | 50 MB | Sculpt, 3D scan, photogrammetry |
| 5,000,000 | 250 MB | Raw scan, high-resolution sculpt |
An ASCII (text) STL of the same shape is several times bigger again. If your file opens in a text editor and starts with solid, saving it as binary alone makes it much smaller.
When it becomes a problem
- Tinkercad refuses imports over its file size limit (25 MB at the time of writing).
- Email and chat: most email services stop at 20–25 MB per message.
- Slicers open big files, but slowly, and slicing a 5-million-triangle mesh can take minutes.
- Model sites have upload limits, and huge files put downloaders off.
How many triangles does a print need?
Far fewer than most files have. A typical FDM nozzle is 0.4 mm wide and layers are 0.1–0.3 mm thick, so differences smaller than about 0.05 mm don’t show in the plastic. Sculpts and scans are usually made for screens and renders, where every tiny facet counts. Keeping 10–25% of the triangles is often invisible in a print; flat mechanical parts can go lower still, because their flat faces need hardly any triangles.
Shrink it without losing the shape
- Open Reduce STL and drop the file.
- Pick how many triangles to keep (start at 25%). It merges triangles where the surface is flat and keeps them where it curves, then tells you the largest change it made, in millimetres.
- Compare the original and the reduced model, with the triangles shown. If the deviation is under about 0.05 mm, the print won’t show it.
- Download the smaller STL.
Everything runs in your browser; a 200 MB scan isn’t uploaded anywhere.
Other ways to make it smaller
- Save as 3MF. 3MF stores each corner once and compresses the whole file. In our tests the Stanford bunny went from 3.5 MB as STL to 1.1 MB as 3MF, and a 59,000-triangle vase from 2.9 MB to 0.66 MB, with no change to the shape. Convert STL to 3MF. Tinkercad doesn’t import 3MF, though, so reduce the triangles for that.
- Zip it for email. STL compresses reasonably well, and nothing about the model changes.
- Export again with coarser settings, if you have the original CAD file: most CAD programs let you choose the export resolution (tolerance or angle).