STL printing guide
STL Scale Factor Explained: Percentages, Target Sizes, and Units
Substantially updated 2026-10-31. First published 2026-09-06.
A uniform scale factor is target size divided by current size. Multiply the factor by 100 to obtain the final scale percentage. Enter 150% to make a model one and a half times its current size; 50% makes it half-size.
Calculate the factor and final percentage
Factor = target / current. Final percentage = factor × 100. Percentage change = (factor - 1) × 100. Compare lengths in the same physical unit and use a finite current length greater than zero. A 50% increase means a final 150%, not a final 50%.
Worked conversions
| Current to target | Factor | Final percentage | Change |
|---|---|---|---|
| 40 to 60 mm | 1.5 | 150% | 50% larger |
| 40 to 20 mm | 0.5 | 50% | 50% smaller |
| 508 to 20 mm | 1 / 25.4 | About 3.937007874% | Only after confirming an inch/mm error |
Keep the exact ratio for the operation; round only its displayed explanation. The last row is a correction hypothesis, not a rule for every oversized file. Establish the intended STL unit before applying it.
Scaling an already scaled copy
Calculate from the dimensions of the file you are changing. If a 40 mm model was saved at 150%, that copy is now 60 mm. To reach 50 mm from the saved copy, use 50 / 60, or about 83.333333% of its current size. Applying 125% to that copy would instead produce 75 mm; 125% is the percentage needed from the original 40 mm file. Successive factors multiply, so record which input each percentage refers to.
What uniform scaling changes
Uniform scaling changes every length by the factor, every area by its square and the volume of unchanged closed geometry by its cube. A factor of 1.5 changes 8,000 mm³ to an expected 27,000 mm³. It does not imply the same ratio for sliced material, supports or print time.
Uniform scaling changes coordinates without reducing triangle count. It also does not fill holes or remove overlapping faces. The geometric calculation assumes the same closed shape; unavailable or ambiguous enclosed volume is not a zero-volume result.
A factor calculated from overall width also changes holes, walls and mating clearances. If one dimension must change while a mounting feature stays fixed, edit the design rather than scaling the entire mesh. Separate axis factors can distort circles and are a different design operation.
Apply the operation and verify the saved copy
Use STL Scale for the operation. The saved-copy scaling guide owns the complete workflow and printable record. Neither entering a percentage nor choosing a coordinate conversion adds a unit declaration to STL. Reopen the exported bytes using the intended receiving convention.