Choose percentage, target size or coordinate conversion
To change a 20 mm width to 30 mm, use 30 / 20 = 1.5, or 150%. Target dimension applies the same factor to every axis: a 20 mm cube becomes 30 x 30 x 30 mm. Its enclosed volume grows by 1.5³ = 3.375, from 8,000 to 27,000 mm³ if it remains valid.
STL does not declare units. Interpreting 20 coordinates as inches describes 508 mm; it does not add unit metadata. This tool's conversion rescales coordinates for the chosen target convention. Confirm what the receiving slicer assumes before exporting.
Keep the original. Download the changed STL, reopen it in the dimensions checker and confirm the intended physical size. Follow the practical scaling workflow for that saved-copy check. Scaling also changes hole sizes, wall thickness and mating clearances; it does not repair topology. ASCII/binary files up to 25 MiB are processed locally.
Try the known 20 mm cube. This is original example geometry, not a print test.