Scale STL to an Exact Size and Measure It (Worked Example)

The bracket you downloaded is 64 mm long, but the gap on your shelf is 80 mm. The miniature is lovely at 28 mm and you want it at 75 mm for a desk display. Or you just need to know whether the two mounting holes are really 50 mm apart before you drill the wall. Learning to scale an STL to an exact size, and to measure it properly afterward, turns "close enough" into "fits first time".

Two ways to scale: percent or target size

Both give the same result when used uniformly. Target size just saves you the arithmetic: the percent is target / current x 100.

Worked example: a bracket from 64 mm to 80 mm

The bracket measures 64 x 22 x 30 mm. You want it 80 mm long (X).

• Scale factor: 80 / 64 = 1.25, so 125%.
• New size: 80 x 27.5 x 37.5 mm.
• Its two 4.5 mm screw holes become 5.6 mm, and their 40 mm spacing becomes 50 mm.
• Its 3 mm thick wall becomes 3.75 mm.
• Volume grows by 1.25³ = 1.95x. A 12 g print becomes about 23 g. Filament grows by roughly the same factor, a little less because the shell share shrinks.

Those knock-on effects are the reason to measure after scaling. If the bracket must use M4 screws on a 40 mm rail spacing, uniform scaling broke it; you need the original file edited in CAD instead, or a different design.

What scaling does and doesn't do

• Everything scales: holes, text, clearances, wall thickness, snap-fit gaps.
• Tolerances don't stay put: a 0.2 mm print-in-place clearance becomes 0.1 mm at 50% and may fuse.
• Thin features may disappear when you scale down: a 0.8 mm fin at 40% is 0.32 mm, below what a 0.4 mm nozzle can print.
• Threads won't match standard screws after scaling, except at 100%.

When non-uniform scaling makes sense

Scaling one axis only, for example making a box taller without making it wider, distorts circles into ovals and changes wall thickness differently on each side. It is fine for simple boxes and spacers, risky for anything with holes. If you need a different proportion, edit the source design when you can.

Measuring the result

The info panel's width x depth x height is the bounding box: the overall size. It doesn't tell you the distance between two holes or the thickness of one tab. For that you measure point to point.

In STL Viewer, Measure (Premium) works like this:

1. Open the model and tap Measure in the tools.
2. "Tap a point on the model": tap the first feature, for example the edge of a hole.
3. "Tap a second point": tap the matching feature.
4. Read the Distance and the ΔX, ΔY and ΔZ differences. For two holes on the same rail, ΔX should carry the spacing and ΔY, ΔZ should be near zero.
5. Tap Clear to measure something else.

Pinch in before tapping; a point on a curved hole edge is easier to place accurately when zoomed. Without Premium you can watch a short ad to take one measurement. On very large models shown as a simplified view, the app notes that the measurement is approximate. The measure guide has screenshots.

Checking hole spacing in practice

To measure center-to-center spacing, measure from the left edge of one hole to the left edge of the other: the distance is the same as center to center, and edges are easier to tap than an invisible center. In the bracket example, after scaling to 125%, you should read 50.0 mm, and ΔY close to 0.

Scaling and exporting on the phone

Premium's Scale & export lets you choose By percent or To a size, pick the side (axis) for a target size, and choose the units in the exported file; it reminds you that most slicers expect millimeters. It shows the exported size before you save, then writes a new binary STL from the full model (even if the screen shows a simplified view), which you can share straight to your PC or slicer. It doesn't overwrite the original. See the scale and export guide.

It also converts inch models to millimeters in one step; if that's your problem, read the inches vs millimeters guide first.

Comparing the original and the scaled copy

With Compare (also Premium), open the scaled STL next to the original: dimensions, volume and triangles are listed side by side, so you can confirm 64 → 80 mm and see the volume nearly double. For a filament check on the new size, the free estimate works on the exported file; see how much filament a print will use.

Measure, Section, Scale & export and Compare come with Premium, available as a one-time purchase or a yearly subscription. Get STL Viewer on Google Play to try the free features first.

What each Premium tool does is summarized on the STL Viewer website.