CURRENT SIZE → DESIRED SIZE
STL scale calculator.
The percentage you need.
Find the uniform percentage to apply to your current model. Compare a known measurement with its intended size, use mm, cm or inches, and preview the resulting overall dimensions.
CHECK THE RATIO
Calculate your STL scale percentage
Use the same known feature for both measurements. Choose each measurement's unit; the calculator converts them to a common unit before dividing.
These examples compare the current and intended sizes in the same unit. A unit mismatch is a possible cause; confirm a reference measurement before applying a correction.
- Linear scale factor
- 0.1×
- Scale percentage relative to current size
- 10%
80 mm ÷ 800 mm = 0.1. Multiply by 100 for the percentage. A scale of 100% leaves the measured size unchanged.
The result is relative to the geometry you just measured. Apply it once with that geometry treated as 100%. If your slicer already has a scale applied, use its exact-size fields or establish a new 100% baseline first; replacing an existing percentage with this result can give the wrong size.
This calculator does not open, edit, rescale, or export an STL file. Uniform scaling changes every linear dimension, including holes and spacing. If only one feature should change, use a local design edit instead.
Results are rounded for display and copying. Keep the original, apply the confirmed factor in your modelling software, then reopen the export and verify the reference measurement and overall size.
A KNOWN MEASUREMENT FIRST
How to calculate an STL scaling percentage
Measure the current size
Choose one known feature or overall dimension and record its current size in your modelling software. Use the same feature for the intended measurement.
Inspect an STL's overall dimensions →Calculate the ratio
Enter the current and desired measurements with their units. Divide desired length by current length after converting both to the same unit, then multiply by 100.
Apply and verify
Apply that factor uniformly to the measured geometry. Treat that geometry as a fresh 100% baseline, or use your software's exact-size fields. Check another dimension before exporting.
Diagnose an STL unit mismatch →THE FORMULA
Desired ÷ current × 100.
For a model that measures 800 mm but should measure 80 mm, the linear factor is 80 ÷ 800 = 0.1. The relative scale is 10%. A 20 × 30 × 40 mm bounding box becomes 2 × 3 × 4 mm at that factor.
Different unit labels do not necessarily mean different physical lengths. A current measurement of 1 inch and a desired measurement of 25.4 mm are equal, so the factor is 1 and the percentage is 100%.
The percentage describes a change from the measured geometry. It is not automatically the final absolute percentage in a slicer whose model is already scaled.
WHEN IT HELPS
Catch a size mismatch.
A model that is exactly 25.4 times too small needs a factor of 25.4, or 2540%. A model exactly 10 times too big needs a factor of 0.1, or 10%. Confirm the mismatch against a known dimension rather than choosing a preset from appearance alone.
STL has no reliable stored unit. Software may interpret the same coordinates differently. Selecting the correct import unit can address the cause; scaling is an alternative when you have established the current and intended sizes.
Try a 20 mm test cube with known dimensions →BEFORE YOU RESIZE
STL scaling questions
The calculator supplies a uniform size ratio. It does not modify geometry or determine whether the part will fit.
Can this calculator resize or download my STL?
No. Enter measurements to calculate a factor and percentage. No file is opened or changed, and there is no STL export. Apply the result in your modelling software, then reopen and check the export.
Does 10% mean reduce the size by 10%?
No. Scaling to 10% makes each dimension one tenth of its measured current size. Reducing the size by 10% means scaling to 90%. A 100% scale leaves the size unchanged.
What if my slicer already shows 150%?
This result is relative to the size you entered, not necessarily the slicer's original import. Replacing 150% with the calculated relative percentage can give a different result. Use exact target dimensions, or establish the measured geometry as a new 100% baseline before applying the factor.
Will uniform scaling change hole diameters and clearances?
Yes. It changes every linear dimension, including the outside envelope, hole diameters and hole spacing. If only a hole or another local feature needs changing, use a specific design edit instead of uniformly scaling the whole part.
Change an STL hole size without scaling everything →Are the optional X, Y and Z values measured automatically?
No. You enter the current overall bounds in one unit. The preview multiplies each bound by the same factor and keeps that unit. Bounds depend on the model's orientation and do not measure wall thickness or individual features.
CONFIRM THE SOURCE SIZE