3D print scale calculator: the percentage for an exact size

The formula

Every mode comes down to one scale factor, and the slicer wants it as a percentage:

slicer % = scale factor × 100

To hit an exact size, with both sizes in the same unit:

scale factor = target size / current size

To move a model from scale 1:X to scale 1:Y:

scale factor = X / Y

To print a real object at 1:X:

printed size = real size / X

Uniform scaling multiplies X, Y and Z by the same factor, so the shape stays the same. Volume grows with the cube of the factor:

volume factor = scale factor³

How do you scale a 3D print to an exact size?

Divide the size you want by the size the slicer shows now, then multiply by 100. That is the percentage to type in the scale box with uniform scaling on. A model 80 mm wide that must be 100 mm wide needs 125%, and its 60 mm and 40 mm sides become 75 mm and 50 mm.

Worked example: 80 mm wide to 100 mm wide

These are the calculator’s defaults. The slicer shows the model at 80 × 60 × 40 mm.

1. Scale factor: 100 / 80 = 1.25.
2. Slicer percentage: 1.25 × 100 = 125%.
3. New size: 80 × 1.25 = 100 mm, 60 × 1.25 = 75 mm, 40 × 1.25 = 50 mm.

Reset the scale to 100% before you read the size. The percentage you type then starts from the same point as the size you read.

Worked example: a target in inches

Say the same model must stand exactly 5 in tall. One inch is exactly 25.4 mm, per the US National Institute of Standards and Technology (NIST), so 5 in is 127 mm. The current height is 40 mm.

1. Scale factor: 127 / 40 = 3.175.
2. Slicer percentage: 317.5%.
3. New size: 254 × 190.5 × 127 mm.

Set the target’s unit to inches and the calculator converts it. Enter the current sizes in the slicer’s millimeters.

Typing the size instead of the percentage

Each of the four slicers below also has size fields in millimeters. With uniform scaling on, typing 100 into the X size field gives the same result as typing 125%. The percentage is the safer number to write down, because it does not depend on which axis you measured.

Where the scale box is in each slicer

Select the model first. In all four slicers, S opens the scale tool.

Slicer	Where the percentage goes	Uniform scaling
Bambu Studio	Object Operations panel: “Scale ratios” in %, “Size” in mm	“uniform scale” checkbox
OrcaSlicer	Object operations panel: “Scale ratios” in %, “Size” in mm	“Uniform scale” checkbox
PrusaSlicer	Object manipulation panel in the right sidebar: “Scale factors” in %, “Size” in mm	Padlock button next to the fields
UltiMaker Cura	Scale tool on the left toolbar: X, Y, Z in mm and in %	“Uniform Scaling” checkbox

PrusaSlicer’s panel has an orange reset arrow that puts the scale back to 100% in one click. With uniform scaling off, each axis takes its own percentage and the model stretches out of shape.

How do you convert between model scales?

Divide the old scale’s ratio number by the new one and multiply by 100. Going from 1:72 to 1:35 is 72 / 35, or 205.71%, so the model grows to just over twice its size. Going back from 1:35 to 1:72 is 35 / 72, or 48.61%. A larger second number always means a smaller model.

Worked example: 1:72 to 1:35

Set the mode to model scale, pick 1:72 for the model now and 1:35 for the print. The model shows 80 × 60 × 40 mm at 1:72.

1. Scale factor: 72 / 35 = 2.0571.
2. Slicer percentage: 205.71%.
3. New size: 164.57 × 123.43 × 82.29 mm.
4. Volume: 2.0571³ = 8.705 times as much plastic for a solid part.

Check that the file really is at 1:72 before you trust the result. Measure one known length in the slicer, such as a wingspan or a hull length, and divide the real length by it. A file labeled 1:72 that is actually 1:70 puts every result 2.9% out.

Common conversions

From	To	Slicer percentage
1:144	1:72	200%
1:72	1:48	150%
1:72	1:35	205.71%
1:48	1:24	200%
1:35	1:12	291.67%
HO (1:87.1)	N (1:160)	54.44%
N (1:160)	HO (1:87.1)	183.7%
O (1:48)	HO (1:87.1)	55.11%
OO (1:76.2)	HO (1:87.1)	87.49%

Model railway scales

The railway presets use the ratios in Standard S-3.2 from the National Model Railroad Association (NMRA), which gives one ratio per scale.

Scale	Ratio
Z	1:220
N	1:160
TT	1:120
HO	1:87.1
OO	1:76.2
S	1:64
O (US)	1:48

A box marked 1:87 differs from 1:87.1 by about 0.1%, or 0.1 mm on a 100 mm model. If your kit states another ratio, pick “Other” and type it.

For tabletop miniatures sold as 28 mm or 32 mm, use the miniature scale calculator, which works from figure height instead of a fixed ratio.

Real-world size at a scale

Divide the real size by the ratio number. A 12 m object at 1:72 prints 12,000 / 72 = 166.67 mm long (6.562 in). A 40 ft object is 40 × 304.8 = 12,192 mm long, since the foot is exactly 304.8 mm (NIST). In HO that prints at 12,192 / 87.1 = 139.98 mm.

If the file is drawn at full size, the slicer percentage is 100 divided by the ratio: 1.389% for 1:72. For a file at any other size, enter the length the slicer shows now in the optional field. If that same 12 m length shows as 150 mm, the percentage is 166.67 / 150 = 111.11%.

Scaling changes volume and weight by the cube

Double every side and the part holds 2³ = 8 times the plastic. Halve every side and it holds one eighth. The 125% example above has 1.953 times the volume, and the 1:72 to 1:35 example has 8.705 times.

That cube applies exactly to a solid part. A sliced part scaled up weighs less than the cube suggests. Its walls and top and bottom layers keep their set thickness, so the shell grows roughly with the square of the factor and only the infill grows with the cube. The print weight calculator works out the grams for the new size with your walls and infill. Print time grows with the extra plastic too, and the print time calculator estimates the new figure.

Scaling also changes every feature on the model. A 5 mm hole at 125% becomes 6.25 mm, so screw holes, pegs and snap fits stop matching their hardware. Scaling down shrinks walls too, and a wall thinner than the slicer’s line width may not print at all.

Checking the build volume

Enter your printer’s build volume and the calculator flags any axis that ends up too long. It also gives the largest uniform scale that fits, which is the smallest of build size divided by model size across the three axes.

Worked example: 1:72 to 1:24 on a 180 mm cube

The 80 × 60 × 40 mm model at 1:72 needs 72 / 24 = 300% to reach 1:24, which makes it 240 × 180 × 120 mm. On a 180 × 180 × 180 mm build volume, X is 60 mm too long. The largest uniform scale that fits is 300% × 180 / 240 = 225%.

The check compares axis to axis as the model sits now. Rotating it on the plate, or splitting it in the slicer and gluing the parts, can fit a model the straight check rejects.