Units: mm vs Meters for OBJ to STL

STL is a list of triangles with coordinates and a face normal. It has no unit field. The number 10 might be 10 millimeters or 10 meters. Programs fill that silence with a habit. FDM slicers almost always treat one unit as one millimeter. Modeling apps often do not. The gap between those habits is how a keychain becomes a part the size of a room, or a room-sized prop becomes a speck on the bed.

The 1000× case

A meter is 1000 millimeters. If an OBJ stores a 0.08 meter figurine as the number 0.08, and you leave the converter on millimeters, the STL still says 0.08. The slicer prints 0.08 mm. You will not find it on the plate. The inverse is the file people remember: an object modeled as “1 unit = 1 meter” but exported as the number 1, then read as 1 mm, is tiny; an object whose coordinates were already in the thousands because someone exported raw meter-scene numbers without scaling is enormous.

If the OBJ was truly in meters, set the unit menu to meters. The converter multiplies every coordinate by 1000 before it writes the STL. A 0.08 model unit becomes 80 mm. The bounding box should then read about 80 mm on the axis you expect. That box is the whole point of the control. A unit menu without a measurement is a guess you cannot see.

Centimeters are the 10× cousin. An architectural mesh in centimeters, left on millimeters, prints at one tenth of the intended size. Inches multiply by 25.4. A 2 inch part left on millimeters becomes a 2 mm part. None of these factors are stored in the STL afterward. They are baked into the coordinates. If you change your mind, convert again. Do not keep the wrong file and “remember” the scale in your head.

Worked example: the 10-unit sample cube

The built-in sample is a 10×10×10 cube (coordinates from −5 to 5 on each axis). With millimeters selected and scale 1, the bounding box reads about 10 × 10 × 10 mm. With meters selected, every coordinate is multiplied by 1000, so the same cube reads about 10000 × 10000 × 10000 mm (10 m on each edge — a 1000× linear jump). That is what the unit menu is for. Download only after the box matches the size you intend.

How to tell which unit you have

Look for a dimension you already know. A miniature base that is supposed to be 40 mm, a phone stand that is supposed to be 80 mm wide, a bracket hole that is supposed to be 5 mm. Drop the OBJ with millimeters selected and read the box. If the known edge is right, you are done. If it is about 1000× small, the file was in meters. If it is about 10× small, try centimeters. If it is about 25.4× small, try inches.

Blender’s scene unit is a clue, not a proof. People set the scene to meters and then type “40” thinking millimeters, or the reverse. The exporter writes the numbers it has. The unit menu on this site asks what those numbers mean, which is not always what the scene panel says. When they disagree, believe a known millimeter measurement.

Apply object scale in Blender before you export. A scale of 0.01 on the object can hide a unit conversion inside the object transform. If that scale is not applied, you can “fix” units here and still be off, because the vertices were never as large as the viewport implied. Apply scale, export a fresh OBJ, and convert that file.

Extra scale on the converter is a different knob. It is a multiplier after the unit conversion. Leave it at 1 when you are only translating meters into millimeters. Use 1.05 or 0.98 when you want a deliberate fit adjustment, and write that choice down. Using extra scale to mean “this file was meters” will confuse you next week. The meters option already means that.

Bed, axis, and a second scale in the slicer

Y-up versus Z-up changes which axis is height. It should not change the longest dimension of a cube, but it does change which number is Z on a figurine. Read height after you pick the up axis, not before. Sitting the mesh on the bed moves Z so the minimum is 0. It does not resize the part. If the height was wrong, the bed shift will not repair it.

Do the scale in one place. If you set meters here, do not also type 1000% in Cura. The two fixes multiply. The usual clean path is: correct unit on this page, extra scale 1, then a slicer scale of 100% unless you are adjusting for shrinkage you understand. Confirm the slicer’s reported size against the bounding box. They should match.

What still will not be in the file

After a correct scale, the STL is still unitless. Another program can misread it. The triangle count is unchanged by scale: a 12-triangle cube stays 12 triangles at 10 mm or at 10 m. Scale does not add detail and does not close holes. A 1000× size error and a broken shell are separate problems. The binary vs ASCII choice also does not change size. ASCII is not a more accurate unit system. It is the same corners written as text.

If you want the click-by-click Blender export around this check, use how to convert OBJ to STL. If the numbers look right and you are deciding whether to keep the source file, use OBJ vs STL.