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Compare STL Files: See Every Difference Between Two 3D Models

Drop the original model and the changed one. The page lines them up automatically, measures the distance from every point of one surface to the other, and colours it: green where they match within your tolerance, red where material was added, blue where it was removed. You get the biggest differences, mean, RMS and a report.

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How to compare two STL files

  1. 1

    Drop both models

    The reference (the original, the CAD file or the version you trust) and the compared model (the new version, the scan or the remesh). STL, OBJ, 3MF or GLB. Nothing is uploaded.

  2. 2

    Let them line up

    Best fit turns and moves the compared model until its surface sits on the reference as closely as it can. Pick Centres only or As saved when the files already share a position.

  3. 3

    Read the heatmap

    Green is within your tolerance. Yellow to red means the compared model has extra material there, cyan to dark blue means it is missing material. Press Focus on a spot to fly to it.

  4. 4

    Save the result

    Download a PNG report with the numbers, a coloured GLB, the compared model moved onto the reference as an STL, or a CSV of every point and its distance.

What the colours mean

Every point is coloured by its distance to the other model. The sign tells you which way: the compared model either sticks out past the reference or lies inside it.

GreenThe two surfaces are closer than your tolerance. For most uses, this part is the same in both files.
Yellow, orange, redThe compared model sticks out past the reference: material was added, a feature is bigger, or the part is longer there. Red is the most.
Cyan, blue, dark blueThe compared model lies inside the reference: material was removed, a hole is bigger, or a feature is shorter. Dark blue is the most.
The scaleThe strongest colour is reached at the larger of twice your tolerance and the distance that 98% of the surface stays under, so one stray point does not wash out the rest. The exact numbers are in the panel.

What the numbers mean

Distances are measured both ways, from the compared model to the reference and back, so a removed feature shows up as well as an added one. Shares are weighted by surface area, not by the number of triangles.

Max over / max underThe largest distance where the compared model has extra material, and the largest where it is missing material.
Mean and mean distanceMean keeps the sign, so a part that grew everywhere shows a positive mean. Mean distance ignores the sign and says how far apart the surfaces are on average.
RMSThe root mean square of the distance, a common single number for "how different" in inspection reports. Big local changes raise it more than the mean.
Within / over / under toleranceThe share of the surface area that is within your tolerance, sticks out past it, or lies inside it.
Largest gap (Hausdorff)The largest distance from any point of either model to the other model. It is the strictest single measure of how far apart two shapes are.
Fit errorHow closely the best-fit alignment matched the surfaces, as an RMS distance. A large value next to small changes means the models may be too different for the automatic fit.

Found the difference? Fix it or make a new version

Close holes in a scan before you compare it, rescale a part that came out in the wrong unit, or turn a photo into a new 3D model with AI and check it against your design. The AI generator uses credits; this comparer does not.

Image to 3D with AI →Repair an STLFix the scale

More free 3D printing tools

Wall thickness checkerSTL repairScale STLSTL editorSTL viewer

Compare STL files FAQ

Drop the original STL in the Reference slot and the new one in the Compared slot. The page aligns them with a best fit, measures the distance between the two surfaces at every vertex and shows a colour heatmap with the numbers: the largest changes, the mean, the RMS and how much of the surface is within your tolerance.
It first matches the main directions and centres of the two shapes, trying the possible flips, then refines the best start with an iterative closest point fit: it repeatedly pairs points on the compared surface with the nearest points on the reference and solves for the rotation and shift that bring them closest. It does not scale or mirror the model, so a part that was scaled will show that as a difference.
The one you trust: the CAD export, the original design or the earlier version. Distances are measured from the compared model to the reference, and red and blue are named from the compared model's point of view (extra or missing material). The Swap button exchanges the two.
Yes. Scan the printed part, drop the design file as the reference and the scan as the compared model, and set the tolerance your part needs. The heatmap shows where the print warped, shrank or has extra material. A scan with holes still works; the holes are simply not measured.
Use the accuracy you care about. Around 0.1 mm suits resin prints and machined parts, 0.2 to 0.5 mm suits FDM prints and most scans, and larger values suit rough remeshes or game assets. Changing the tolerance recolours the model and updates the numbers at once without measuring again.
STL and OBJ files do not store units. The page assumes millimetres; change Model units and every number is shown in the right unit. Both files must use the same unit. If one model is ten or a thousand times bigger than the other, one of them was exported in a different unit: scale it first, for example with the free Scale STL tool.
STL (binary and ASCII), OBJ, 3MF and GLB up to 100 MB and 1.5 million triangles each. Several parts in one file are compared together. Coarse models are split into smaller faces first so the heatmap shows detail. The models do not need to have the same triangles or the same number of triangles.
No. Both files are read, aligned and measured in your browser, in a background worker, and never leave your device. The tool is free with no signup.

Distances are measured in your browser from each vertex to the nearest point of the other surface, after the alignment you chose. They are a visual and numeric guide, not a certified inspection report.