# Free online STEP viewer

> View STEP and STP CAD files in your browser without SolidWorks, Fusion 360 or FreeCAD. The file is tessellated on your device with OpenCASCADE — free, no upload, no account.

Source: https://freestlviewer.com/step-viewer/  
Updated: October 22, 2026

Open .step and .stp CAD files without installing CAD software. The geometry is converted on your own device with OpenCASCADE, so the file is never uploaded. The tool is interactive: open https://freestlviewer.com/step-viewer/ and drop a file onto the page.

## What a STEP file is

STEP (ISO 10303, "Standard for the Exchange of Product model data") is the
neutral format CAD programs use to hand solid models to each other. A `.step`
or `.stp` file does not contain triangles. It describes exact geometry — planes,
cylinders, B-spline surfaces and the edges where they meet — as a boundary
representation (B-rep), along with assembly structure and, in the common AP214
and AP242 variants, colours.

Exact geometry is what makes STEP valuable to engineers, and also what makes it
hard to view: something has to turn those mathematical surfaces into triangles
before a screen can draw them. Usually that "something" is a full CAD package.

## How Mes3D opens STEP in a browser

Mes3D uses **OpenCASCADE** (through the open-source occt-import-js WebAssembly
build), the same geometry kernel behind FreeCAD and the Mes3D iPhone app:

1. The first time you open a STEP file, the browser downloads the OpenCASCADE
   engine (about 7 MB). Later files reuse it.
2. The file is parsed and tessellated in a background worker, so the page stays
   responsive while a large assembly is processed.
3. Surfaces are meshed with a deflection of 0.1 % of the model's bounding box,
   which keeps curved faces smooth without exploding the triangle count.
4. Geometry comes out in millimetres, and body colours stored in the file are
   kept.

None of this involves a server. The CAD file stays on your machine, which is
usually the whole point when the file came from a client or a supplier under
NDA.

## How to view a STEP file here

1. Drag the `.step` or `.stp` file onto the drop zone above, or click **Choose
   3D file**.
2. Wait a moment while it is tessellated — small parts take a second, large
   assemblies longer.
3. Rotate, zoom and pan. Use the preset views and orthographic projection to
   look straight down an axis, which is how drawings are read.
4. On a computer, measure point-to-point distances in millimetres, read the
   overall width, height and depth, and open the analysis panel for volume,
   surface area and estimated weight.

## What you get — and what you do not

| | Mes3D | Full CAD (FreeCAD, Fusion 360, SolidWorks) |
| --- | --- | --- |
| View the 3D shape | Yes | Yes |
| Overall dimensions, point-to-point distance | Yes | Yes |
| Volume and estimated weight | Yes (from the mesh) | Yes (exact) |
| Feature tree, sketches, parameters | No — STEP does not carry them anyway | Only for native files |
| Edit and re-export STEP | No | Yes |
| Install and account | None | Install required; some need an account |

Two practical notes:

- **Measurements are taken on the mesh.** With a 0.1 % deflection the error on
  a curved face is tiny compared with the part, but if you need a tolerance on a
  bore, use CAD.
- **Volume comes from the tessellated solid.** It is accurate for closed bodies;
  an assembly with overlapping parts counts the overlap twice.

## Common problems

**Nothing appears.** Some exporters write only wireframe or surface data
without closed faces. Re-export from the source CAD as a solid ("AP214" or
"AP242" with solids) and try again.

**It takes a long time.** Very large assemblies — hundreds of parts with
fasteners and threads — produce millions of triangles. The browser handles
files up to 200 MB, but a fast computer makes a big difference here.

**The model is tiny or huge.** Mes3D reads STEP in millimetres. If the part was
modelled in inches and exported with the wrong unit, scale is off by 25.4 — see
[STL units, mm vs inches](https://freestlviewer.com/blog/stl-units-mm-vs-inches/) for how to spot it.

## On a phone

Mobile browsers can open STEP files here too, but tessellating a large
assembly is heavy work for a phone. The Mes3D app opens STEP attachments from
Mail or Files directly — on iPhone and iPad with OpenCASCADE running natively —
and adds measuring and cross-sections on the device, plus AR on iPhone and iPad.
There is a native Mac version as well.

## Frequently asked questions

### How can I open a STEP file without CAD software?

Drop the .step or .stp file into Mes3D in any modern browser. It converts the CAD geometry into a mesh on your device using OpenCASCADE and shows it in 3D, with measuring and dimensions. No CAD licence, installation or account is needed.

### Is my STEP file uploaded?

No. The OpenCASCADE engine is downloaded once (about 7 MB) and runs inside your browser in a background worker. The CAD file is read from your disk and processed locally.

### What is the difference between .step and .stp?

None — they are two extensions for the same ISO 10303-21 format. Mes3D opens both.

### Can I edit the STEP file or export it back to STEP?

No. Mes3D is a viewer: it turns the exact B-rep surfaces into triangles for display and measurement. To change features or dimensions you need CAD software such as FreeCAD, Fusion 360 or SolidWorks.

### Does Mes3D open IGES files?

Not at the moment. Mes3D opens STEP and STP CAD files; IGES (.igs, .iges) is not supported.