# STEP Volume Changed After Conversion: What to Check

If two CAD files both report a volume but the numbers differ, check units, body scope, missing components, healing and solid validity before blaming rounding.

This guide covers general CAD workflows. CADProps supports STEP, IGES, SolidWorks parts and ZIP assemblies, STL, OBJ, 3MF and static GLB/GLTF. Validated conversions are downloadable. Partial assemblies carry warnings; properties cover loaded geometry only.

A STEP conversion can succeed and still deserve investigation. If the source system reports one volume and the converted or re-imported STEP reports another, do not dismiss the difference as “just a translator issue” and do not immediately repair the model until the numbers match.

First establish whether you are measuring the **same physical geometry in the same units and scope**. A real volume difference can come from a missing body, a suppressed component, an open shell that was healed, a unit mistake, or geometry that changed during translation.

CADProps reports volume only when usable closed solid geometry is available. It does not replace an unavailable volume with a bounding-box estimate.

## Start with scale, because volume magnifies unit mistakes

A linear scale error becomes much larger in volume. If a part that should be 1 inch wide is interpreted as 1 mm, the linear error is 25.4× and the volume error is `25.4³`, more than sixteen thousand times.

Before comparing volume:

1. compare one known length;
2. confirm the displayed unit;
3. check the overall X, Y and Z extents;
4. only then compare volume.

If the known length is wrong by exactly 25.4, investigate inch/mm interpretation before anything else. Healing a model cannot fix a unit mismatch.

## Make sure both sides include the same bodies

A volume number is only the sum of what the software loaded and counted.

Typical scope differences include:

- one export includes hidden or construction solids and the other does not;
- a multi-body part was exported as only the active body;
- an assembly conversion omitted unresolved components;
- a simplified representation or configuration was exported instead of the released state;
- duplicate bodies were imported on one side;
- one application counts a body that another rejected as invalid.

For an assembly, compare the component and body counts before comparing the total volume. A plausible overall bounding box does not prove that every internal component is present.

## Healing can change topology

CAD translators often need to sew faces, adjust tolerances or repair topology so that imported surfaces form usable solids. Open CASCADE documents a set of shape-healing operations for correcting problems in exchanged geometry. [Open CASCADE Shape Healing](https://dev.opencascade.org/doc/overview/html/occt_user_guides__shape_healing.html).

Repair is not automatically bad. A tiny gap created by numerical differences between CAD kernels may need sewing before a solid can be recognized. But repair is still an operation on the model. If the volume changes materially after healing, the question is not “which number looks nicer?” but “what geometry was changed or newly closed?”

Do not keep increasing sewing tolerances until a volume appears. An overly permissive tolerance can join edges that were not meant to meet.

## A volume exists only under geometric preconditions

Open CASCADE's B-Rep property routines assume appropriate topology and orientation for volume calculations. [BRepGProp geometry properties](https://dev.opencascade.org/doc/refman/html/class_b_rep_g_prop.html). A shaded surface model can look like a solid without satisfying those conditions.

That gives three different cases:

| Result | Meaning |
| --- | --- |
| Same volume within the precision needed for the job | Translation likely preserved the measured solid scope, but still check critical geometry |
| Both sides have volume but differ materially | Investigate scale, body scope and changed/healed geometry |
| One side has volume and the other says unavailable | One representation may no longer be a valid closed solid, or the reader may not have loaded all required geometry |

“Unavailable” is not zero. The [missing STEP volume guide](https://www.cadprops.com/guides/why-step-volume-is-zero/index.md) covers that separate failure mode.

## Compare geometry, not only one scalar

A single volume can match while a feature moved. Two different shapes can have the same volume. Use a small verification set:

- one or two known dimensions;
- body/component count;
- total volume;
- overall extents;
- important hole diameters and mating faces;
- assembly placement if the handoff is an assembly.

For two STEP revisions, use the [CAD comparison workflow](https://www.cadprops.com/guides/compare-cad-revisions/index.md) rather than treating volume difference as a complete change detector.

## Example: a housing gains 1.8% volume

Suppose the native CAD reports 125,000 mm³ and the re-imported STEP reports 127,250 mm³. The difference is 1.8%.

That percentage alone tells you almost nothing about the cause. A missing bore, a filled opening, one accidentally included boss, or a healing operation that closes a gap can all move the total. Check the known dimensions and visually isolate the changed region before accepting either value.

If the dimensions are instead 25.4× different, stop: you are looking at a scale problem, not a subtle B-Rep repair issue.

## A release check after conversion

Use this sequence whenever a converted STEP becomes a manufacturing deliverable:

1. Keep the source file and the exported STEP together with the revision.
2. Reopen the STEP in an independent reader or the actual downstream CAD/CAM system.
3. Verify one known dimension and the expected body count.
4. Compare volume only after units and scope match.
5. Investigate any material difference rather than normalizing it away.
6. Check critical holes, faces and assembly placements.
7. Record whether any healing or repair was performed.

The [CAD conversion checklist](https://www.cadprops.com/guides/cad-conversion-checklist/index.md) is a broader version of this process.

A conversion is successful when the receiving workflow gets the geometry it needs, not merely when a file with a `.step` extension was written.

## Related reading

- [CAD File Conversion Checklist: Units, Solids & Accuracy](https://www.cadprops.com/guides/cad-conversion-checklist/index.md)
- [How to Calculate Volume from a STEP File](https://www.cadprops.com/guides/calculate-step-volume/index.md)
- [Compare CAD Revisions: Geometry and Property Checks](https://www.cadprops.com/guides/compare-cad-revisions/index.md)
- [Reduce STEP File Size: Keep Required CAD Geometry](https://www.cadprops.com/guides/smaller-step-files/index.md)
- [STEP Assembly Flattened After Import? Check Structure and Names](https://www.cadprops.com/guides/step-assembly-flattened-after-import/index.md)
- [STEP File Volume Zero or Missing? Check Solid Geometry](https://www.cadprops.com/guides/why-step-volume-is-zero/index.md)

Original article: https://www.cadprops.com/guides/step-volume-changed-after-conversion/

HTML page: https://www.cadprops.com/guides/step-volume-changed-after-conversion/