I'm trying to scale a lot of dynamically created game objects in Unity3D to the bounds of a sphere collider, based on the size of their current mesh. Each object has a different scale and mesh size. Some are bigger than the AABB of the collider, and some are smaller. Here's the script I've written so far:

private void ScaleToCollider(GameObject objectToScale, SphereCollider sphere) {
    var currentScale = objectToScale.transform.localScale;
    var currentSize = objectToScale.GetMeshHierarchyBounds().size;

    var targetSize = (sphere.radius * 2);

    var newScale = new Vector3 {
        x = targetSize * currentScale.x / currentSize.x,
        y = targetSize * currentScale.y / currentSize.y,
        z = targetSize * currentScale.z / currentSize.z

    Debug.Log("{0} Current scale: {1}, targetSize: {2}, currentSize: {3}, newScale: {4}, currentScale.x: {5}, currentSize.x: {6}",
        objectToScale.name, currentScale, targetSize, currentSize, newScale, currentScale.x, currentSize.x);

    //DoorDevice_meshBase                   Current scale: (0.1, 4.0, 3.0), targetSize: 5, currentSize: (2.9, 4.0, 1.1), newScale: (0.2, 5.0, 13.4), currentScale.x: 0.125, currentSize.x: 2.869114
    //RedControlPanelForAirlock_meshBase    Current scale: (1.0, 1.0, 1.0), targetSize: 5, currentSize: (0.0, 0.3, 0.2), newScale: (147.1, 16.7, 25.0), currentScale.x: 1, currentSize.x: 0.03400017

    objectToScale.transform.localScale = newScale;

And the supporting extension method:

public static Bounds GetMeshHierarchyBounds(this GameObject go) {
    var bounds = new Bounds(); // Not used, but a struct needs to be instantiated

    if (go.renderer != null) {
        bounds = go.renderer.bounds;
        // Make sure the parent is included
        Debug.Log("Found parent bounds: " + bounds);
    foreach (var c in go.GetComponentsInChildren<MeshRenderer>()) {
        Debug.Log("Found {0} bounds are {1}", c.name, c.bounds);
        if (bounds.size == Vector3.zero) {
            bounds = c.bounds;
        } else {
    return bounds;

After the re-scale, there doesn't seem to be any consistency to the results - some objects with completely uniform scales (x,y,z) seem to resize correctly, but others don't :|

Its one of those things I've been trying to fix for so long I've lost all grasp on any of the logic :| Any help would be appreciated!


First idea:

Rescale each object to (1.0, 1.0, 1.0) before applying your algorithm. But then I thought that maybe the objects look good when they are non-uniformly scaled to begin with.

Second idea:

I think that the problem is that the algorithm is mixing scale and size values. If your var newScale = new Vector3(...) code only works for uniform scales of 1.0, then the currentScale.<x/y/z> parts in it could be replaced by 1.0 which means that they are effectively doing nothing.

Suggestion (which I haven't tried):

Calculating the targetSize / currentSize (for each of x,y,z) will give you the newSizeRatio for each dimension. Take the minimum of them and multiply the scale vector by it:

float newSizeRatioX = targetSize.x / currentSize.x;
float newSizeRatioY = targetSize.y / currentSize.y;
float newSizeRatioZ = targetSize.z / currentSize.z;

float minimumNewSizeRatio = Math.Min(newSizeRatioX, Math.Min(newSizeRatioY, newSizeRatioZ));

Vector3 newScale = currentScale * minimumNewSizeRatio;

I believe this will work because:

  • Multiplying all dimensions with the same ratio will probably produce a uniformly scaled object.
  • Multiplying with the minimum ratio will guaranty that the object will fit inside the circle.

I hope this works :-)

  • \$\begingroup\$ Hey vrinek! Thanks for your reply. I actually sorted this about 2 hours after I posted this (and forgot to come back and update it!). I will post my solution, but it's similar to your last example - everything now resizes correctly, and in addition I've applied a reduction factor too - so I can resize to 50% (opr whatever) of the sphere radius. \$\endgroup\$
    – Spikeh
    Jun 6 '14 at 15:00

use a mesh collider instead !

i just had the same problem.

but then i just used a mesh collider instead of a sphere collider and the problem almost solved itself

just be sure to update the mesh collider shared mesh as well:

var meshInstance = Instantiate(meshes[type]);
gameObject.GetComponent<MeshFilter> ().mesh = meshInstance;
gameObject.GetComponent<MeshCollider> ().sharedMesh = meshInstance;

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