Used to approximate a proper mass value for all the colliders in a given Rigidbody

RigidbodyMassCalculator.cs

using UnityEngine;
using System.Linq;

[RequireComponent(typeof(Rigidbody))]
public class RigidbodyMassCalculator : MonoBehaviour {

	public float density = 1f;
	public bool recalculateOnAwake = true;
	
	Rigidbody rb;

	void Awake () {
		rb = GetComponent<Rigidbody>();
		if( recalculateOnAwake )
			RecalculateMass();
	}

	public void RecalculateMass() {
		Collider[] cols = GetComponentsInChildren<Collider>();
		float volume = cols.Sum( x => GetVolume( x, transform.localScale ) );
		rb.mass = volume * density;
	}

	static float GetVolume( Collider c, Vector3 scale ) {
		if( c is BoxCollider )
			return BoxVolume( Vector3.Scale(( c as BoxCollider ).size, scale) );

		if( c is MeshCollider ) // Approximate. Might want to use actual convex polytype volume
			return BoxVolume( Vector3.Scale( c.bounds.extents * 2, scale ) );

		float uniScale = scale.x * scale.y * scale.z;

		if( c is SphereCollider )
			return SphereVolume( ( c as SphereCollider ).radius * uniScale );

		if( c is CapsuleCollider ) {
			CapsuleCollider cc = c as CapsuleCollider;
			return CapsuleVolume( cc.height * scale[cc.direction], cc.radius * uniScale );
		}

		if( c is CharacterController ) {
			CharacterController chc = c as CharacterController;
			return CapsuleVolume( chc.height * scale.y, chc.radius * uniScale );
		}

		Debug.Log( "Invalid attempt to get volume of " + c.GetType().Name, c.gameObject );
		return 0f;
	}

	static float BoxVolume( Vector3 size ) {
		return size.x * size.y * size.z;
	}

	static float SphereVolume( float r ) {
		return r * r * r * Mathf.PI * ( 4f / 3f );
	}

	static float CylinderVolume( float h, float r ) {
		return h * r * r * Mathf.PI;
	}

	static float CapsuleVolume( float h, float r ) {
		float cylHeight = h - 2 * r;
		float sphereVol = SphereVolume( r );
		// If radius is twice as large or bigger than h - it degenerates into a sphere
		if( cylHeight <= 0 )
			return sphereVol;
		float cylVol = CylinderVolume( cylHeight, r );
		return sphereVol + cylVol;
	}

}