Answers for "How is Rigidbody 2D Auto Mass calculated?"
http://answers.unity.com/questions/1271858/how-is-rigidbody-2d-auto-mass-calculated.html
The latest answers for the question "How is Rigidbody 2D Auto Mass calculated?"Answer by MelvMay
http://answers.unity.com/answers/1272193/view.html
Unity uses Box2D for its 2D physics. Box2D wants to calculate the mass by assigning a collider a density and multiplying that density by the collider's area to give the mass. This is how Box2D works however Unity has historically presented an explicit mass value.
UseAutoMass essentially works how Box2D wants to work i.e. when it is set to true, you can set a density on each collider and the collider's area multipled by this density will give that colliders mass. The Rigidbody2D adds-up the mass for all colliders attached to it and this is the final 'auto mass'.
I would suggest you go to the Box2D source code here: [https://github.com/erincatto/Box2D/tree/master/Box2D/Box2D/Collision/Shapes][1]
If you look at each shape's implementation (.cpp file) and look for a function named "ComputeMass()". You'll see it calculate the shape's area and multiply it by its density.
Note that the function name is slightly misleading as it's calculating the 'mass data' which means calculating not only the mass but the rotational inertia and the local center of mass.
Hope this helps.
[1]: https://github.com/erincatto/Box2D/tree/master/Box2D/Box2D/Collision/ShapesWed, 16 Nov 2016 08:19:23 GMTMelvMayAnswer by UNDERHILL
http://answers.unity.com/answers/1272168/view.html
https://docs.unity3d.com/ScriptReference/Rigidbody2D-useAutoMass.html
collider2d.density * areaWed, 16 Nov 2016 07:02:41 GMTUNDERHILL