How do I set mass of an object? (Physics)

Firstly, you need to square 64, so 64 x 64 = 4096

So why does Construct think the density is 81? 81 is the square of 9, so I'm guessing Construct thinks the box is 9 x 9 for some reason - maybe it's resized in the layout?

I'm curious too so am about to experiment, but if you can also upload the project, that might help.

EDIT - I just recreated your steps and mine's reporting a mass of 81.9199979305! No idea why! I'll keep experimenting...

EDIT 2 - added a sprite with size = 64 x 64 (instead of 1 x 1) and get the same mass...

As an implementation detail 1 meter = 50 pixels in the physics behavior. We shouldn’t need to worry about that except the conversion was inconsistently applied to all the values when you get values. So often values are off by a factor of 50.

64x64/50 =81.92