Element information will report the volume of a morph.
So place a morph block under the stair with a higher height than the highest point of the understair space.
SEO the morph to the bottom of the stair using Subtraction with Upward Extrusion.
Get the volume of the morph with Element Information.
Viola !
(Anybody else remember 'viola'?)
David Collins
Win10 64bit Intel i7 6700 3.40 Ghz, 32 Gb RAM, GeForce RTX 3070
AC 27.0 (4001 INT FULL)