Using sketchup for calculating volume

I have created a model of my swimming pool trying to narrow down the exact volume.

It is an irregular shaped pool and I cannot close two faces. Is there any possible way to force this?

I have tried redrawing the lines, welding and makeface with no luck

The edges must not be co-planer so draw an edge diagonally and you should get two faces to close the hole.

Once you have all faces made, and it is a “watertight” enclosed space, window select all of it, right-click the selection, and choose “Make Group” from the context menu.

You should then see the volume displayed in the Entity Info inspector.