Calculate how many cubic meters of insulation you need for a wall, roof or floor, based on length, width and thickness.
The volume is area times thickness, once the thickness is converted from millimeters to meters:
Insulation products are normally sold in packs labeled with how many square meters they cover at a given thickness — check the packaging for the exact thickness you're using, since pack sizes vary a lot between products and manufacturers.
It depends on the building part and climate zone. Exterior walls are often insulated with 150–250 mm, roofs with 300–400 mm, and interior walls (sound only) with 70–100 mm. Check your local building code for minimum requirements.
Yes, calculate each layer separately with the correct thickness for that layer, then add the volumes together.