Mortar volume equals sand volume.
An ideal mortar mix has a lime volume equal to the space between the grains of sand, so to meassure the space, place a known volume of dry sand in a bucket and add meassured water until the water level reaches the top of the sand. Then calculate sand to lime ratio.
Calculate area of walls to be plastered then multiply by thickness of plaster equals volume of mortar. Apply a fudge factor to avoid shortfall. Plaster volume equals sand volume. Apply sand/lime ratio for amount of lime needed.
