Mortar Calculator

Mortar is estimated per masonry unit. A standard 8×8×16 concrete block takes roughly one-eighth of a cubic foot of mortar once you count the bed joint under it, the head joint beside it, and normal waste. A modular brick, with its thinner 3/8-inch joints, takes a little less — about one-ninth of a cubic foot. Multiply your unit count by that per-unit figure and you have the mortar volume.

To turn volume into bags, divide by the yield of a bag of mortar mix. An 80-pound bag of pre-blended mortar makes about half a cubic foot, so a hundred blocks — about twelve and a half cubic feet of mortar — works out to roughly twenty-five to twenty-eight bags depending on your waste allowance. Bag counts round up, because partial bags are not sold.

One honest caveat: buying finished mortar mix in bags takes more bags than mixing your own from masonry cement and sand, where you buy the cement and add sand separately. If you are site-mixing, use this cubic-foot figure to size your cement and sand instead of the bag count.