Counting blocks the way the yard does
A concrete block wall is counted by face area. Each block, together with its share of the standard ⅜″ mortar joint, covers a fixed patch of wall: a standard 8 × 8 × 16 block presents a 16″ × 8″ face, and with the joint that works out to 1.125 blocks per square foot of wall. Divide the wall's area by the block's face area (height × length, joint included) and you have the count before waste — this calculator does exactly that, for metric walls too, then adds your waste allowance and rounds up, because blocks are bought whole.
Face dimensions here are the nominal height × length of the block plus one ⅜″ joint, the convention behind every published block table.
The mortar estimate — and its assumption
Mortar is estimated with the widely published rule of thumb of about 3 bags of 80 lb mortar mix per 100 standard blocks laid with ⅜″ joints (the figure quoted by contractor references such as Housecall Pro). Thicker blocks (12″) use roughly a third more per block; thinner 6″ blocks a little less, and the calculator scales for the size you pick. Treat the bag count as a planning figure, not gospel: bag-yield tables that count every scrap of squeeze-out run higher, and masons lose mortar to the board, droppings and tooling — order an extra bag or two rather than stopping a wall mid-course for one bag.
Worked example
A 10 ft × 8 ft garden wall is 80 sq ft. At 1.125 blocks per sq ft that is 90 blocks before waste; with the default 10% allowance it is 99 blocks to order, and about 3 bags of 80 lb mortar mix by the rule above.
How to use this calculator
- Enter the wall's length and height (feet or metres).
- Pick the block size — 8 × 8 × 16 is the standard CMU.
- Set a waste allowance: 5% for a plain straight wall, 10% with corners, openings or lots of cuts.
- Read the order quantity, then sanity-check the concrete for footings or core fill with the Concrete and Cubic Yards calculators.
What this count leaves out
Openings are not deducted — if your wall has doors or windows, subtract their area from the wall (length × height of each opening) before entering the dimensions, or accept the surplus as your breakage stock. The count also excludes lintels, bond beams, rebar and core-fill concrete, which are estimated separately.