Worked example — 20 ft × 8 ft CMU wall, one 20 ft² door
Gross 160 ft² minus 20 ft² door is 140 ft² net. At 112.5 blocks per 100 ft² that is 158 units before waste. Round up after your waste input; add mortar bags from the yield you typed, not from a second guess.
Net wall, then face size
Measure finished wall length and height. Deduct doors and windows that will not be masonry. Unusual returns, soldier courses and custom units belong on their own lines.
An 8×16 CMU with a 3/8 in joint is taken as a 8×16 nominal face — 0.89 ft², commonly planned as 112.5 units per 100 ft². Modular brick is a different face. Do not run both through one calculator.
Mortar after the count, not instead of it
Mortar bags come from a stated yield, such as 5.5 ft³ of mortar per 100 ft² of wall as a joint rule of thumb, or a manufacturer’s units-per-bag figure. Construclytic keeps that yield visible so you can replace it.
Lintels, grout fill, rebar and ties are extras. Waste for cuts and breakage stays a percentage, not a hidden second joint.
Do it in order
- Measure net wall area after openings.
- Pick CMU or brick and use the nominal face that includes the joint.
- Round to whole units, then estimate mortar from the selected yield.
Planning boundary
These counts are planning quantities. They do not design the wall or guarantee a mason’s production rate.
Sources
This guide is original worked arithmetic for the calculator linked above. It does not cite a laboratory test or a supplier price. The method, the internal check and the planning limit are on How we calculate.
Why not actual block size?
Because the joint is already in the nominal module. Adding it again overstates the wall.
Does this size the wall?
No. Wind, seismic ties, grout cells and bearing stay with the design.
What about retaining-wall block?
Use the retaining-block calculator. It counts exposed face, not a CMU running bond.