About the Mortar Calculator
This mortar calculator estimates how much mortar you need to lay a brick or concrete block (CMU) wall. Enter the wall length and height, subtract any doors or windows, pick the masonry unit and joint thickness, and it returns the number of units, the volume of mortar in cubic feet, and how many bags of pre-mixed mortar to buy.
It is meant for DIYers building a garden or retaining wall, homeowners checking a mason’s material list, and contractors who want a quick take-off. Instead of a fixed “bags per 100 blocks” rule, it calculates the actual volume of the bed and head joints around each unit, so changing the joint size or unit type changes the answer the way it does on site.
Hollow concrete block is assumed to be laid with face-shell bedding (mortar only on the two face shells), which is standard for ungrouted block walls; brick and 4-inch block are laid on a full bed. The result is the true joint volume plus a waste allowance of roughly 10–25%. Bagged-mortar labels quote much lower coverage to allow for beginners, dropped mortar and full-bedded courses, so the notes also show the bag-label count as an upper bound.
With the default inputs, the bags of mortar is 4. Change any value above to recalculate instantly.
How to use the mortar calculator
- 1Measure the wall length and height in feet.
- 2Add up the area of any openings and enter it to subtract.
- 3Choose the brick or block you are laying and the joint thickness.
- 4Keep a 10–25% waste allowance, more for beginners.
- 5Enter the yield from your mortar bag to get the number of bags to buy.
Formula and method
Each unit plus its share of joint occupies a face of (L + t) × (H + t) square inches, so dividing the net wall area by that face gives the unit count. The mortar face around one unit is that face minus the unit itself, which simplifies to t × (L + H + t); multiplying by the mortared depth d gives cubic inches of mortar per unit.
For hollow block the depth is the two face shells only (about 2 in for 6-inch block and 2.5 in for 8- and 12-inch block), which is face-shell bedding. Brick and 4-inch block use the full bed depth. Total cubic inches ÷ 1,728 gives cubic feet, the waste allowance is added, and the result is divided by the bag yield printed on the mortar bag and rounded up. Grouted cores, collar joints and full-bed block are not included.
- A
- Net wall area (sq ft)
- L, H
- Actual unit length and height (in)
- t
- Joint thickness (in)
- d
- Mortared bed depth (in)
Worked examples
30 × 4 ft garden wall in 8-inch block
A 120 sq ft wall holds 135 standard blocks, since each block with a 3/8 in joint covers 16 × 8 in. Face-shell joints use about 22 cubic inches of mortar per block, or 1.73 cu ft in total; with 20% waste that is 2.08 cu ft, so four 0.67 cu ft bags.
20 × 10 ft modular brick veneer with a window
The net area is 200 − 21 = 179 sq ft. Modular brick with 3/8 in joints runs 6.86 bricks per sq ft, about 1,228 bricks. Each brick uses roughly 14 cubic inches of mortar on a full bed, giving 11.88 cu ft with 20% waste — 18 bags at 0.67 cu ft each.
6-inch block foundation, 15% waste
A 240 sq ft wall needs 270 blocks. With thinner 1 in face shells the bed depth is 2 in, so mortar comes to 2.77 cu ft before waste and 3.18 cu ft after 15%, which is five bags.
Frequently asked questions
How many bags of mortar do I need per 100 blocks?+
The joints of 100 face-shell-bedded 8 × 8 × 16 blocks hold only about 1.3 cu ft of mortar, roughly 2–3 bags of 80 lb pre-mix with waste. Bag labels are much more conservative: QUIKRETE says an 80 lb bag lays up to 13 blocks, or about 8 bags per 100. Real use falls between, depending on skill, mortar dropped into cores and full bedding of starter courses.
How much mortar do I need for 1,000 bricks?+
Modular brick with 3/8 in joints has about 8 cu ft of joint volume per 1,000 bricks. With a 20% allowance that is close to 10 cu ft, or about 15 bags of 80 lb pre-mix; a bag label figure of 37 bricks per bag would suggest 27 bags, so experienced masons land near the first figure and beginners nearer the second.
What type of mortar should I use for a block wall?+
Type S is the usual choice for walls below grade, retaining walls and structural block because of its higher strength. Type N is common for above-grade veneer and interior or non-load-bearing walls. Follow local code or your engineer for structural work.
How thick should mortar joints be?+
The standard joint for both brick and concrete block is 3/8 inch. Unit sizes are designed around it, so a modular brick plus a 3/8 in joint fills exactly 8 inches of length.
Why is there so much mortar waste?+
Mortar is dropped, struck off the joints, and stiffens on the board before it can be used. Experienced masons may waste 10%, while DIY projects often waste 25% or more, so it is safer to buy an extra bag.