Block walls are made of two things: blocks and the mortar that bonds them. The block count is easy, but estimating mortar is harder, because the amount depends on how the joints are bedded, how thick the block is and how much falls on the ground or dries unused. This calculator works out the volume of mortar in the joints from the geometry of the wall, adds an allowance you can adjust, and converts that into 80 lb bags.
How to use the block mortar calculator
- Enter the wall size (length and height) and choose the block face and thickness.
- Select the bedding. Face-shell bedding is the standard for hollow block; full bedding fills the whole top of the block.
- Set the waste allowance. The default is 30%.
- Enter the bag yield from the label of the mortar you are buying.
Mortar formula
Mortar per block = (block length × bedded width + block height × bedded width) × joint thickness
Face-shell bedded width = 2 × 1.25 in = 2.5 in · full bedding = block thickness
Total mortar = blocks × mortar per block × (1 + allowance)
The first term is the bed (horizontal) joint and the second is the head (vertical) joint, both 3/8 inch thick. The 1-1/4 inch face-shell thickness is the minimum specified for 8-inch hollow concrete masonry units in ASTM C90.
Worked example: a 20 × 8 ft wall
- Blocks: 160 sq ft ÷ 0.8889 = 180 blocks.
- Mortar per block with face-shell bedding: (16 × 2.5 + 8 × 2.5) × 0.375 = 22.5 cubic inches.
- Joint volume: 180 × 22.5 = 4,050 in³ = 2.34 cu ft.
- With a 30% allowance: 2.34 × 1.30 = 3.05 cu ft.
- Bags at 0.6 cu ft each: 3.05 ÷ 0.6 = 5.1, so 6 bags.
The same wall laid with full bedding needs 7.15 cu ft in the joints, almost three times as much.
Mortar for common walls
8-inch block, standard face-shell bedding, 30% allowance and 0.6 cu ft bags.
| Wall | Blocks | Joint mortar (cu ft) | With allowance (cu ft) | 80 lb bags |
|---|---|---|---|---|
| 10 × 4 ft | 45 | 0.59 | 0.76 | 2 |
| 20 × 4 ft | 90 | 1.17 | 1.52 | 3 |
| 20 × 8 ft | 180 | 2.34 | 3.05 | 6 |
| 30 × 8 ft | 270 | 3.52 | 4.57 | 8 |
| 40 × 8 ft | 360 | 4.69 | 6.09 | 11 |
Why estimates vary so much
Published figures for mortar per 100 blocks vary by a factor of two or more, because they assume different bedding, joint thickness, mortar droppings and bag sizes. For that reason the calculator shows the geometric volume separately from the allowance, and the bag yield is editable. If you are buying mortar for a large job, lay a small test section and note how many bags it used.
- Strike or tool joints soon after laying; they pack the mortar and shed water.
- Mix small batches that can be used within about an hour or two, and re-temper only as the mortar manufacturer allows.
- Do not forget the bed under the first course, which is usually full width.
Start with the number of blocks in the Concrete Block Calculator. If the cores will be filled, add the Concrete Block Fill Calculator. For loose ingredients mixed on site, see the Concrete Mix Calculator and the Sand Calculator.
Frequently asked questions
How much mortar do I need for 100 concrete blocks?
With standard face-shell bedding and 3/8-inch joints the joints themselves hold about 1.6 cubic feet per 100 blocks; with a 30% allowance for droppings and waste this is about 2.1 cubic feet. Published rules of thumb differ, so treat the result as an estimate.
What is face-shell bedding?
Mortar is placed only on the two outer face shells of the block (and the end flanges for head joints) instead of across the whole top. It is the normal method for hollow block. Full bedding is used for solid units and for the first course on a footing.
How many bags of mortar do I need?
Divide the mortar volume, including your allowance, by the yield printed on the bag, then round up. The yield changes between brands and types, so edit the yield box to match your bag.
Why is a waste allowance needed?
Mortar falls from the trowel, is trimmed off joints, dries in the mixing tub and is squeezed out of the joints. Allowances of 25% to 40% are common, and beginners should use more.
What mortar type should I use?
Type S is a common choice for below-grade and structural block walls and Type N for above-grade general use, but your plans or local code decide. This calculator estimates quantity only.