Mortar for a Wall
Enter the wall face area (one side), unit type, and joint size.
Concrete
Mortar Calculator
Quick answer: mortar = units × joint volume per unit × (1 + allowance). An 8x8x16 block with 3/8 in face-shell joints holds 22.1 in³, so 450 blocks (400 ft² of wall) need 5.8 ft³ in theory, 8.7 ft³ with a 50% allowance — about 13 bags of 80 lb premix.
Work out mortar for concrete block (CMU) and brick walls from the wall area, unit size, and joint thickness. Results are in 80 lb premixed bags, with a masonry-cement-and-sand alternative for mixing your own.
Face-shell bedding on a block
How to calculate mortar for block and brick
Rather than rely on a single rule of thumb, this calculator measures the mortar that actually sits in the joints. Every unit carries one bed joint (along its length plus one joint) and one head joint (its height). Multiply that joint area by the joint thickness and by the width of mortar — the face shells on hollow block, or the full bed depth on brick.
j × w × (L + j + H) × units × (1 + allowance)
w = 2 × face shell for block, bed depth for brick; inches → ÷ 1,728 for ft³
- Count units: wall area × units per ft². With 3/8 in joints an 8x8x16 block makes a 16 × 8 in module, so 144 ÷ 128 = 1.125 blocks per ft².
- Joint volume per block: 2 face shells × 1.25 in × 0.375 in × (16 + 7.625 in) = 22.1 in³ = 0.0128 ft³.
- Theoretical mortar: units × joint volume. That is about 1.28 ft³ per 100 blocks.
- Add an allowance for droppings, extruded mortar that is struck off, full bedding on the first course, and leftover batches. The default is 50%.
- Convert to bags: ÷ 0.67 ft³ for 80 lb premix bags, or ÷ 3 ft³ per bag of masonry cement mixed 1 : 3 with sand.
Checking the geometry against the field rule
A commonly quoted rule is about 3 bags of 80 lb premixed mortar per 100 concrete blocks. Three bags at 0.67 ft³ is 2.0 ft³ — about 1.57 times the 1.28 ft³ the face-shell joints hold. The difference is the mortar that never ends up in a joint, which is why the default allowance is 50%. Expect to need more if you full-bed every course, use 1/2 in joints (1.72 ft³ of joint volume per 100 blocks), lay block on a hot, windy day, or are new to masonry.
Worked example
A 50 ft × 8 ft garage wall of 8x8x16 block with 3/8 in joints.
- Blocks: 400 ft² × 1.125 = 450
- Joints: 450 × 22.15 in³ ÷ 1,728 = 5.77 ft³; + 50% = 8.65 ft³
- Premix: 8.65 ÷ 0.67 = 13 bags of 80 lb mortar mix (the 3-per-100 rule gives 13.5)
- Or mix your own: 8.65 ÷ 3 = 3 bags of masonry cement and about 8.7 ft³ (0.32 yd³) of mason sand
Mortar per 100 square feet of wall
3/8 in joints, single wythe, 50% allowance included in the last three columns.
| Unit | Units per 100 ft² | Joint volume (theoretical) | Mortar to plan for | 80 lb premix bags | Masonry cement bags (1 : 3) |
|---|---|---|---|---|---|
| 8x8x16 block, face-shell bedded | 112.5 | 1.44 ft³ | 2.16 ft³ | 3.2 | 0.7 |
| Modular brick | 686 | 5.53 ft³ | 8.29 ft³ | 12.4 | 2.8 |
| Standard brick | 655 | 5.47 ft³ | 8.21 ft³ | 12.3 | 2.7 |
| Utility brick | 300 | 3.69 ft³ | 5.53 ft³ | 8.3 | 1.8 |
Mortar mixing and ordering tips
- Brick uses far more mortar per square foot than block. A brick wall has about six times as many units per square foot, and brick is bedded across its full depth, so a modular brick wall needs roughly four times the mortar of the same area of block.
- The first course is full-bedded. The starting course of block is usually laid in a full mortar bed on the footing, and cells that will be grouted have their webs mortared. Raise the allowance for short walls where the first course is a large share of the total.
- Pick the type for the job. Type N suits most above-grade walls and veneer; Type S is the usual choice below grade and for retaining walls. Do not mix types or brands within one wall.
- Mix only what you can use. Mortar should generally be used within about 2½ hours of mixing; re-temper with a little water only to replace evaporation, and discard mortar that has started to set.
- Measure sand by volume consistently. Use the same bucket or shovel count for every batch so color and strength stay uniform along the wall.
- Bag yield varies. Brands print different yields, and wetter mixes go a little further. If your bag lists a yield, enter it in the options.
Mortar calculator FAQ
How many bags of mortar do I need for 100 blocks?
For 8x8x16 block with face-shell bedding and 3/8 in joints, the joints hold about 1.28 cu ft per 100 blocks. Adding a 50 percent allowance gives about 1.9 cu ft, or 3 bags of 80 lb premixed mortar, which matches the common rule of about 3 bags per 100 block.
How much mortar does an 80 lb bag make?
An 80 lb bag of premixed Type N or Type S mortar mix yields roughly 0.67 cu ft of wet mortar, and somewhat more with extra mixing water. This calculator uses 0.67 cu ft so the bag count leans high; check the yield printed on your brand's bag.
How much sand do I mix with a bag of masonry cement?
Masonry cement mortar is commonly proportioned at 1 part masonry cement to 2-1/4 to 3 parts damp, loose sand by volume, with one bag counted as 1 cu ft. At 1 to 3, one bag plus 3 cu ft of sand makes roughly 3 cu ft of mortar, since the cement mostly fills the gaps between sand grains.
How much mortar do I need per square foot of block wall?
For 8x8x16 block there are 1.125 blocks per square foot, and face-shell joints hold about 0.014 cu ft per square foot in theory. With a 50 percent allowance, 100 sq ft of wall needs about 2.2 cu ft of mortar, or a little over 3 bags of 80 lb premix.
What type of mortar should I use for block walls?
Type N is the general-purpose choice for above-grade walls and veneer. Type S has higher strength and is typically used below grade, in foundations and retaining walls, and where wind or soil loads are higher. Follow the project specification or local code if one applies.
How we built this calculator: results use the standard volume, area, and coverage formulas shown on this page, with typical U.S. material densities, bag yields, and waste factors that you can override. Written and checked by the YardsCalculator editorial team. Last reviewed: October 2026.