How to use the mortar calculator
- Choose wall size or unit count. If you have already used the brick or block calculator, enter the number of units.
- Pick what you are laying. Bricks and blocks use very different amounts of mortar.
- Check the bag yields under Adjust assumptions against the bag you are buying.
The mortar formula
Units = wall area × units per sq ft (6.75 modular bricks or 1.125 blocks)
Bags of mortar mix = units × (1 + waste) ÷ units per bag
Mix your own: 1 bag masonry cement + 3 cu ft sand ≈ 3 cu ft of mortar
Worked example
A 20 × 4 ft garden wall
- In modular brick: 80 sq ft × 6.75 = 540 bricks; with 10% waste ÷ 30 per bag = 20 bags.
- In 8 × 8 × 16 block: 80 × 1.125 = 90 blocks; with 10% ÷ 12 per bag = 9 bags.
Mortar types
| Type | Strength | Typical use |
|---|---|---|
| Type N | Medium | Above-grade walls, veneer, garden walls |
| Type S | High | Below grade, retaining walls, patios |
| Type M | Highest | Foundations and heavy loads, as specified |
| Type O | Low | Interior, non-load-bearing, repointing soft brick |
Common mistakes to avoid
- Believing the bag chart. Plan for fewer units per bag than printed.
- Mixing too much at once. Mortar stiffens in about 90 minutes; mix what you can lay.
- Using concrete sand. Mortar needs fine masonry sand.
- Retempering old mortar. Adding water to stiff mortar weakens it.
Frequently asked questions
How many bricks does a bag of mortar lay?
Makers quote up to about 35 to 40 standard bricks per 80 lb bag of mortar mix with 3/8 in joints. On real jobs, with waste and full joints, 25 to 30 is a safer planning figure, which is the default here.
How many blocks does a bag of mortar lay?
About 12 to 13 standard 8 × 8 × 16 blocks per 80 lb bag of mortar mix. A 20 ft × 4 ft block wall needs 90 blocks, so about 9 bags with 10% extra.
Can I mix my own mortar?
Yes. A common mix is one 70 lb bag of masonry cement to about 3 cubic feet of masonry sand, which makes roughly 3 cu ft of mortar. The calculator gives both options.
Type N or Type S mortar?
Type N is the usual choice for above-grade, non-load-bearing walls and veneer. Type S is stronger and used below grade, in retaining walls and where specified. Follow your project specification.
Why is my mortar estimate higher than the bag says?
Bag charts assume perfect 3/8 in joints and no waste. Thicker joints, dropped mortar, mortar that stiffens before use and filling cores all add up, so we plan with a lower count per bag.