How this calculator works
A standard 8 × 16 inch concrete block (with its mortar joint) covers about 0.89 square feet of wall, so a square foot of wall takes about 1.125 blocks. An 80 lb bag of mortar mix makes about 0.6 cubic feet and lays roughly 35 blocks, including waste.
Worked example: a 20 × 4 ft wall (80 ft²) takes 90 blocks and 3 bags of 80 lb mortar mix at 35 blocks a bag. A wall 40 × 8 ft needs 360 blocks and 11 bags. For 100 blocks it is 3 bags.
Mortar for common block counts (35 blocks per bag)
Bags of 80 lb.
| Blocks | Bags | Wall area (8 × 16 in blocks) |
|---|---|---|
| 50 | 2 | 44 ft² |
| 100 | 3 | 89 ft² |
| 200 | 6 | 178 ft² |
| 300 | 9 | 267 ft² |
| 500 | 15 | 444 ft² |
| 1000 | 29 | 889 ft² |
Tips before you buy
- Use the mortar type the plans call for: Type N for general use above grade, Type S for below grade and walls that need more strength.
- Buy a few extra bags. Mortar is cheap, and running out halfway through a course leaves a visible join.
- Keep the bags dry and off the ground. Damp mortar mix is spoiled.
- Wet very dry, absorbent blocks lightly before laying them in hot weather.
- Bedding the first course on a full bed of mortar and checking level at each course matters more than the quantity.
Tools & materials for this project
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Frequently asked questions
How many bags of mortar for 100 concrete blocks?
3 bags of 80 lb at about 35 blocks a bag (2.9 bags, rounded up).
How much mortar for a block wall 20 ft long and 4 ft high?
3 bags of 80 lb mix for 90 blocks.
How many blocks per square foot of wall?
About 1.125 for standard 8 × 16 inch blocks: one block covers about 0.89 ft² of wall, joints included.
How much sand and cement make a bag of mortar?
A bag of mortar mix already contains sand and cement (and usually lime). If you mix your own, 1 part cement to 3 parts sand by volume is typical for mortar.
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Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.