How Many Bags of Mortar for Concrete Blocks? Wall Size Chart

An 80 lb bag of mortar mix lays roughly 35 standard concrete blocks (8 × 16 inch), so a wall 20 ft long and 4 ft high, with 90 blocks, takes 3 bags. For 100 blocks it is 3 bags. The block mortar calculator works from the wall size or the block count, with the blocks per bag adjustable.

Mortar by number of blocks

At 35 blocks per 80 lb bag:

Mortar by wall size

A bigger wall, 40 ft long and 8 ft high, needs 360 blocks and 11 bags.

What changes the amount

• Joint thickness: 3/8 inch is standard. Thicker joints use more.
• Skill and waste: mortar that falls or dries on the board is wasted, which is why we count only about 35 blocks per bag.
• Filling the cores: grout or concrete for the hollow cores is extra, and it is a lot more material than the mortar.
• Block size: 8×8×16 is standard. Other sizes change the count.

Mixing and laying

1. Mix in small batches that you can use in about 90 minutes.
2. Add water a little at a time until the mortar slides off the trowel and stays on the block.
3. Lay the first course on a full bed of mortar, level in both directions.
4. Butter the end of each block, set it, and tap it to level. Strike the joints when they are thumbprint hard.
5. Keep the bags dry and off the ground.

See also the concrete block calculator for the number of blocks, and the retaining wall materials guide. Estimates only: follow the mortar maker's instructions and the plans.

Frequently asked questions