How many bags of mortar per 100 blocks depends entirely on which bag you buy. Eight bags of pre-blended mortar, or three bags of masonry cement plus about a fifth of a yard of sand. For the same 100 block and the same wall, the two answers differ by a factor of three and a half because they are not the same product. Almost every answer to this question online gives one number without saying which bag it is talking about.
There are two bags, and they look alike
Walk into the yard and you can buy mortar two ways.
- Pre-blended mortar, sold as Mason Mix or mortar mix in 60, 80, and 94 pound bags. Cement, lime, and sand are already in the bag. You add water.
- Masonry cement, sold in 70 pound bags. Cement and lime only. You buy sand separately and mix it in on-site, at roughly 1 part cement to 2.25 or 3 parts sand.
The second bag contains no sand, which is most of the volume of finished mortar, so it goes a great deal further. That is the whole reason the numbers diverge, and it is why a figure quoted without the product name is useless.
The bag is rated in units, not in area
Quikrete publishes both yields, and both are stated as a count of masonry units rather than square feet of wall:
| Product | Bag | Block per bag | Brick per bag | Per 100 block | Per 1,000 brick |
|---|---|---|---|---|---|
| Mason Mix, pre-blended | 80 lb | 13 | 37 | 8 bags | 28 bags |
| Masonry cement, Type S | 70 lb | 46 | 144 | 3 bags | 7 bags |
The masonry cement column assumes the sand that goes with it. Quikrete rates one bag mixed with 240 pounds of sand at 2.75 cubic feet of mortar, which lays 46 blocks or 144 bricks. For 100 blocks, that is about 520 pounds of sand, or a fifth of a cubic yard, on top of the three bags of cement.
The trade body is less generous than the bag. CMHA TEK 04-02A, its note on estimating masonry materials, puts a 70 lb bag of masonry cement at roughly 30 hollow units laid with face shell bedding, whereas Quikrete claims 46. That is the difference between 3.3 bags per 100 block and 2.2, so if you are mixing on site, buy against the lower yield.
Brick takes about twice the mortar that block does
The unit counts hide something useful. Convert them to wall area, and the two materials no longer look comparable:
- An 80 lb bag of Mason Mix lays 13 blocks, and a standard 8 by 8 by 16 block has an area of 0.89 square feet, so that bag covers 11.6 square feet of wall.
- The same bag lays 37 modular bricks at 0.148 square feet each, which is 5.5 square feet of wall.
So a square foot of brickwork takes roughly twice the mortar of a square foot of blockwork. The masonry cement figures say the same thing: 40.9 square feet of block against 21.3 of brick per bag, a ratio of 1.9.
The reason is joint length. A block face is six times the area of a modular brick face, so covering a wall in blocks means far fewer units and far less joint to fill. Anyone estimating a brick job based on a block rule of thumb, or the reverse, is off by a factor of two before they start.
Mortar waste is not unit waste
Those yields are bare yields. The Brick Industry Association, in Technical Note 10, says to add at least 5 percent to the masonry unit count and 15 to 25 percent to the mortar. Mortar receives a much larger allowance because of what happens to it on site: droppings off the board, the tail end of a batch that stiffens before it goes down, and what stays in the mixer.
Twenty percent on 100 blocks turns 8 bags of Mason Mix into 10 and leaves the masonry cement figure at 3, because 2.17 bags rounds up to 3 either way. That is the other quiet advantage of the cement and sand route on a small job: the rounding wastes less.
BIA also records the trade version of all this: “Masons frequently use a rule of thumb that eight bags of masonry cement will lay 1000 modular bricks. This is a very rough estimate and includes an unspecified amount of waste.” Quikrete’s own figure works out to 7, so the rule of thumb is the yield plus roughly 15 percent, which is exactly the allowance BIA recommends elsewhere in the same document.
Working out how many bags of mortar per 100 blocks
Bags of mortar
( wall area ÷ face area of one unit ) ÷ units per bag × 1.2
A standard block face is 0.89 square feet, while a modular brick face is 0.148. The mortar waste allowance is 1.2, which falls between the BIA’s recommended 15 and 25 percent range. Round up.
Take a 10 by 8 foot block wall, which is 80 square feet. Divide by 0.89, and it is 90 blocks. Ninety divided by 13 is 6.9 bags of Mason Mix, and adding 20 percent on top makes 9 bags. On the masonry cement route it is 90 divided by 46, which is 2 bags, 2.4 with waste, so 3 bags plus about 570 pounds of sand.
Nine bags against three. Both are correct. The difference is that one is 720 pounds of material, while the other is 210 pounds of cement and a quarter yard of sand you have to source and mix.
Which to choose is not really an estimating question. Pre-blended is consistent, needs no sandpile or proportioning, and costs more per unit laid. Masonry cement is cheaper and gives you control over the mix, and it needs a mixer, somewhere to put the sand, and someone who knows the proportions. Below a few hundred units, the bag is less hassle.
Count the units first
Frequently asked
Eight 80 lb bags of pre-blended Mason Mix, or three 70 lb bags of masonry cement plus about a fifth of a cubic yard of sand. Quikrete rates an 80 lb bag of Mason Mix at 13 standard blocks and one bag of masonry cement mixed with 240 lb of sand at 46 blocks. Add 15 to 25 percent for mortar waste, and the pre-blended figure becomes 10 bags.
Thirteen standard 8 by 8 by 16 blocks for an 80 lb bag of pre-blended Mason Mix, or 46 for a 70 lb bag of masonry cement once it is mixed with 240 lb of sand. The gap is the sand: the pre-blended bag already contains it and the masonry cement bag does not, so it goes much further per bag.
About 28 bags of 80 lb pre-blended Mason Mix at 37 bricks per bag, or 7 bags of 70 lb masonry cement at 144 bricks per bag, plus roughly 0.6 cubic yards of sand. The Brick Industry Association notes the mason rule of thumb as eight bags of masonry cement per 1,000 modular bricks, which is the yield plus an allowance for waste.
About twice as much mortar is used per square foot of wall. An 80 lb bag of Mason Mix lays 13 blocks, covering 11.6 square feet, or 37 modular bricks, covering 5.5 square feet. The reason is joint length: a block face is six times the area of a modular brick face, so a block wall has far fewer units and far less joint to fill for the same area.
Fifteen to 25 percent, which is much more than you add to the units themselves. The Brick Industry Association recommends at least 5 percent on the brick count but 15 to 25 percent on the mortar, because mortar is lost off the board, stiffens before it is used, and stays behind in the mixer. Twenty percent is a reasonable default.