Block Mortar Calculator

How many bags of mortar 100 blocks take

Start with the number every mason knows: a standard 8x8x16 block needs about 21.8 cubic inches of mortar at a 3/8 inch joint, and an 80 lb bag of preblended mortar covers 13 of them. That works out to 7.7 bags for 100 blocks, or 8 bags once you add a 5% waste allowance. Round up and buy 9.

Two published figures bracket that answer. The QUIKRETE Mortar Mix No. 1102 data sheet says one 80 lb bag lays up to 13 standard blocks. Table 2 of CMHA TEK 04-02A, Estimating Concrete Masonry Materials, puts preblended mortar at 16 blocks per 80 lb bag. The gap comes from how each source treats waste, board loss and the extra mortar a real wall soaks up at corners, so treat 13 as the working number and 16 as the best case.

That range is why bag counts on the internet disagree so wildly. A rule of 30 blocks per bag is describing masonry cement, which is only half the mix: you still have to add sand. A rule of 12 blocks per bag is describing a wall with heavy tooling and a lot of waste. Neither is wrong, but only one of them is a bag of preblended mortar.

The joint is the whole calculation

Mortar does not fill the block. It fills the gap between blocks, and that gap is a thin layer on the face shells plus a small vertical band at each end. For a 3/8 inch joint on a standard unit, the bed joint takes 14.65 cubic inches and the two head joints take 7.15, for 21.8 total.

Because the mortar follows the face shells, the width of the block drops out of the math. A 4 inch block and a 12 inch block have the same face size, the same joint length and the same mortar demand. That single fact explains why a 6 inch wall and a 12 inch wall use the same number of bags for the same block count, which surprises people who assume a thicker wall needs a thicker mortar bed. It does not. The bed is on the shells, and the shells are 1.25 inches wide no matter how deep the block is.

What does change the math is anything that adds joint length. Corners, pilasters, bond beams and control joints all interrupt the running bond and add mortar. So does a wall that steps up a slope, because every step adds a fresh bed joint. The calculator takes a block count, so add 2% to 3% to the count for a wall with several corners and let the waste field carry the rest.

Joint thickness moves the bag count fast

Joint thickness is the one number on a block wall that a beginner can get wrong without noticing. A 1/8 inch difference sounds small, but the mortar volume scales with it directly, so the bag count scales with it too.

Code and good practice both point at 3/8 inch for concrete block. That is the thickness that puts the 7-5/8 inch actual height back to the 8 inch nominal module, so eight courses add up to 64 inches instead of 61. Anything much thicker costs mortar and weakens the wall, because a thick joint shrinks more as it cures. Anything much thinner is hard to tool and leaves high spots that make the next course rock.

For a first wall, aim at 3/8 inch and expect to be a little over. The trowel does the measuring, not a ruler, and the block itself varies by up to 1/8 inch from the plant. That is another reason the calculator rounds up to whole bags.

Block size and shape: what changes, what does not

Face size decides how many units a wall needs, and the unit count drives the mortar. Width does not enter into it. The table below holds the joint at 3/8 inch and shows how the bag count per 100 square feet of wall changes with the unit.

Two rows carry the surprise. A half-length unit uses only 14.3 cubic inches of mortar, a third less than a full block, yet a wall of them needs 11.4 bags per 100 square feet instead of 8.7. Half-length blocks double the number of head joints without halving the bed, so the total goes up. Half-height units are worse, at 14.3 bags per 100 square feet, because every extra course adds a full bed joint across the wall.

This matters when you are coursing out a wall to hit a door head or a beam. The last few courses often use half-height units, and that small area can eat half a bag more than the square footage suggests. Order the specials with the rest of the pallet and add one bag to the total.

Worked example: a 100-block garden wall

A 10 by 10 foot single wythe wall in 8x8x16 block, 3/8 inch joints, one pallet of 100 blocks, 5% waste and mortar at $9 a bag.

• Mortar per block: 2 faces x 1.25 in deep x 0.375 in thick x 23.25 in of joint run = 21.8 cubic inches. That joint run is the block face of 15.625 in plus the face height of 7.625 in.
• Mortar for 100 blocks: 2,180 cubic inches, or 1.26 cubic feet
• Bags at 13 blocks per bag: 100 / 13 = 7.7 bags
• With 5% waste: 8.08 bags, so buy 9
• Cost at $9 a bag: $81.00

Nine bags is 720 lb of material, which fits in the back of a pickup and takes two trips if you are loading a car. Buy the whole pallet at once if you can, because bag lots can vary slightly in color and a wall built from two lots can show it.

Now the same wall in half-height units to course out the top: 225 units per 100 square feet means the wall needs 225 blocks and about 15 bags, not 9. That is the kind of jump worth catching before you drive to the yard. The calculator handles it by switching the unit type, and the bag count updates with the block count.

Waste, spillage and the weather

Waste on a block job is not a rounding error. It shows up in four places, and only one of them is obvious.

• Droppings. Mortar pushed out of the joint falls inside the wall and onto the footing. On a hollow wall that can be 3% to 5% of the mix.
• Board loss. Whatever is left in the tub when the batch sets is thrown away. Small batches waste more, because the film on the tub is a bigger share.
• Breakage. Blocks chip at the corners and split when cut. CMHA adds 5% to the unit count for exactly this, and the same allowance covers the mortar that goes with the broken units.
• Weather. Hot, dry and windy conditions pull water out of the mortar and force you to discard stiff batches early. A 95 degree day with a breeze can double the discard rate.

Five percent is the right default for a straight wall on a flat footing. Use 10% for a wall with several corners, a stepped footing, or a first attempt. The extra bag is cheap next to a second trip to the store, and unopened bags keep for months in a dry garage.

One more allowance is worth making on long walls: the first course beds on a full mortar bed rather than a face shell bed, which takes noticeably more mortar. Add about one bag per 30 feet of wall length to cover it.

Mixing water, working time and the retemper rule

An 80 lb bag of preblended mortar wants about 5 quarts of clean water, and 6 quarts is the ceiling. Put most of the water in the tub first, add the dry mix, and let the mixer run for a full 5 minutes. The mix is right when a half inch of mortar sticks to a trowel held nearly vertical without sliding off.

Working life is shorter than most beginners expect. Preblended mortar stays usable for roughly 1.5 to 2 hours in mild weather, and much less in sun or wind. Once it starts to stiffen, the batch is done. Adding water to a batch that has begun to set does not restore it, it just weakens the cement bonds that have already formed, and the wall pays for it later. Mix smaller batches and keep the board covered with a damp towel.

Type matters too. QUIKRETE Mortar Mix meets ASTM C270 and C1714 as Type N, which is right for above grade walls. Load bearing walls, foundations and anything below grade want Type S at 1,800 psi instead of 750. The bag count does not change with the type, only the strength and the price.

Site mix versus bagged mortar

Bagged mortar is the right answer for anything under a few hundred blocks. Above that, site mixing starts to win on cost, and CMHA TEK 04-02A gives the yields to plan with.

The site mix numbers assume face shell bedding and include nominal waste, the same basis as the bagged figures, so they compare directly. One batch of Type S lays 46 blocks, which is 2.2 bags per 100 blocks of mortar materials rather than 7.7 bags of preblended mix, plus the sand. On a 1,000 block job that difference pays for the mixer rental and then some.

Below 300 blocks the arithmetic flips. Sand comes by the ton, portland by the bag, and the leftover pile is a nuisance. Bagged mortar also keeps its proportions exact, which matters more than price when the wall is small and every joint is visible. Run the numbers for your own job, then buy one extra bag either way.