Block Mortar Calculator: How Many Bags Do You Need?

How many blocks does a bag of mortar lay?

The figures below come from CMHA TEK 04-02A, the industry estimating reference. They assume face shell bedding, which is the normal method for hollow concrete masonry.

Two things about this table surprise people.

The first is that block width does not appear anywhere in it. A 6 inch block and a 12 inch block are bedded on their face shells, and the face shells are the same depth on both. The mortar sits on the shells, not across the full width, so the quantity per block is the same. Width changes the grout quantity, not the mortar quantity.

The second is that a stronger mortar does not mean more bags. It means fewer blocks per batch, because the batch itself is smaller. Type M uses a quarter part of lime to one part cement; Type O uses two parts. The Type O batch is larger, so it lays more blocks. That is a difference in batch size, not in how thick the joints are.

What the mortar type actually changes

Mortar type is a specification decision, not an estimating one. Choose the type the project requires, then calculate quantity from that choice.

ASTM C270 sets these minimum 28 day compressive strengths: Type M at 2,500 psi, Type S at 1,800 psi, Type N at 750 psi, Type O at 350 psi.

Higher is not better. Mortar should generally be weaker than the units it bonds, so that movement cracks travel through the joint rather than through the block. A joint can be repointed. A cracked unit cannot.

Some situations remove the choice. Empirical design of foundation walls requires Type M or S. Glass unit masonry requires Type N or S. Seismic Design Categories D, E and F require portland cement lime or mortar cement, Types S or M. Outside those cases, Type N or S covers most block work above grade.

For the full comparison, see the mortar guide for block walls.

Face shell bedding is the basis for these numbers

Every figure on this page assumes face shell bedding: mortar applied to the two face shells of the unit, with the webs left bare.

Full mortar bedding, where mortar covers the webs as well, uses noticeably more material. If your project specifies full bedding, which it may for the starting course on a footing or for solid units, these quantities will run low. Add for it rather than relying on the waste percentage to absorb the difference.

Joint thickness is the other assumption. These figures are based on the standard 3 over 8 inch joint. That joint is not a tolerance allowance, it is the reason nominal and actual block dimensions differ. A nominal 8 by 8 by 16 inch block actually measures 7 and 5 over 8 by 7 and 5 over 8 by 15 and 5 over 8 inches, and the joint makes up the rest. See the cinder block dimensions reference for the full size breakdown.

Site mixed or preblended

Preblended mortar arrives with the cement, lime and sand already proportioned. You add water. It is sold in 80 lb bags and in bulk bags of roughly 3,000 lb, and it is the simpler option for small and medium jobs because there is nothing to proportion on site.

Site mixed mortar means batching portland cement, hydrated lime and sand yourself by volume. It is cheaper per unit of mortar at volume, and it is the only practical option on large jobs, but the mix has to be controlled. A batch that drifts from the specified proportions no longer meets the type it was specified as.

Masonry cement or mortar cement plus sand sits between the two. Mortar cement differs from masonry cement in one important way: it carries a minimum bond strength requirement, which masonry cement does not.

This calculator handles all three routes. For site mixed batches it reports the ingredient volumes in cubic feet, because that is how ASTM C270 and the CMHA estimating tables express them.

Why published bag counts disagree

Run the same wall through several online mortar calculators and the bag counts will not match. The spread is not small.

Some sites publish 13 blocks per 80 lb bag. The CMHA estimating figure is 16. On a 500 block wall that is the difference between 39 bags and 32, a 22 percent overbuy.

The reason is usually that the lower figures come from a manufacturer's conservative bag coverage rather than from an industry estimating table, or that full bedding has been assumed without saying so. Neither is wrong on its own terms. Both are wrong if the page does not tell you which basis it used.

This calculator uses the CMHA TEK 04-02A figures and says so next to every result. If your supplier's bag states a different coverage, use the supplier's number and treat this as the cross check.

Worked example

A 30 foot long, 8 foot high wall of standard 8 by 8 by 16 inch block, face shell bedded, with preblended 80 lb bags, at 5 percent waste.

Wall area is 30 times 8, which is 240 square feet.

A conventional 8 by 16 inch unit face covers 8 over 9 of a square foot, so a wall takes 1.125 units per square foot. That gives 240 times 1.125, or 270 blocks.

At 16 blocks per bag, 270 blocks needs 270 divided by 16, which is 16.875 bags.

Adding 5 percent waste gives 17.72 bags, which rounds up to 18 bags.

Note that the waste allowance is applied to the mortar, not to the block count, because the block count in this example is the actual number of units being laid. If you are also ordering the blocks, apply waste separately to those. CMHA uses 119 units per 100 square feet as the block figure including a 5 percent allowance, against 112.5 without.

What this calculator does not cover

Special units are not included. Pilaster, corner, bond beam and lintel units are estimated separately and subtracted from the stretcher count. Neither are collar joints between wythes, parging, or repointing existing work, all of which consume mortar on a different basis.

Grout is a different material and a different calculation entirely. Grout fills the cells; mortar bonds the units. If you are filling cores, use the block fill calculator. For block quantities, use the cinder block calculator.

Sources

CMHA TEK 04-02A, Estimating Concrete Masonry Materials, mortar and unit estimating tables.

CMHA TEK 09-01A, Mortars for Concrete Masonry, ASTM C270 mortar types, property requirements and code restrictions.