Why Masonry Calculators Disagree

The problem

Put the same wall through five masonry calculators and you will get five answers. For small jobs the spread is an annoyance. On a 500 block wall it is the difference between one delivery and two, or between a half empty pallet and a shortfall on a Friday afternoon.

Some of the spread is legitimate. Some of it is not. The difficulty is that almost no calculator tells you which basis it used, so you cannot tell a defensible difference from an error.

This page sets out four figures where published values diverge, what each basis actually measures, and which one to order by. Every number here is traceable to a named standard or a manufacturer's own published data sheet. We do not name the sites that publish the outlying figures, because the point is not who is wrong, it is how to tell.

1. Grout volume per block

How much grout does one 8 inch concrete block hold?

The two defensible figures are close together, and that proximity is the point.

The geometric figure comes from the unit's own dimensions: two face shells and three webs at the ASTM C90 minimum thicknesses, subtracted from the specified unit volume. Because C90 minimums produce the thinnest permissible shells, this calculation yields the largest cell volume the standard allows. It is a theoretical ceiling, carrying no field waste.

The CMHA figure comes from TEK 09-04A, Table 2, the industry estimating table. It is higher because it includes a waste allowance and accounts for overfill into face shell bedded joints.

Order to the CMHA figure. Understand the geometric one, because it tells you the floor beneath it.

The figures below those two cannot be reconciled with either basis. A published value of 0.014 cubic feet per block is roughly 23 times below the CMHA figure. There is no bedding method, waste assumption or unit configuration that produces it.

2. Grout quantity changes with spacing, not just wall size

The largest single error in grout estimating is not the per block figure. It is assuming every cell gets filled.

Most reinforced residential block walls are partially grouted, meaning only the cells containing reinforcement are filled. Reinforcement spacing is set by the structural drawings, commonly between 16 and 48 inches on center.

At 8 inch wall thickness the spread between filling all cells and filling at 48 inches on center is 5.9 times. A calculator that does not ask for grout spacing cannot be right for a partially grouted wall except by coincidence.

Bond beams are the exception in the other direction. They are fully grouted regardless of the wall's vertical spacing, and CMHA Table 2 excludes them, so their volume is added on top of whatever the table gives.

3. Mortar coverage per bag

How many blocks does one 80 pound bag of mortar lay?

Here the disagreement is entirely legitimate, and it is the clearest example of why the basis matters more than the number.

CMHA publishes a trade average across the industry for face shell bedding. Manufacturers publish a yield for their own specific product, set deliberately on the conservative side so a buyer does not run short mid course. Neither is an error. They are answering slightly different questions.

Order to the data sheet printed on the bag you are actually buying. Use the CMHA figure as a sanity check on the total.

One detail that follows from this: mortar type does not change the bag count. QUIKRETE publishes identical yields for its Type S Mason Mix and its Type N Mortar Mix. The type changes the proportions inside the bag, not how much mortar a joint holds.

4. ASTM C90 minimum compressive strength

The current minimum net area compressive strength for loadbearing concrete masonry units under ASTM C90 is 2,000 psi. The figure of 1,900 psi still circulates widely and is superseded.

This one matters less for estimating and more for what it reveals. We found a site publishing both figures, on two different pages, without acknowledging the difference. That is what happens when reference content is assembled from other reference content rather than from the standard.

How to read any masonry calculator

Four questions will tell you whether a result is usable.

Which basis? Industry estimating table, geometric calculation, or manufacturer data sheet. If the page does not say, you cannot check the answer.

Does it ask for grout spacing? If it only asks for wall dimensions and thickness, it is assuming full grouting, and for most reinforced walls that is wrong by a factor of three to six.

Does waste get applied once? Estimating tables usually include a waste allowance already. Adding your own on top double counts it.

Is the source named next to the number? Not in a footer, not in a disclaimer, next to the figure it produced. A constant you cannot trace is a constant you cannot check.

Our sources

Every figure on this page comes from one of the following. All are freely available.

CMHA TEK 09-04A, Grout for Concrete Masonry, Table 2, grout volume estimation. CMHA TEK 04-02A, Estimating Concrete Masonry Materials, unit and mortar estimating. CMHA TEK 09-01A, Mortars for Concrete Masonry, ASTM C270 mortar types. CMHA CMU-TEC-001-23, Concrete Masonry Unit Shapes, Sizes, Properties, and Specifications, Table 2, reproducing ASTM C90 physical requirements. QUIKRETE and Sakrete product data sheets, published yields and mixing water.

Every constant used anywhere on this site is listed on the sources page with its origin, generated from the code itself so it cannot drift from what the calculators actually use. The methodology page sets out how each tool derives its result.

If you find a figure here you believe is wrong, we want to know. Corrections go to the address on the contact page and we publish what changes.