Mortar calculator

Published answers for this differ by 3.7 times

Search how many bags of mortar 100 blocks takes and you will be told 2.25, 3, 7 and 8.3. They are not rounding errors of each other. They are two different products with different yields, and the bag is the only place that says which one you have.

Both figures are printed by the same manufacturer:

So 100 blocks is 8 bags of one and 3 bags of the other. 46 ÷ 13 = 3.5, 144 ÷ 37 = 3.9, and the range of published answers is 3.7 — the spread is the two products, with nothing else left to explain.

The reason is not subtle once you see it. A preblended bag is mortar: binder, lime and sand, complete. A bag of masonry cement is binder only. Its own yield sentence says so — one bag "and 240 pounds of Mason's Sand will yield approximately 2.75 cu ft of mortar". You are comparing a bag of finished mortar against a bag of the ingredient that makes four bags of it.

Which is right for you: under about 30 bags, preblended, every time — you are buying consistency and one trip. Above that, masonry cement plus sand is dramatically cheaper and is what a mason on the job will be using. The wall does not care; both meet ASTM C 270.

The conditions our number holds under

State these and the figure stops being folklore:

• 80 lb bag of preblended mix, or a 70 lb bag of masonry cement with the full 240 lb of sand. Neither manufacturer publishes a yield for any other bag size, so this page does not scale one by weight.
• Standard 8 × 8 × 16 in block, or standard brick. Nothing else is published.
• The word on the sheet is "up to" — it assumes clean 3/8 in joints and face-shell bedding on block. Full beds and a first course levelled on a thick bed use more.
• No waste. The allowance above is yours to set.

Worth knowing how optimistic "up to" is. Cross the masonry cement yield with the theoretical figures in BIA Technical Note 10, Table 4: BIA puts 9.5 cu ft of mortar in 100 sq ft of standard brickwork at 655 brick, which is 25 cu in per brick. The 2.75 cu ft that lays 144 bricks works out at 33 cu in — about a third more. BIA says outright that its numbers exclude waste. That gap, roughly 30%, is the difference between joint volume on paper and mortar leaving a mixer, and it is a reasonable sanity check on any waste allowance you set here.

Do not put your block waste allowance through the mortar sum

The expensive mistake, and an easy one. Your unit count carries a breakage allowance — 5% or so for block that arrives cracked and block you cut in half. Neither consumes a full unit's worth of joint; a cut block uses less mortar.

So the count you type above is the units going into the wall, and the waste field here is a separate allowance on the mortar itself. Feed an already inflated block count into a mortar divisor and you pay the waste twice.

Mixing it yourself

Site-mixed mortar is cement, hydrated lime and sand batched by volume to the ASTM C 270 proportion specification: per 1 part portland cement, lime is 1/4 part for Type M, over 1/4 to 1/2 for Type S, over 1/2 to 1-1/4 for Type N, over 1-1/4 to 2-1/2 for Type O. Sand is 2-1/4 to 3 times the combined cement and lime volume, damp and loose. A sack of portland cement is 94 lb and one cubic foot, which is what makes those ratios work with a bucket. Mix to proportion and no testing is required; mix to the property specification instead and the 28-day minimums apply — M 2500, S 1800, N 750, O 350 psi.

Type N is the general-purpose choice above grade, Type S below grade and where the wall is reinforced. Stronger is not better: mortar harder than the units pushes cracking into the brick instead of the joint. Preblended mix takes about 5 quarts of water per 80 lb bag, and any mortar has to be placed within 2-1/2 hours of the original batch.

The jobs this page cannot size, and why

Everything above is one operation: a count of units laid, divided by a published units-per-bag yield. That is the entire model, and it is why whole categories of masonry work have no route through this page — there is no count to divide.

Stone veneer and repointing. No manufacturer publishes a yield for either — veneer depends on stone thickness and how much of the back you butter, repointing on how deep you rake out. Anyone quoting square feet per bag for those has estimated, not measured.

Mortar spread as a coat or a bed rather than a joint. Parging a foundation, a scratch coat, a setting bed under stone or tile, a chimney crown. Those are a thickness over an area — a volume — and neither of the two yields above is expressed in a way that can answer them. Nor can the unit count see them: a first course levelled on a deliberately thick bed is still one course of units here, and the extra mortar has to come out of the waste allowance you set.

Filling the cores of a block wall. That is grout, a different material with a different consistency, poured rather than laid, and it is not in either yield. The volume depends on your specific unit's core dimensions, which the manufacturer publishes and we do not guess at — the same reason the concrete block calculator leaves it out.

Anything that is not a masonry mortar. Both products above are ASTM C 270 / C 91 masonry mortars for laying units. A firebox, a flue liner or a fire pit interior is refractory work and is outside every figure on this page.

Two of the outputs also stay put when the job goes to zero, on purpose. Units one bag lays and bags per 100 units are properties of the sack and the unit, not of your wall, and the site-mix rows are a ratio — they read the same with nothing entered so you can check them against what is in front of you.

Four ways this answer can be confidently wrong

The bag count is only as narrow as the yield behind it, and that yield describes two units made by one manufacturer:

• Your unit is probably not one of the two published ones. The dropdown offers standard block and standard brick because those are what the yields name. A queen, a utility, a half-high block, a 12 in block or a split-face unit is a different amount of joint per unit — and the direction is not obvious, because bigger units mean fewer, larger joints and less mortar per square foot, not more. For anything other than these two, work from wall area on the brick calculator, which carries a mortar volume per profile.
• The yields are one manufacturer's. Another brand's preblended bag, or its masonry cement, can publish different figures and a different sand weight. The point of this page is that you read the sack; that includes reading whose it is.
• The recipe is a ratio, not an order. The site-mix outputs are parts per 1 cu ft of portland cement, straight from the proportion specification. They do not scale with the unit count and there is no path on this page from your wall to a number of sacks of cement. Batch to the ratio, mix what you can place, and note that the specification means sand damp and loose — measuring it dry, or compacted in the barrow, is a different batch from the one specified.
• Nothing cross-checks the type against the bag. Select Type S in the recipe field while buying a Type N bag and the page reports both without complaint. They are two independent questions here, and only you can see the sack.