Where mortar actually goes in a wall
Mortar occupies the joints, and nothing else. In running bond every unit sits in a repeating rectangular cell measuring (L + j) by (H + j), where L and H are the specified unit dimensions and j is the joint. Subtract the unit's own face area from that cell and what is left is the joint area: one bed joint, one head joint, and the small square where the two meet. Multiply by the depth the mortar is bedded to and you have the volume for one unit.
That bedded depth is the fork in the road. Solid brick is bedded full, so the depth is the whole width of the unit — 3⅝ in for a modular brick. Hollow concrete units are normally face-shell bedded, meaning the mortar sits only on the two shells at the front and back of the unit. On an 8 in block with 1¼ in shells that is 2½ in of the 7⅝ in width, so face-shell bedding uses about a third of the mortar full bedding would. TMS 602 permits face-shell bedding for hollow units but requires full bedding at the starting course on the foundation and around cells that are to be grouted.
The consequence is arithmetic you can check on the back of a mortar board: 8.06 ft³ of mortar per 1,000 modular brick laid full-bed at 3/8 in, and 1.28 ft³ per 100 face-shell bedded 8 in block. Those are the geometric floors. What you actually buy is higher, and the next section explains why.
ASTM C270 types and what the proportions mean
ASTM C270 is the specification for mortar for unit masonry, and it lets you specify a mortar in either of two ways: by proportion or by property. The proportion specification is the one masons work from, and for cement-lime mortars it fixes the parts of portland cement to hydrated lime and then sets the aggregate at not less than 2¼ and not more than 3 times the sum of the cementitious volumes. Working at the upper limit of 3 gives the familiar recipes: Type M is 1 : ¼ : 3¾, Type S is 1 : ½ : 4½, Type N is 1 : 1 : 6, and Type O is 1 : 2 : 9.
Read those left to right and you can see what changes: as you move from M to O, the lime share rises and the cement is spread through more sand. More cement gives more compressive strength and less workability; more lime gives better workability, better water retention, and a mortar soft enough to accommodate small movements and to be repointed without damaging the units. That trade-off — not "stronger is better" — is how you choose a type. A hard Type M mortar in a soft historic brick wall will pull the faces off the brick, because the mortar joint is supposed to be the sacrificial element.
In broad practice: Type N is the general-purpose above-grade choice, Type S is used where more flexural bond strength or below-grade durability is wanted, Type M is reserved for masonry in contact with earth, and Type O is limited to interior, non-load-bearing work. Where a project specification names a type, use that one — the choice interacts with unit strength and with the assumed f′m of the assembly.
Worked example: 500 8 in block, face-shell bedded, Type S
Five hundred nominal 8 × 8 × 16 concrete masonry units, 3/8 in joints, face-shell bedded, Type S mortar, with a 25% waste allowance.
- Cell area. (15.625 + 0.375) × (7.625 + 0.375) = 16.000 × 8.000 = 128.000 in².
- Joint area. 128.000 − (15.625 × 7.625) = 128.000 − 119.141 = 8.859 in².
- Bedded depth. Two face shells at the ASTM C90 minimum of 1.25 in = 2.500 in.
- Mortar per block. 2.500 × 8.859 ÷ 1,728 = 0.012817 ft³.
- For 500 block. 0.012817 × 500 = 6.409 ft³; with 25% waste, 8.01 ft³.
- Sand. Yield is taken as the damp loose sand volume, so 8.01 ft³ of sand — about 0.40 tons at 100 lb/ft³, or roughly a third of a cubic yard.
- Portland cement. Type S puts 4.5 parts of sand to 1 part of cement, so 8.01 ÷ 4.5 = 1.78 ft³ = 1.78 bags of 94 lb portland.
- Hydrated lime. Half the cement volume: 0.5 × 1.78 = 0.89 ft³. A 50 lb bag of lime is 1.25 ft³, so 0.71 bags.
- Or buy it pre-blended. 8.01 ÷ 0.60 ft³ per bag = 14 bags of 80 lb mortar mix.
Sanity-check the cement figure against the batch: 1.78 bags of portland, 0.89 ft³ of lime and 8.01 ft³ of sand is 1 : 0.5 : 4.5 by volume, exactly the Type S proportion. That is the check worth doing whenever a mix looks wrong on the board.
How to read the result and how much to actually order
The volume this calculator reports is joint geometry plus your allowance, and the geometry is the smaller part of what a wall consumes. Mortar is furrowed and squeezed out, it falls off the board and off the trowel, the starting course on the footing is bedded full even on hollow units, cells that will be grouted are bedded full, joints are struck and sometimes repointed, and a batch that stiffens past its working life goes in the barrow. That is why the default allowance here is 25% rather than the 5% you would use on units — and why long-standing masons' rules of thumb for bags per hundred block sit well above the geometric figure.
Order sand by the cubic yard: 27 ft³ to the yard, so the 8 ft³ in the worked example is under a third of a yard, and no supplier will deliver that. On small jobs buy bagged sand or pre-blended mix; the break-even against a bulk load and a mixer is usually somewhere in the low hundreds of block. Order cement and lime whole: bags do not keep well once opened, and lime in particular carbonates.
Water is the one number to treat loosely. Mortar is mixed to a workability the mason judges by feel — C270 does not fix a water-cement ratio, and retempering with water during the working life of a batch is normal and permitted. The figure here is a starting point in the mixer, roughly a gallon and a quarter for each 80 lb bag of pre-blended mix. Add the last of it slowly.
ASTM C270 cement-lime proportions and what they yield
| Type | Portland : lime : sand | Mortar per 94 lb bag of portland (ft³) | Portland per ft³ of mortar (lb) | Lime per ft³ of mortar (lb) | Typical use |
|---|---|---|---|---|---|
| M | 1 : ¼ : 3¾ | 3.75 | 25.1 | 2.7 | Masonry in contact with earth |
| S | 1 : ½ : 4½ | 4.50 | 20.9 | 4.4 | Load-bearing walls, at or below grade |
| N | 1 : 1 : 6 | 6.00 | 15.7 | 6.7 | General above-grade work, veneer |
| O | 1 : 2 : 9 | 9.00 | 10.4 | 8.9 | Interior, non-load-bearing only |
Cement weight per cubic foot of mortar is 94 lb divided by the yield; lime is 40 lb per cubic foot times the lime part divided by the yield.
Masonry cement and mortar cement are different products
The proportions above are for cement-lime mortar mixed from portland cement, hydrated lime and sand. ASTM C270 also recognises mortars made with masonry cement (ASTM C91) and mortar cement (ASTM C1329), which are proprietary blends already containing the plasticiser, and which are proportioned differently — typically one bag of the blended cement to 2¼–3 volumes of sand. Do not add lime to a masonry cement mix, and do not assume the strength classes are interchangeable: mortar cement is required where higher flexural bond strength is specified, and some codes restrict masonry cement in high seismic design categories.
Mistakes that leave you short or leave you with a bad wall
- Estimating from joint geometry alone. The geometric volume is a floor. Board loss, the full-bedded starting course and repointing all take material the joints never see.
- Using face-shell bedding everywhere on a block wall. The base course and every grouted cell are bedded full under TMS 602, so real consumption is between the two figures this calculator gives.
- Choosing the strongest mortar available. Mortar should be the sacrificial element. A hard mortar in a soft or historic unit transfers movement into the units and spalls their faces.
- Adding lime to masonry cement. It is already plasticised. Adding lime changes the proportion the product was designed and tested at.
- Measuring sand by the shovel. Shovel counts vary with the mason and with how damp the sand is. Batch by box or by bucket if the mix matters.
- Ignoring bulking in damp sand. Sand at a few percent moisture occupies noticeably more volume than dry sand, which is exactly why the proportion specification is written for damp loose sand.
- Retempering after the working life. Adding water to bring back a batch that has begun to set does not restore it; it produces weak mortar. Discard it.
Where this sits in a masonry takeoff
Mortar is the third line on a masonry estimate, after the units and before the grout. Get the unit count first from the concrete block calculator or the brick calculator — both deduct openings and add waste, and both report a mortar figure using the same geometry as this page. Then come back here to split that volume into cement, lime and sand, or into bags.
Grout is a separate material and a separate order. It fills the cells around vertical reinforcement, it is specified by ASTM C476, and its volume dwarfs mortar on any reinforced wall. The bars themselves come out of the rebar spacing calculator and their tonnage out of the rebar weight calculator. Below the wall, the footing volume comes from the concrete footing calculator.
Two limits to state plainly. First, this calculator gives quantity and proportion; it does not verify that a mortar meets the property requirements of ASTM C270, which is a laboratory matter. Second, mortar type is part of a structural assembly's assumed strength, so on any engineered wall the specification governs and the mason's preference does not.