The job
Modelled, not recorded. Every figure here is the answer the Mortar Mix Calculator (ASTM C270 Types M, S, N, O) gives for these inputs, written down in both unit systems and re-run whenever the site is built. No job was carried out, so there is no measured overrun on this page.
The job is the mortar for a run of brick veneer laid by a small crew who mix on site in a drum mixer, which is how mortar is bought on any job bigger than a pallet of premixed bags. The volume, 20 ft³ (0.57 m³), is the mortar the wall needs as the brick count worked it out; this calculator starts where that one stops, turning a volume of mortar into the materials to make it.
The mortar is Type N — general purpose, exterior above grade, the general-purpose type for exterior work above grade. The calculator takes the proportion specification of ASTM C270, which gives each type as parts by volume of portland cement and hydrated lime to sand, uses the middle of each type's lime range as the nominal mix, and holds the sand between the standard's bounds of two and a quarter and three times the binder.
The sand ratio is left at 3 parts of sand to one part of binder, the top of the standard's range and the leanest mix it allows, and the allowance at the calculator's 10 per cent, for mortar dropped from the board and left in the mixer at the end of the day.
The inputs, exactly as entered
| Input | Value | Why |
|---|---|---|
| Mortar needed | 20 ft³ (0.57 m³) | The mortar the wall needs, taken from the brick count's own mortar estimate rather than guessed, entered in cubic feet. |
| Mortar type | Type N — general purpose, exterior above grade | Left at the calculator's own default. |
| Sand per part of binder | 3 | Left at the calculator's own default. |
| Waste allowance | 10 | Left at the calculator's own default. |
The working, step by step
1. Mortar after waste
22 ft³ (0.62 m³)
The allowance goes on the volume first: 22 ft³ of mortar to make. Everything after this step is a share of that figure, so a change to the allowance moves every material in proportion.
2. Sand
22 ft³ (0.62 m³)
By the convention the calculator states in its sources, the wet mortar occupies the same volume as its sand, because the cement and lime paste fills the spaces between the grains rather than adding to them. So the sand is 22 ft³, equal to the mortar, and that is the headline. The convention moves with the sand's grading, which is why a mason checks the first batch against the board.
3. Cement
3.91 ft³ (0.11 m³)
The binder is the sand divided by the sand ratio, and the portland cement is its share of that binder for Type N: 3.91 ft³. Cement is the expensive and the decisive ingredient, so it is the one to measure by the sack rather than by the shovel.
4. Hydrated lime
3.42 ft³ (0.1 m³)
The rest of the binder is hydrated lime, 3.42 ft³, at the middle of the range ASTM C270 allows Type N. The lime is what makes the mortar workable on the trowel and gives it some give as the wall moves; a richer lime mix is weaker and more forgiving.
5. 94 lb sacks (1 ft³ each)
4 sacks
A US sack of portland cement is a cubic foot by definition, as the calculator's last source notes, so the cement volume rounded up is 4 sacks. That exact relationship is why US mortar recipes are written in shovels per sack.
Sand needed
22 ft³
0.623 m³
Buy 4 sacks of portland cement, hydrated lime for 3.42 ft³, and 22 ft³ of masonry sand, which is 0.623 m³. Sand is bought by the ton or the yard from a yard and the lime in bags, so both are rounded up at the counter; the calculator gives the volumes to round. Masonry cement or mortar cement in a single bag replaces the cement and lime together, and is counted from its own bag's instructions rather than from these proportions.
What to buy
| Quantity | Figure | What it is for |
|---|---|---|
| Sand needed | 22 ft³ (0.623 m³) | Washed masonry sand for the whole batch, allowance included. |
| Cement | 3.91 ft³ (0.11 m³) | Portland cement for the binder; bought by the sack, as the row below counts it. |
| Hydrated lime | 3.42 ft³ (0.1 m³) | Hydrated lime for masonry mortar, bought in bags; read the bag's volume or weight against this figure. |
| 94 lb sacks (1 ft³ each) | 4 sacks | Whole sacks of portland cement, each a cubic foot. |
The allowance, and what it does here
The 10 per cent goes on the mortar volume before any material is split out, so it reaches the sand, the cement and the lime alike. It covers mortar dropped from the board, the last of a batch that stiffens before it is used, and what is left on the mixer's drum. It does not cover a wall built with fuller joints than the brick estimate assumed, which is a difference in the volume itself and belongs in the first input.
The common mistake
Adding the cement and lime on top of the sand
A recipe of one part cement, some lime and three of sand looks as if it should make more mortar than the sand alone, and a mixer who adds the parts together orders about a quarter too little sand for the volume wanted, or makes far more mortar than the wall needs. The binder paste mostly fills the gaps between sand grains, so the batch comes out close to the volume of its sand. Size the sand from the mortar volume, as the calculator does, and work the cement and lime from the sand.
Change the job, and see what moves
The same volume, in Type S
Mortar type: Type S — structural, exterior at or below grade → 22 ft³ (0.623 m³)
Mix Type S — structural, exterior at or below grade instead, for work at or below grade, and the sand stays at 22 ft³, but the cement rises to 5.33 ft³ and the lime falls to 2 ft³: 6 sacks. A stronger mortar is a richer one, not a bigger one. Which type a wall needs is set by its specification and the code, not by preference: a mortar stronger than the brick is not better.
The same mortar, with less sand per part of binder
Sand per part of binder: 2.25 → 22 ft³ (0.623 m³)
Take the sand ratio down to 2.25 parts, the bottom of the standard's range, and the sand still matches the mortar at 22 ft³; the binder grows instead, to 5.21 ft³ of cement and 4.56 ft³ of lime. A richer mix is stickier and stronger, and it takes 6 sacks of cement for the same volume of mortar.
What to run next
- Brick Calculator
The mortar volume a wall needs comes from its brick count and joint, which this calculator works out first.
- Lime Mortar Mix Calculator (NHL and Lime Putty)
For older soft brick, a lime mortar without portland cement is often specified instead, with its own proportions.
- Tuckpointing Mortar Repair Volume Calculator
Repointing an existing wall needs a much smaller volume, worked from the joint depth and length rather than from the bricks.
Sources
The method and every constant behind these figures belong to the Mortar Mix Calculator (ASTM C270 Types M, S, N, O), version 1.0.0, last verified 2026-09-15. It rests on:
- Proportions are the ASTM C270 PROPORTION specification, parts by volume per 1 part portland cement: Type M 1/4 lime, Type S over 1/4 to 1/2, Type N over 1/2 to 1 1/4, Type O over 1 1/4 to 2 1/2. The midpoint of each band is used as the nominal and both ends are shown
- Aggregate is not less than 2 1/4 and not more than 3 times the sum of the cementitious volumes, which is why the sand ratio is an input bounded at those limits rather than a fixed figure
- Wet yield is taken as the sand volume, because the binder paste occupies the voids between sand grains rather than adding to them — a trade convention stated here rather than a measured property, and it moves with the sand's grading
- A US sack of portland cement is 94 lb and one cubic foot by definition, so that bag volume is exact. The 25 kg metric bag volume assumes a loose bulk density near 1,500 kg per cubic metre and is an assumption, not a definition
Frequently asked questions
- How much sand, cement and lime for 20 ft³ of Type N mortar?
- With the calculator's 10 per cent allowance: 22 ft³ of sand, 3.91 ft³ of portland cement (4 sacks) and 3.42 ft³ of hydrated lime, at 3 parts of sand to one of binder.
- Why is the sand the same as the mortar volume?
- The cement and lime paste fills the voids between the sand grains, so the mixed mortar comes out close to the volume of its sand. The calculator takes that as its convention, and says so in its sources.
- How much more cement does Type S need?
- For the same volume, 5.33 ft³ against 3.91 ft³ for Type N, with less lime; the sand is unchanged at 22 ft³.