Why is a stronger mortar not a better mortar?

Because the mortar is meant to be the weakest part of the wall, and making it strong moves the damage somewhere you cannot repair. Masonry moves — thermally, with moisture, with small settlements — and that movement has to go somewhere. A mortar weaker than the units cracks at the joint, which is a repointing job. A mortar stronger than the units cracks the UNITS, which is a rebuild, and on a soft historic brick it also traps moisture and spalls the faces. That is why Type O exists and why repointing an old soft wall in Type M or S does real damage rather than none. The rule that survives across markets is the same one British mortar designations encode: choose the weakest mortar that will do the job, not the strongest available.

What does the lime actually do?

Three things that cement cannot, which is why it is in the proportion table rather than being an optional extra. It makes the mortar WORKABLE — plastic and sticky enough to stay on a trowel and to squeeze into a joint without falling out — and workability is what lets a bricklayer get a full bed rather than a hollow one. It holds WATER against the suction of a dry brick, so the cement can hydrate instead of being drained before it sets. And it gives the set mortar some capacity to heal: lime slowly recarbonates, and fine cracks in a limey mortar close over time in a way a straight cement mortar's do not. The trade-off is strength and set speed, which is exactly the axis the four C270 types are arranged along.

Why does the calculator say the sand volume is the mortar volume?

Because the binder does not add to the pile, it fills the gaps in it. Loose sand is roughly a third voids, and the cement and lime paste occupies those voids rather than sitting between the grains and pushing them apart. So a batch made with a cubic metre of sand yields close to a cubic metre of mortar, and adding the three component volumes together — sand plus cement plus lime — over-states the yield by something like half. It is the same reason a concrete mix's yield is not the sum of its ingredients. The convention is close enough for ordering and is stated on this page rather than hidden, because sand grading moves it: a well-graded sand has fewer voids and yields a little more, a single-sized sand a little less.

How does an ASTM type map onto a British mortar designation?

Loosely, and the mapping is by intent rather than by equivalence. British practice numbers mortar designations from (i) — the strongest, cement-rich — down to (iv) and (v), which are lime-rich and soft, and recent standards specify mortar by compressive strength class instead. The ordering corresponds: an ASTM Type M sits near a designation (i), Type S near (ii), Type N near (iii), Type O near (iv). What does not correspond is the arithmetic, because the proportion tables differ and the sand ratio is expressed differently, so converting a specification by looking up the nearest row produces a mortar that is approximately right and provably nothing. A specification written against one standard should be batched against that standard.

Materials & Quantities

Mortar Mix Calculator (ASTM C270 Types M, S, N, O)

Batch quantities of cement, lime and sand for a mortar volume, by ASTM C270 type, using the proportion specification — with what each type is for.

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Every formula citedNamed published standards
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SettingsSettings for this calculationUS

Market

Imperial · sales tax

Who is doing the work?

The standard allowance most suppliers and estimating guides assume for ordinary work.

Mortar needed

The volume of wet mortar the job needs.

Take it from a bricklaying or blockwork quantity rather than guessing: mortar volume per square metre of wall depends on the unit size and the joint thickness, and a 10 mm joint in blockwork is a very different quantity from the same joint in brick.

Mortar type

The ASTM C270 type, which sets how much lime goes in per part of cement.

More lime means a weaker, more workable and more forgiving mortar. Stronger is not better: mortar should be weaker than the units it beds, so that movement cracks the joint rather than the brick, and a joint is repointable where a cracked brick is not.

Sand per part of binder

Volumes of sand per combined volume of cement and lime.

ASTM C270 allows 2 1/4 to 3, and the bounds here are those limits. Three is the usual working figure and the most economical; dropping toward 2 1/4 gives a richer, stickier mortar that some bricklayers prefer in cold weather or with very absorbent units.

Waste allowance

Percentage added for droppings, board waste and part batches.

Mortar has a working life of a couple of hours and cannot be retempered indefinitely, so the last of a batch is often thrown. Ten per cent covers ordinary walling; raise it for pointing and for small intricate work where each batch is part-used.

Sand needed

38.5 ft³

Medium confidence

Type N: 1 part cement to 0.875 lime to 3 sand, by volume. The standard allows 0.5 to 1.25 parts lime for this type — use the ratio your specification names if it gives one. Suits most above-grade exterior and interior load-bearing walls, and the usual default. This is the proportion specification and does not certify a strength.

Cement: 6.84 ft³
Hydrated lime: 5.99 ft³
Sand: 38.5 ft³
94 lb sacks (1 ft³ each): 7 sacks
Mortar after waste: 38.5 ft³

Then change the inputs to see how far the answer moves.

Show calculation logic

How this was calculated

Formula source(s)

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

Inputs used

Mortar needed: 35 ft³
Mortar type: Type N — general purpose, exterior above grade
Sand per part of binder: 3
Waste allowance: 10

Intermediate steps

Cement: 6.84 ft³
Hydrated lime: 5.99 ft³
Sand: 38.5 ft³
94 lb sacks (1 ft³ each): 7 sacks
Mortar after waste: 38.5 ft³

Final result38.5 ft³

Confidence note: Type N: 1 part cement to 0.875 lime to 3 sand, by volume. The standard allows 0.5 to 1.25 parts lime for this type — use the ratio your specification names if it gives one. Suits most above-grade exterior and interior load-bearing walls, and the usual default. This is the proportion specification and does not certify a strength.

What this calculation does not cover

This is the PROPORTION specification of ASTM C270 and it certifies no strength. A mortar batched to these volumes has not been tested, and the compressive strengths in the standard's property table belong to laboratory specimens prepared a different way. Never state that a proportion-batched mortar meets a property specification.
Yield is taken as the sand volume, which is a trade convention rather than a measured property. Sand grading moves it: a well-graded sand has fewer voids for the paste to fill and yields slightly more than its own volume, a single-sized sand rather less.
Sand is assumed batched DAMP and by volume, as it is on site. Damp sand bulks by a fifth to a third, so a volume batch of dry sand puts materially more sand into the mix than intended — which is a weaker mortar, not a stronger one.
The lime figure is the midpoint of the band the standard allows for that type. Where a specification names a ratio, use theirs; the band is wide for Types N and O and the ends behave noticeably differently.
Masonry cement and mortar cement are not covered. Both replace the separate cement-and-lime pair with a single bagged product, their C270 proportion rows are different, and substituting one for the other at these volumes gives the wrong mix.
Nothing here addresses colour, admixtures, or matching an existing mortar. Repointing historic masonry is a different problem in which a cement-based mortar of any C270 type may be the wrong answer entirely.

Estimated cost — your price

Price per ft³ ($)

This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.

Part of a bigger job

This trade is one line of a job takeoff. Run the whole job and every other trade comes back with it, off the same measurements.

Repointing brickwork (tuckpointing)Check a quote
also measuresScaffold Face Area and Hire Quantity Tuckpointing Mortar Repair Volume Lime Mortar Mix (NHL and Lime Putty)Caulk & Sealant

Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Sources checked 2026-09-15 · v1.0.0

Regulatory standards & verification citations4

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

Which documents these citations point at

Standards referenced: ASTM C270 (ASTM International, United States).

Cite this page

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Tools and safety for this job

To mix adhesive or mortar and mix mortar. Generic types, no brands, no prices.

Protection this work requires

Cutting or grinding concrete, masonry, screed, tile or fibre-cement board releases respirable silica: cut wet or extract at the tool, and wear a P2/N95 respirator at minimum — a nuisance dust mask does not filter it.
Wet cement and lime burn skin and eyes painlessly until the damage is done: waterproof gloves to EN 374, safety glasses whenever the mix can splash, and never kneel in wet mix in permeable trousers.

Show the 5 tools this job needs

Essential

Mixing bucket or tub
Paddle mixer
Shovel and spade
Wheelbarrow

Recommended

Cement mixer

what each tiling tool is for and what each masonry tool is for, and the spec that decides which to buy where one does.

Worked example: Brick garden wall — step 5 of 8

How to calculate mortar mix (ASTM C270 types M, S, N, O) in 5 steps

Mortar neededThe volume of wet mortar the job needs.
Mortar typeThe ASTM C270 type, which sets how much lime goes in per part of cement.
Sand per part of binderVolumes of sand per combined volume of cement and lime.
Waste allowancePercentage added for droppings, board waste and part batches.
Sand neededThe tool computes the sand needed from those figures and shows the formula, its sources, and a confidence rating alongside it.

Sand needed by mortar needed

Page defaults, not your figures above.

Mortar needed	Sand needed (ft³)
20 ft³	22
30 ft³	33
40 ft³	44
50 ft³	55
60 ft³	66
70 ft³	77

Frequently asked questions

Why is a stronger mortar not a better mortar?: Because the mortar is meant to be the weakest part of the wall, and making it strong moves the damage somewhere you cannot repair. Masonry moves — thermally, with moisture, with small settlements — and that movement has to go somewhere. A mortar weaker than the units cracks at the joint, which is a repointing job. A mortar stronger than the units cracks the UNITS, which is a rebuild, and on a soft historic brick it also traps moisture and spalls the faces. That is why Type O exists and why repointing an old soft wall in Type M or S does real damage rather than none. The rule that survives across markets is the same one British mortar designations encode: choose the weakest mortar that will do the job, not the strongest available.
What does the lime actually do?: Three things that cement cannot, which is why it is in the proportion table rather than being an optional extra. It makes the mortar WORKABLE — plastic and sticky enough to stay on a trowel and to squeeze into a joint without falling out — and workability is what lets a bricklayer get a full bed rather than a hollow one. It holds WATER against the suction of a dry brick, so the cement can hydrate instead of being drained before it sets. And it gives the set mortar some capacity to heal: lime slowly recarbonates, and fine cracks in a limey mortar close over time in a way a straight cement mortar's do not. The trade-off is strength and set speed, which is exactly the axis the four C270 types are arranged along.
Why does the calculator say the sand volume is the mortar volume?: Because the binder does not add to the pile, it fills the gaps in it. Loose sand is roughly a third voids, and the cement and lime paste occupies those voids rather than sitting between the grains and pushing them apart. So a batch made with a cubic metre of sand yields close to a cubic metre of mortar, and adding the three component volumes together — sand plus cement plus lime — over-states the yield by something like half. It is the same reason a concrete mix's yield is not the sum of its ingredients. The convention is close enough for ordering and is stated on this page rather than hidden, because sand grading moves it: a well-graded sand has fewer voids and yields a little more, a single-sized sand a little less.
How does an ASTM type map onto a British mortar designation?: Loosely, and the mapping is by intent rather than by equivalence. British practice numbers mortar designations from (i) — the strongest, cement-rich — down to (iv) and (v), which are lime-rich and soft, and recent standards specify mortar by compressive strength class instead. The ordering corresponds: an ASTM Type M sits near a designation (i), Type S near (ii), Type N near (iii), Type O near (iv). What does not correspond is the arithmetic, because the proportion tables differ and the sand ratio is expressed differently, so converting a specification by looking up the nearest row produces a mortar that is approximately right and provably nothing. A specification written against one standard should be batched against that standard.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.