Mortar calculator guide
The calculator above turns a wall into the mortar that goes on the order: whole bags of pre-blended mortar by ASTM type, or the cement, lime and sand for a mix-your-own batch. Describe each wall the way you already know it — a brick or block count, a wall area, or a length and height with the openings deducted — and the answer updates as you type, metric or imperial, entirely in your browser.
The guide below answers the bag question first, then shows where the mortar volume comes from — the joints themselves, not a hidden per-square-metre constant — what every assumption in the card means, and how the same arithmetic reads by hand on both sheets.
Wall elevation
Unit & joint section
The joints the calculator measures. The mortar (in colour) is the bed and head joints between the units — its volume scales with the joint thickness, which is exactly what the engine computes.
How much mortar do I need?
About 2.7 bags of 25 kg pre-blended mortar per 100 standard bricks, and about 10 bags per 100 standard 100 mm blocks — that is what this calculator returns on its metric sheet at a 10 mm joint. On the imperial sheet, with 80 lb bags, the same walls come out at about 1.4 bags per 100 US modular bricks and about 11.4 bags per 100 8 in concrete blocks.
The two sheets differ because they describe different products, not because the arithmetic changes. A US modular brick is smaller than a UK metric brick, so it holds less mortar in its joints, and an 80 lb bag lays about half again as much as a 25 kg one. An 8 in concrete block runs the other way: a bigger face and a deeper bed than a 100 mm block, so it takes more mortar per unit.
The rule of thumb behind all four figures is one line — units in the wall, times the mortar in one unit's joints, plus waste, divided by what one bag lays, rounded up. Mixing your own instead of buying bags, the same 100 UK metric bricks need about 0.045 m³ of dry materials, which a 1:4 masonry-cement mix splits into about 0.009 m³ of cement — one 50 kg bag — and about 0.036 m³, or 58 kg, of sand.
The calculator checks itself against the trade's published mortar quantities, not against a bag's coverage claim. For a US modular brick the Brick Industry Association gives 8.1 ft³ of mortar per 1000 brick in the wall plus 15–25% for waste; for an 8 in concrete block NCMA gives 8.5 ft³ per 100 ft² of wall including 10% waste. The figures above land within a few per cent of both. A manufacturer's units-per-bag claim measures something else — a conservative coverage rate for an unstated unit at an unstated joint — and runs two to four times the mortar the joints hold, which is why it is shown here for reference and not used as the reference. Under Assumptions the published rate is a field you own: it opens on the trade figure for the two products that have one, blank for every other, and a blank field means nothing is cross-checked rather than something being checked against a number that was never about it.
| Wall | Bag | Units one bag lays | Bags per 100 units | Manufacturer claim, 80 lb bag |
|---|---|---|---|---|
| UK metric brick, half-brick, 10 mm joint | 25 kg | ≈ 41 | 2.67 | — |
| UK metric brick, one-brick, 10 mm joint | 25 kg | ≈ 20 | 2.80 | — |
| Concrete block 100 mm, 10 mm joint | 25 kg | ≈ 11 | 9.98 | — |
| US modular brick, half-brick, ⅜ in joint | 80 lb | ≈ 78 | 1.41 | 35–37 |
| Concrete block (CMU) 8 in, ⅜ in joint | 80 lb | ≈ 10 | 11.42 | 12–13 |
How this calculator works
You describe the wall; the calculator does the take-off. Add a section per wall and switch it between Count, Area and Dimensions — a unit count you already have, an area you already measured, or a length and height with doors and windows to deduct. The purchase form, mortar type, bag size and mix sit under Method, and every constant the answer depends on sits under Assumptions.
Mortar is never a per-square-metre constant here. It is derived from the joint itself, unit by unit, reconciled with published rates by a visible allowance, inflated by the mortar waste, and only then divided by what one bag lays. The formulas below are those steps in the order the engine applies them.
Every looked-up value — the joint, the bag yield, the mortar allowance, the mortar and breakage waste, the bulking factor, the mix ratio, the lime and aggregate parts, and the cement, lime and sand densities — is an editable field with its source behind the question mark beside it. Switch between metric and imperial at any time and every label, threshold and result follows. Sections are summed first and rounded once, so the section rows always add up to the bag count in the answer card.
Five steps, in the order the engine applies them. Lengths are in metres and volumes in cubic metres; the imperial sheet does the same arithmetic in feet and inches.
Units in the wall
units = net area × leaf ÷ ((work L + joint) × (work H + joint))- Net area is the wall's length × height less every opening. Work L and work H are the unit's own length and height as moulded; one joint is added to each, because a unit occupies its own size plus a joint of wall.
- Leaf is 1 for a half-brick or single-block wall and 2 for a one-brick wall, which is two skins bonded together.
- A UK metric brick at a 10 mm joint gives 59.26 bricks per m²; a US modular brick at a ⅜ in joint gives 6.86 per ft².
Mortar in one unit's joints
mortar per unit = (work L × joint + work H × joint) × leaf depth- The two terms are the bed joint under the unit and the head joint at one end. Leaf depth is the unit's own thickness — or its length on a one-brick wall, which is bonded through.
- A UK metric brick at a 10 mm joint gives 0.000287 m³; a US modular brick at a ⅜ in joint gives 13.42 in³, or 0.00777 ft³.
Wet mortar for the job
wet mortar = units × mortar per unit × (1 + allowance)- Allowance is the mortar allowance — 10% for brick and 80% for block by default. A solid brick is bedded exactly as the joints are drawn, so its allowance carries only part of the 15–25% the Brick Industry Association asks you to add. A hollow block is bedded on its face shells and loses mortar into its cores, so NCMA's ordering rate sits well above any joint volume.
- Breakage waste is deliberately not in this line. It buys spare units; it does not lay extra mortar.
Bags to buy
bags = ⌈ wet mortar × (1 + waste) ÷ bag yield ⌉- Waste is the mortar waste — spillage, droppings and drying loss, 10% by default. Bag yield is the wet mortar one bag lays: 0.013 m³ for a 25 kg bag, 0.667 ft³ for an 80 lb bag.
- The brackets round up to whole bags, once, on the project total — so several sections never each pay for their own part bag.
Or mix your own
dry mortar = wet mortar × bulking × (1 + waste)- Bulking is the wet-to-dry multiplier for voids and bulking, 1.30 by default; the same mortar waste applies, so the two purchase forms describe the same wall.
- The dry volume is then split by the mix proportions — cement and lime become whole bags at their bag yields, sand becomes a weight at its density.
Estimate your mortar in five steps
- Choose units and purchase formPick metric or imperial, then choose whether you are buying pre-blended bags or mixing your own; both are always worked out.
- Pick the mortar typeChoose the ASTM type — M, S, N or O — and the bag size, or for a mix the masonry ratio or the portland-lime proportions.
- Add the wall sectionsAdd one section per wall and enter a unit count, a wall area, or a length and height with any openings to deduct.
- Check the assumptionsOpen Assumptions and confirm the joint, bag yield, mortar allowance, waste, bulking and densities against what you are actually buying.
- Read the answer, export or printRead the bags to buy, the cement, lime and sand for a mix and the optional cost, then export to CSV or print the take-off sheet.
How the bag count is worked out
A pre-blended bag is a single product — mortar cement and graded sand, just add water — so one figure decides the count: the bag's yield, meaning the wet mortar it lays. The calculator seeds that from the bag size you pick and lets you overwrite it, because yields differ by brand and by mortar type.
The seeded yields come from a dry bulk density of about 1920 kg/m³, or 120 lb/ft³ — the density behind the widely published 0.667 ft³ yield of an 80 lb bag. The table shows what each bag size holds on that basis and how far it goes on a brick and a block wall at the default joint and allowances.
The bags-per-100-units line under the answer is a smooth ratio, not the lumpy purchase count: it divides the raw bag figure by the units actually placed in the wall, so a job that rounds 18.99 bags up to 19 still reports 2.67 bags per 100. Read that line against the rule of thumb on your own bag, and order on the whole-bag figure.
| Bag | Yield (metric) | Yield (imperial) | Bricks laid | Blocks laid |
|---|---|---|---|---|
| 25 kg | 0.013 m³ (13 L) | 0.459 ft³ | ≈ 41 | ≈ 11 |
| 60 lb | 0.0142 m³ (14 L) | 0.501 ft³ | ≈ 45 | ≈ 12 |
| 80 lb | 0.0189 m³ (19 L) | 0.667 ft³ | ≈ 60 | ≈ 16 |
How cement, lime and sand are worked out
Mixing your own starts from the same wet mortar and converts it to a dry volume: multiply by the bulking factor — 1.30 by default — for the voids and bulking dry materials carry, then add the same mortar waste the bagged path applies. Whatever the mix, that dry volume is what gets split.
A masonry-cement batch splits it two ways on a 1:n cement-to-sand ratio: cement is 1 ÷ (1 + n) of the dry volume and sand is n ÷ (1 + n). A portland-lime batch splits it three ways — 1 part portland cement, the ASTM type's lime parts, and sand at the aggregate multiplier times the cement-plus-lime volume. Cement and lime are counted as separate bags because they are bought as separate products; sand is converted to weight at its density, because that is how it is sold.
The shares are fixed by the proportions alone, so the table reads the same on either sheet. Note what the right-hand column says: at the default 3× aggregate multiplier every ASTM type puts sand at three quarters of the batch, and the mortar type only decides how the remaining quarter is divided between cement and lime.
| Mix | Parts by volume | Cement | Lime | Sand |
|---|---|---|---|---|
| Masonry cement, strong | 1 : 3 | 25.0% | — | 75.0% |
| Masonry cement, general | 1 : 4 | 20.0% | — | 80.0% |
| Masonry cement, soft | 1 : 5 | 16.7% | — | 83.3% |
| Masonry cement, very soft | 1 : 6 | 14.3% | — | 85.7% |
| Portland and lime, Type M | 1 : ¼ : 3¾ | 20.0% | 5.0% | 75.0% |
| Portland and lime, Type S | 1 : ½ : 4½ | 16.7% | 8.3% | 75.0% |
| Portland and lime, Type N | 1 : 1 : 6 | 12.5% | 12.5% | 75.0% |
| Portland and lime, Type O | 1 : 2 : 9 | 8.3% | 16.7% | 75.0% |
ASTM mortar type reference
ASTM C270 defines four common mortar types by strength, and the type you pick sets both the strength the wall can carry and, for a mix-your-own portland-lime batch, the lime and sand proportions. Type M is the strongest and belongs below grade; Type S is the high-strength choice at or below grade and against hard units; Type N is the general-purpose above-grade mortar and the common residential default; Type O is a soft, low-strength mortar for interior work and pointing on soft units.
Stronger is not better. A mortar harder than the units it beds will not yield, so movement cracks the brick instead of the joint — which is why Type O and Type N still exist for soft and historic masonry, and why the table's typical-use column matters more than its strength column.
| Type | Cement : lime : sand | Min. strength | Typical use |
|---|---|---|---|
| Type M | 1 : ¼ : 3¾ | 2500 psi (17.2 MPa) | Below-grade, foundations and retaining walls — the highest-strength type. |
| Type S | 1 : ½ : 4½ | 1800 psi (12.4 MPa) | At or below grade, exterior walls and hard units — high strength. |
| Type N | 1 : 1 : 6 | 750 psi (5.2 MPa) | General purpose — above-grade exterior and interior load-bearing walls. The common residential choice. |
| Type O | 1 : 2 : 9 | 350 psi (2.4 MPa) | Interior non-load-bearing walls and pointing on soft units — low strength. |
Worked examples
Each example reproduces exactly what the calculator returns for the inputs listed, so you can check the method by hand. Both run a 10% mortar allowance on brick, 10% mortar waste, 10% breakage on the unit count, a 1.30 bulking factor and a 1:4 masonry-cement mix.
Each example reproduces the calculator's output for the given inputs.
A 5 m × 2.4 m half-brick wall
Metric · UK metric brick · 10 mm joint · 25 kg bags- Given
- Wall 5 m × 2.4 m, no openings
- UK metric brick 215 × 102.5 × 65 mm, half-brick leaf
- 10 mm joint, 10% mortar allowance, 10% mortar waste, 10% breakage
- 25 kg pre-blended bag yielding 0.013 m³
- Or a 1:4 masonry mix: bulking 1.30, 50 kg cement at 1440 kg/m³, sand 1600 kg/m³
- Formula
bags = ⌈ units × mortar per unit × (1 + allowance) × (1 + waste) ÷ bag yield ⌉- Steps
- Net area: 5 × 2.4 = 12.00 m²
- Coordinating face 225 × 75 mm → 1 ÷ (0.225 × 0.075) = 59.26 bricks per m²
- Bricks placed: 12.00 × 59.26 = 711.1 — and 783 to buy once 10% breakage is added
- Mortar per brick: (0.215 × 0.010 + 0.065 × 0.010) × 0.1025 = 0.000287 m³
- Wet mortar: 711.1 × 0.000287 × 1.10 = 0.2245 m³
- With 10% mortar waste: 0.2245 × 1.10 = 0.2469 m³
- Bags: 0.2469 ÷ 0.013 = 19.0 → 19 bags; the same raw figure over the 711.1 bricks placed is 2.67 bags per 100
- Mixing instead: dry 0.2245 × 1.30 × 1.10 = 0.3210 m³ → cement 0.0642 m³, sand 0.2568 m³
- Cement bags: ⌈ 0.0642 ÷ 0.0347 ⌉ = ⌈ 1.85 ⌉ = 2 bags; sand 0.2568 × 1600 = 411 kg
- Result
- 19 × 25 kg bags of pre-blended mortar and 783 bricks — or 2 × 50 kg cement bags and 0.41 t of sand for a 1:4 mix
A 16 ft × 8 ft half-brick wall
Imperial · US modular brick · ⅜ in joint · 80 lb bags- Given
- Wall 16 ft × 8 ft, no openings
- US modular brick 7⅝ × 3⅝ × 2¼ in, half-brick leaf
- ⅜ in joint, 10% mortar allowance, 10% mortar waste, 10% breakage
- 80 lb pre-blended bag yielding 0.667 ft³
- Or a 1:4 masonry mix: bulking 1.30, 94 lb cement at 90 lb/ft³, sand 100 lb/ft³
- Formula
bags = ⌈ units × mortar per unit × (1 + allowance) × (1 + waste) ÷ bag yield ⌉- Steps
- Net area: 16 × 8 = 128 ft²
- Coordinating face 8 × 2⅝ in = 0.1458 ft² → 1 ÷ 0.1458 = 6.86 bricks per ft²
- Bricks placed: 128 × 6.86 = 877.7 — and 966 to buy once 10% breakage is added
- Mortar per brick: (7.625 × 0.375 + 2.25 × 0.375) × 3.625 = 13.42 in³ = 0.00777 ft³
- Wet mortar: 877.7 × 0.00777 × 1.10 = 7.50 ft³
- With 10% mortar waste: 7.50 × 1.10 = 8.25 ft³
- Bags: 8.25 ÷ 0.667 = 12.4 → 13 bags, or 1.41 bags per 100 bricks
- Mixing instead: dry 7.50 × 1.30 × 1.10 = 10.73 ft³ → cement 2.15 ft³, sand 8.58 ft³
- Cement bags: ⌈ 2.15 ÷ 1.045 ⌉ = ⌈ 2.05 ⌉ = 3 sacks; sand 8.58 × 100 = 858 lb
- Result
- 13 × 80 lb bags of pre-blended mortar and 966 bricks — or 3 × 94 lb cement sacks and 0.43 short ton of sand for a 1:4 mix
Common mistakes
Six errors that turn up again and again on a mortar take-off, and what each one costs.
- Ordering bags against the dry volume. A bag's yield is the wet mortar it lays, while the cement, lime and sand figures are dry volumes — 30% larger by default, because of the bulking factor. Count bags off the dry number and you over-order by about a third.
- Reading a manufacturer's units-per-bag figure onto a one-brick wall. Those figures count a single wythe, and a one-brick wall is two skins bonded together, so the same face area takes roughly twice the units and twice the mortar. The calculator's bags-per-100 line counts both leaves, so it barely moves — 2.80 against 2.67 — while the wall's total bag count doubles.
- Leaving the mortar allowance at the brick figure on a block job. A hollow block is bedded on its face shells and loses mortar into its cores, so the published ordering rate sits far above the joint volume — which is why the block default is 80% and the brick default is 10%. Leave it at 10% and a blockwork order lands about 40% short.
- Mixing a Type N ratio and expecting Type S strength. Type N is 1 : 1 : 6 by volume and specified at 750 psi; Type S is 1 : ½ : 4½ and 1800 psi. The proportions are the specification, so a batch mixed to Type N is a Type N mortar whatever the detail called for.
- Buying sand by volume when the supplier sells it by weight. The tool reports both — the mix split gives a volume, and the sand density turns it into tonnes or short tons — but only the weight matches what a bulk bag is priced by. At the default 1600 kg/m³ a cubic metre of sand is 1.6 t.
- Dropping the mortar waste because the mix looked generous. Spillage, droppings and what stiffens on the board are real losses: 10% is the default and the floor, and 15–20% is the honest figure for a first wall or for hot, windy weather. Of every line on the sheet, this is the cheapest one to be generous with.
Frequently asked questions
Short answers to the questions people most often ask about estimating mortar, bags, cement, lime and sand with this calculator.
How many bags of mortar do I need?
It depends on the unit size, the joint thickness and the bag yield. At this calculator's defaults a 25 kg bag lays about 41 standard bricks or 11 standard blocks, which is about 2.7 bags per 100 bricks and about 10 per 100 blocks. Manufacturers quote a far more conservative 35–37 bricks or 12–13 blocks per 80 lb bag, but that is a coverage claim rather than a mortar quantity; the calculator is set to the Brick Industry Association and NCMA rates instead. It works the exact number from your own wall, joint and allowances and rounds up to whole bags.
Which ASTM mortar type should I use?
Type N is the general-purpose choice for above-grade exterior and interior walls. Type S suits at-or-below-grade work and harder units, Type M is for foundations and retaining walls, and Type O is a low-strength mortar for interior work and repointing on soft units. Pick the type under Method; it sets the strength and the portland-lime proportions.
Should I buy pre-blended bags or mix my own?
Pre-blended bags are simplest — one product, just add water — and the calculator counts whole bags. Mixing your own from masonry cement and sand, or portland cement, hydrated lime and sand, can be cheaper in volume and lets you match a specific mix. The tool computes both for the same wall so you can compare, and the prices you enter follow whichever form leads the results.
How much cement, lime and sand does the mortar need?
The wet mortar is bulked to a dry volume and split by the mix. A 1:4 masonry-cement batch is 20% cement and 80% sand by volume; a portland-lime Type N batch is 12.5% cement, 12.5% lime and 75% sand. Cement and lime are converted to whole bags at their bag yields, and sand to weight at its density — all editable in Assumptions.
Does the calculator work in metric and imperial?
Both. Switch units at the top and every dimension, yield and result converts — unit sizes in mm or inches, walls in m or ft, mortar in m³ or ft³, sand in kilograms and tonnes or pounds and short tons. The imperial sheet opens on its own round figures, a ⅜ in joint and an 80 lb bag, rather than converted metric ones.
Is the calculator private?
Yes. Every calculation runs entirely in your browser. Nothing you enter is sent to any server, and your project is saved only in your own browser's local storage.