The bags-per-hundred rule
Mortar estimating has a dependable rule of thumb: one 80-lb bag of mortar lays about 33 concrete blocks or 150 bricks, with roughly a quarter cubic yard of sand per bag. The calculator scales that to your count — round up, because a half-batch that dries out mid-wall is wasted either way.
Type S or Type N?
Type S mortar (higher strength) is standard below grade, for retaining walls and any load-bearing CMU work. Type N is softer and better for above-grade brick veneer repair. The bag count barely differs — the choice is about what the wall does, not how much material it takes.
The mortar estimating formula
Mortar estimating runs on a single, well-tested field allowance: one 80-lb bag of mortar lays about 33 concrete blocks or about 150 bricks, assuming standard 3/8-inch bed and head joints. That gives:
bags = number of blocks ÷ 33 (or number of bricks ÷ 150), rounded up
The rounding matters. Mortar is batched, not saved — a half-used batch sets and is thrown away — so it always pays to round up rather than run out two courses from the top. As a rule of thumb, budget about 3 bags per 100 blocks and keep one spare bag on site for the unexpected cut or opening.
What actually changes the bag count
The 33-blocks-per-bag figure is a solid average, but four things move it up or down:
| Factor | Effect on mortar | Why |
|---|---|---|
| Joint thickness | Bigger joints use more | A 1/2-inch joint takes roughly a third more mortar than a 3/8-inch joint. |
| Unit type | Hollow units use more than solid | Mortar troweled across the webs and face shells adds up course by course. |
| Unit size | Face size sets the count | A standard modular brick has far less face area than a block, so one bag lays many more units. |
| Weather | Hot, dry days waste more | Mortar stiffens faster in the sun, so more of the batch is discarded before it is used. |
Because every one of these is approximate, the professional habit is to order close to the calculated count and keep one unopened bag back for the tail of the job rather than ordering a whole extra pallet.
Type S, N or O — which mortar?
Mortar type is a strength-and-flexibility choice governed by ASTM C270, and it barely changes the quantity — it changes what the wall can do. The proportions below are by volume, cement : lime : sand:
| Type | Proportions (by volume) | Where it belongs |
|---|---|---|
| Type M | 1 : 1/4 : 3.75 | Highest strength — below grade, severe exposure. |
| Type S | 1 : 1/2 : 4.5 | Standard for load-bearing CMU, retaining walls, below grade. |
| Type N | 1 : 1 : 6 | Above-grade brick veneer, soft historic units, repointing. |
| Type O | 1 : 2 : 9 | Interior, non-load-bearing, low-stress repairs. |
The pattern is simple: more cement and less lime means more strength and less flexibility. For a load-bearing block wall, Type S is the default; for a brick-veneer repair above grade, Type N lets the wall move without cracking. Choose the type from what the wall does, then use the same 33-blocks-per-bag figure for the quantity.
How to size flagstone mortar from the plan
Three numbers drive the calculation, and all three come off the plan or the stone supplier's sheet: the finished area in square feet; the stone face size and thickness (for example 16 by 12 inches at 1-1/2 inches thick); and the joint width, from 1/4 inch for a tight formal joint up to 1 inch for a relaxed, planted joint. Thickness sets how deep the joint can be; face size sets the stone count and therefore the total joint length.
The stone count follows directly — divide the patio length by the stone length plus the joint, and the width by the stone width plus the joint, then round each up:
Columns = ceil(patio length ÷ (stone length + joint)), rows = ceil(patio width ÷ (stone width + joint))
Because flagstone is quarried and split rather than moulded, every piece is a different size and a straight grid never fits. Add 15% for cuts and offcuts, and dry-lay the whole field before setting anything in mortar — rearranging is free, moving already-set stones is not.
Bedding mortar
Bedding (cu ft) = area (sq ft) × bedding depth (inches) ÷ 12
At 200 square feet and a 1-inch bed that is 16.7 cubic feet, or 0.62 cubic yards. At 2 inches it doubles to 33.3 cubic feet, 1.24 cubic yards. On a Type N or Type S proportional mix of 1 part cement to 6 parts sand, that is 2 bags at 1 inch and 3 bags at 2 inches, with roughly 0.66 and 1.32 cubic yards of sand.
Bonding mortar
Bonding (cu ft) = area (sq ft) × bonding depth (inches) ÷ 12
At 200 square feet and 1/2 inch that is 8.3 cubic feet, 0.31 cubic yards, one 80-lb bag of Type S mortar and about a third of a cubic yard of sand.
Jointing mortar
Joints are the volume people forget, because they look like hairlines. They are not: a 1/2-inch joint around a 1-1/2-inch stone still has to be filled nearly full width for the depth the mortar can support, and the total joint length in a paved area is large. For a 20 by 10 foot patio laid with 16 by 12 inch stones on a 1/2-inch joint, the internal grid alone is about 350 feet of joint:
Jointing (cu ft) = total joint length (ft) × joint width (in) ÷ 12 × fill depth (in) ÷ 12
Filled to half an inch below the stone face, that 350 feet of joint works out to about 1.2 cubic feet, 0.045 cubic yards, and one bag of mortar covers it with room to spare. So jointing mortar is the smallest of the three layers, and on a tight joint you should buy one bag and treat it as the smallest line on the estimate. If a supplier quotes several bags of jointing mortar for a 200 square foot patio, check the joint width and depth they assumed first — a 1-inch joint doubles it, but it still does not reach multiple bags.
What changes the numbers most
| Change | Effect on mortar |
|---|---|
| Bedding depth 1 inch to 2 inches | Doubles the largest layer — the biggest single lever on the estimate. |
| Joint width 1/4 inch to 1 inch | Quadruples jointing volume, but jointing stays the smallest layer. |
| Stone thickness 1 inch to 2 inches | Increases fill depth, so more jointing mortar per foot of joint. |
| Type N to Type S | Changes the mix ratio, not the volume. Type S is 1:0.5:4.5 cement, lime and sand; Type N is 1:1:6. |
Flagstone and paver patios: three layers, not one number
A flagstone patio does not use the same mortar as a block wall, and this is where most mortar calculators stop being useful. A mortared stone patio has three layers, each with its own volume and material:
- Bedding mortar — the screeded bed the stones are pressed into, typically 1 to 2 inches thick over the compacted base — the largest single consumer of mortar.
- Bonding mortar — the thin bed that glues the stone down, often 1/2 inch, usually Type S for exterior exposure.
- Jointing mortar — what fills the gaps after the stones are set. On a tight 1/4-inch joint this is a surprisingly small quantity; on a 1-inch joint it roughly doubles.
The rule of thumb on this page — one 80-lb bag lays about 33 blocks or 150 bricks — comes from unit masonry, where the joint is a thin line between identical units. Stone paving has a much larger joint and a much larger bed, so the rule understates the material you need. The layers also want different types: bedding and bonding take the freeze-thaw cycles, so they want a harder Type S, while jointing has to be pointed without chipping the stone edge and is usually a more workable Type N. Specifying one bagged mortar for all three layers is the most common single mistake on flagstone jobs.
Tools for mixing and laying mortar
Mortar is batched, not saved, so the tools are about working at a steady pace rather than about quantity. This is the small kit a block or brick wall is laid with.
- Masonry trowel — The 10-inch London or Philadelphia pattern spreads both bed and head joints. An even joint is what keeps the bag estimate honest.
- Mortar mixing tub and hoe — Mix the size of batch you can lay before it sets. A half-batch that dries out mid-wall is wasted whatever the calculator said.
- Mortar hawk — Holds the working board of mortar at the wall, so you are not walking back to the tub for every block.
- Mortar jointer — Striking a consistent concave joint sheds water and finishes the wall. A trowel edge alone leaves a smeared face that traps moisture.
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Sources and standards
Where these figures come from
Every formula on this page comes from the published reference material below. The 33-blocks-per-bag allowance sits inside the range used across the industry and is consistent with the mortar-type proportions set by ASTM C270 for standard 3/8-inch joints. Joint thickness, unit absorption and weather all shift the real number, so treat the bag count as a firm planning figure rather than a guarantee.
- ASTM International — C94 for ready-mixed concrete, C150 for cement and C143 for slump testing.
- American Concrete Institute — ACI 318 code requirements and ACI 302 slab construction guidance.
- Portland Cement Association — mix design, yield and curing data published by the US cement industry.
- ASTM C270 — standard specification for mortar for unit masonry (Type M/S/N/O proportions).
- Brick Industry Association — technical notes for clay brick masonry and mortar.
- ASTM C90 — standard specification for loadbearing concrete masonry units.
- ASTM C270 — mortar for unit masonry: Type M 2,500 psi, Type S 1,800 psi, Type N 750 psi, Type O 350 psi minimum 28-day compressive strength, with the Type S 1:0.5:4.5 and Type N 1:1:6 volume proportions used on this page.
- Brick Industry Association — technical notes on mortar selection, joint profile and the rule that mortar must stay softer than the unit it bonds.
- NCMA — National Concrete Masonry Association guidance on flagstone and stone paving installation.
Figures, formulas and reference tables last verified: 24 October 2026.