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.
Three worked examples
Example 1 — 500 concrete blocks
Bags = 500 ÷ 33 = 15.15, so 16 bags — the same answer the calculator returns for 500 blocks. Cross-checking with the per-hundred rule: 500 ÷ 100 × 3 = 15, so 16 bags covers the count with a spare.
Example 2 — a 1,000-block CMU wall
Bags = 1,000 ÷ 33 = 30.3, so 31 bags. By weight that is roughly 2,480 lb of mortar, which is why larger walls are bought on a pallet rather than by the bag. Add a bag or two when the wall is full of openings, because every corner and lintel wastes a partial batch.
Example 3 — 2,000 modular bricks
Bricks use a different per-bag figure — about 150 bricks per 80-lb bag. Bags = 2,000 ÷ 150 = 13.3, so 14 bags. Note that the same 14 bags would lay 14 × 33 ≈ 460 blocks: a brick simply uses far less mortar per unit of wall, which is why a brick-veneer estimate and a block estimate are never interchangeable. For the block count that feeds this number, start with the concrete block calculator.
Where these figures come from
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. Unit masonry itself is covered by ASTM C90 for concrete block and by the technical notes of the Brick Industry Association for clay brick. 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.
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.
As an Amazon Associate I earn from qualifying purchases. The links above are affiliate links — if you buy through one, the price you pay is unchanged and a small commission helps keep these calculators free. Tools are listed because the job calls for them, not because of commission rates.
Sources and standards
Where these figures come from
Every formula on this page is implemented from the published reference material below. Where a value depends on a regional code, the calculator lets you override it rather than assuming one jurisdiction.
- 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.
Figures, formulas and reference tables last verified: 25 September 2026.