CMU joint mortar & site mix estimator

Block Mortar Calculator Mortar Bags & Site Mix for CMU

Estimate how many bags of mortar you need to lay a concrete block wall — pre-mixed 80-lb or 60-lb bags, plus a portland-lime-sand site mix by mortar type N, S, M, or O.

Default sample is ready

Input

20 x 8 ft

80-lb bags

Mortar type

Type S

Assumes standard 8×16 face blocks and 10% waste. Bag coverage is the manufacturers' field rate, including bedding loss — a planning estimate, not a structural mortar spec.

Edit the estimate Count blocks instead

Estimate Block Mortar

Use this for the joint mortar between blocks. To count the blocks themselves, use the standard concrete block calculator instead. Filling cores? Use the block fill calculator .

Unit:

Wall Input

Start from:

Wall Length (ft)

Wall Height (ft)

Mortar Settings

Mortar Type (ASTM C270)

Bag size:

Waste Allowance

10%

Applied to the block count — covers cut blocks, dropped mortar, and full-bed first courses.

Cost Estimation

Enable to enter your local mortar bag price.

How This Block Mortar Calculator Works

Mortar follows the joints, so the calculator works from the block count. Start from a known count, or let it estimate one from the wall face: each nominal 8×16 inch block covers 0.889 square feet, which is 1.125 blocks per square foot (12.5 blocks per square meter in metric). The waste allowance is applied to the block count, because every extra block laid means extra bed and head joints.

Bags are then estimated with the industry field coverage rate: one 80-lb bag of pre-mixed mortar lays about 12 standard blocks, and one 60-lb bag about 9 — so 100 blocks take 9 bags of 80-lb or 12 bags of 60-lb. This is the rate published by pre-mix manufacturers and it already includes normal bedding loss, which is why it runs higher than a pure geometric joint-volume calculation.

The site-mix list converts the same mortar volume into raw materials using the ASTM C270 proportions for your mortar type — portland cement, hydrated lime, and masonry sand by parts. Use this page for the joint mortar in a CMU wall. It does not count the blocks, fill the cores (see the block fill calculator), or cover brick mortar, thinset, or structural mix design.

Wall area & blocks

blocks = ceil(wall area × 1.125)

Each nominal 8×16 inch CMU face covers 0.889 square feet including joints. Metric walls use 12.5 blocks per square meter. Waste is added to this count before the bag math.

Pre-mix bags

bags = ceil(blocks ÷ 12) (80-lb)

Bag size	Blocks per bag	Bags per 100 block
80-lb	~12	9
60-lb	~9	12

Site mix by mortar type

cement : lime : sand (ASTM C270)

Type	Proportions	Strength
O	1 : 2 : 9	350 psi
N	1 : 1 : 6	750 psi
S	2 : 1 : 9	1800 psi
M	3 : 1 : 12	2500 psi

Material densities

volume ÷ unit yield = bags

One 94-lb bag of portland cement ≈ 1 cubic foot. One 50-lb bag of hydrated lime ≈ 1.25 cubic feet (40 lb per cubic foot). Masonry sand runs about 100 lb per cubic foot and is sold by the cubic yard or ton.

Bag coverage follows the field rates published by pre-mix manufacturers (Quikrete and Sakrete quote roughly 12-13 standard blocks per 80-lb bag). Site-mix proportions follow ASTM C270. Last reviewed June 2026.

Example Mortar Estimate

The prefilled sample lays a 20 x 8 ft wall — 180 blocks, or 198 with the 10% waste allowance — in Type S (1800 psi) mortar.

Quick sanity check: 100 standard blocks with no waste take 9 bags of 80-lb pre-mix (100 ÷ 12, rounded up) or 12 bags of 60-lb (100 ÷ 9, rounded up) — the same benchmark row masonry suppliers quote.

80-lb bags

incl. 10% waste

Blocks to lay

198

Site mix

3 + 1

portland + lime bags

Masonry sand

11.22 cu ft

0.416 cu yd

Mortar Planning Checklist

Pre-mix or site mix

Pre-mixed bags are simpler for jobs up to about 100 blocks — just add water. Beyond that, a portland-lime-sand site mix costs noticeably less per cubic foot. This page prices both lists.

Pick the right type

Type N for above-grade general work, Type S for below-grade and load-bearing walls, Type M for foundations, Type O for interior repair only. Follow the plans if a type is specified.

Waste & full joints

First courses are often laid in a full mortar bed, and hot weather dries boards faster. Keep at least 10% waste, and round up to whole bags — short one bag stops the wall.

Big walls: think bulk

Past a few hundred blocks, bagged pre-mix gets expensive and slow. Price masonry cement plus bulk sand, or ask your supplier about silo or ready-mixed mortar delivery.

Frequently Asked Questions

How many bags of mortar do I need per 100 block?

Plan on 9 bags of 80-lb pre-mixed mortar — or 12 bags of 60-lb — per 100 standard 8×16 blocks. That covers the bed and head joints for laying the wall, not core fill. If you have heard "3 bags per 100 block," that figure is for masonry cement, which gets mixed with about 18 shovels of sand per bag. It is a different product than ready-to-use pre-mix, so the bag counts differ.

How many blocks does one 80-lb bag of mortar lay?

About 12 standard 8×8×16 blocks per 80-lb bag, or roughly 9 blocks per 60-lb bag. This is the field coverage rate published by pre-mix manufacturers like Quikrete and Sakrete, and it already includes normal bedding loss — the mortar that squeezes out of the joints or drops off the trowel while laying.

What is the difference between Type N and Type S mortar?

Type N (750 psi) is the general-purpose choice for above-grade, non-load-bearing block walls. Type S (1800 psi) adds compressive and bond strength for below-grade work, foundations, and load-bearing walls. Type M (2500 psi) is for foundations and heavy structural loads, and Type O (350 psi) is limited to interior, non-load-bearing repair work. When in doubt between N and S for a concrete block wall, most masons specify Type S.

Should I buy pre-mixed mortar or mix on site?

For small jobs — up to roughly 100 blocks — pre-mixed bags are simpler: just add water. For larger walls, a site mix of portland cement, hydrated lime, and masonry sand costs noticeably less per cubic foot of mortar. This calculator shows both at once: the pre-mix bag count and the equivalent site-mix shopping list for your selected mortar type.

Does block size change how much mortar I need?

Yes — mortar follows the number of joints, not just the wall area. Smaller blocks mean more blocks and more joints per square foot, so more mortar. This calculator assumes standard 8×16 face blocks at 1.125 per square foot. If you lay 4-inch-high half blocks, expect roughly double the bed joints and noticeably more mortar per square foot.

Does this calculator include core fill or grout?

No. This page only estimates the mortar in the bed and head joints between blocks. Filling the block cores with grout or concrete is a separate volume calculation with different math — use the block fill calculator for that.

How much sand and cement do I need to mix block mortar on site?

Split the total mortar volume by the ASTM C270 proportions for your mortar type. Type S, for example, is 2 parts portland cement, 1 part hydrated lime, and 9 parts sand. Then convert to purchase units: one 94-lb bag of portland is about 1 cubic foot, one 50-lb bag of hydrated lime is about 1.25 cubic feet, and masonry sand is sold by the cubic yard or ton. Always check these against the yield table on your manufacturer’s bag before ordering.

Estimating the rest of the project? Count the blocks themselves with the concrete block calculator , or estimate grout for the cores with the block fill calculator .