Mortar Calculator

This is an estimate, not professional advice. Check your inputs and verify the result against your plans and local building code before you build or order. See terms and disclaimer.

How this calculator works

Enter your dimensions and the result updates instantly. A waste allowance is included so you order slightly over rather than running short.

Worked example

A 20 ft by 8 ft concrete block wall with a 10% waste allowance. Inputs: Wall length 20 ft, Wall height 8 ft, Waste allowance 10 %. Result: 17 .

Mortar for common block walls

Calculated for standard 16 by 8 inch block with a 10% waste allowance. Tap a size to load it above.

Method & assumptions

Mortar is estimated from the number of units in the wall. We work out the wall area, multiply by the units per square foot (1.125 for standard 16 by 8 inch block, 6.75 for modular brick), and then apply the bags-per-unit rule for 80 lb mortar mix with 3/8 inch joints. The brick figure is not a rule of thumb: Brick Industry Association Technical Note 10, table 4, gives 675 modular brick per 100 square feet of wall, which is 6.75 per square foot.

An 80 lb bag of pre-mixed mortar lays 12 standard blocks or 35 modular bricks, both figures taken from the Sakrete Type N Mortar Mix data sheet, which works out to about 9 bags per 100 blocks and 29 bags per 1000 bricks. Note these are bags of mortar mix, with the sand already blended in; the older rules of three bags per 100 blocks or seven per 1000 bricks count bags of masonry cement that you mix with sand on site, so they under-count the pre-mixed bags you actually buy. We add your waste allowance for spillage and the mortar that gets dropped or thrown away, then round up to whole bags because you cannot buy part of a bag. Tighter or wider joints change the figure, so treat the result as a working estimate and keep a spare bag on site.

This assumes a single-wythe wall of standard units. Double-wythe walls, large joints, and tooled or raked joints all use more mortar, while collar joints and grouted cells are separate from the laying mortar counted here. The result covers only pre-mixed mortar with the sand already blended in. Site-batched masonry cement and sand are omitted because their proportions depend on the specified mortar type and constituent materials.

Pro tips and common mistakes

• Keep joints consistent. A steady 3/8 inch joint is what these estimates assume. Fatter joints eat mortar fast and quietly blow your bag count, so tool them the same all the way up.
• Mix only what you can use. Mortar starts to set in about 60 to 90 minutes. Mix in batches you can lay before it stiffens, and do not retemper it more than once.
• Buy a spare bag. Running out mid-wall means a cold joint or a supply run. The waste allowance covers most of it, but one extra bag on site is cheap insurance.
• Match the mortar type to the job. Type N suits most above-grade walls; type S is stronger for below-grade or load-bearing work. Using the wrong type weakens the wall or over-stresses the units.
• Protect the work. Do not lay in freezing weather or full sun without cover. Mortar that freezes or dries too fast loses strength and bond.

Frequently asked questions

Sources and references

These references and structured source records support the method and material data used on this page.

• Brick Industry Association, Technical Note 10: Dimensioning and Estimating Brick Masonry (table 4: 675 modular brick per 100 sq ft)
• Sakrete Mortar Mix Type N, technical data sheet (80 lb bag lays 12 concrete block or 35 standard brick)
• QUIKRETE Mortar Mix No. 1102, product data sheet (80 lb bag, 8 x 8 x 16 in standard block)
• Mortar (Wikipedia)

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