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

Wall area (ft²) = Length(ft) × Height(ft)Units = Area × units per ft² (block 1.125, brick 6.86)Mortar bags = Units × bags per unit × (1 + Waste%)Block: ~1 bag / 12 blocks · Brick: ~1 bag / 36 bricks (80 lb mortar mix)Or mix your own: masonry cement (block ~3/100, brick ~7/1000) + ~1 yd³ sand per 7 bags

Enter your dimensions and the result updates instantly. A waste allowance is included so you order slightly over rather than running short mid-pour, and ready-mix is rounded up to the nearest quarter yard, which is how it is sold.

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, Unit type 1, 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.

Slab size	80 lb mortar bags	Units in the wall
10 ft × 8 ft	9	90	Use →
20 ft × 8 ft	17	180	Use →
30 ft × 8 ft	25	270	Use →
40 ft × 8 ft	33	360	Use →
50 ft × 8 ft	42	450	Use →
20 ft × 4 ft	9	90	Use →

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, 6.86 for modular brick), and then apply the bags-per-unit rule for 80 lb mortar mix with 3/8 inch joints.

An 80 lb bag of pre-mixed mortar lays about 12 standard blocks or about 36 modular bricks, which works out to roughly 8 bags per 100 blocks and 28 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. If you prefer to mix your own instead of buying pre-mix, the calculator also gives the masonry cement (70 lb bags) and the sand: a type N or S mix is roughly one part masonry cement to three parts sand by volume, about one cubic yard of sand per seven bags of cement.

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.

How we check this

Every formula is shown in full and verified with automated tests against manufacturer spec sheets and ASTM and ACI standards. We show our work so you can check it.

Frequently asked questions

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

An 80 lb bag of pre-mixed mortar lays about 12 standard blocks at 3/8 inch joints, so roughly 8 to 9 bags per 100 blocks. The old three-bags-per-100 rule counts masonry cement mixed with sand on site, not pre-mixed mortar mix.

How many bags of mortar per 1000 bricks?

An 80 lb bag of pre-mixed mortar lays about 36 modular bricks, so roughly 28 bags per 1000 bricks at 3/8 inch joints. This calculator works it out from your wall size and unit type.

How much mortar do I need for a block wall?

Find the wall area, multiply by 1.125 for the block count, then divide by about 12 for bags, and add waste. A 20 by 8 foot wall (160 square feet, about 180 blocks) needs about 17 bags with 10% waste.

What is the difference between mortar mix and masonry cement?

Mortar mix comes as one 80 lb bag with the sand already blended in, ready to add water. Masonry cement is cement only, sold to be mixed with separate sand on site. The old rules of thumb (three bags per 100 blocks, seven per 1000 bricks) count masonry cement, so they under-count the pre-mixed mortar bags this calculator estimates.

References

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