Mortar Quantity Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/9/2026
The mortar quantity is calculated with surface area × consumption per m² depending on joint type: about 15 L/m² for a thin joint with concrete blocks, and about 32 L/m² for a standard joint with bricks or blocks. For a 20 m² wall with a thin joint, expect about 300 L of mortar, roughly 4.2 bags of 25 kg cement.
Explanation
The amount of laying mortar needed to build a wall depends mainly on two factors: the total surface area to build and the thickness of the joints between the units. A thin joint, about 1 cm, typical of laying concrete blocks with a thin-bed or traditional mortar applied as a thin bead, consumes noticeably less material than a standard joint of 8 to 12 mm, more common in traditional brick masonry. This calculator applies average coefficients drawn from job-site feedback (15 L/m² for a thin joint, 32 L/m² for a standard joint): actual consumption still varies depending on the exact size of the blocks or bricks used, how regular the laying is, and the mason's experience, so always allow a 10 to 15% margin beyond the estimate for waste and job-site adjustments. The cement bag estimate is based on a common mix of 350 kg of cement per m³ of mortar (about 14 bags of 25 kg per m³); for structural concrete quantities (slab, footing), see our concrete mix calculator, which uses a different mix specific to reinforced concrete.
Example: a 20 m² wall with a thin joint
Inputs
Surface area: 20 m². Joint type: thin (≈1 cm, concrete blocks).
Calculation
Mortar needed = 20 × 15 = 300 L. Cement bags = (300 ÷ 1000) × 14 = 4.2 bags of 25 kg.
Result
This wall needs about 300 L of mortar, roughly 4.2 bags of 25 kg cement (round up to 5 bags when buying).
Frequently asked questions
Why allow an extra margin beyond the calculated result?
The coefficients used are job-site averages: actual consumption varies with the exact size of the blocks, how regular the laying is, and losses during mixing and application. A 10 to 15% margin beyond the estimate avoids running out of material mid-job, an interruption usually costlier than a slight material surplus.
What is the difference between laying mortar and concrete?
Laying mortar (cement, sand, water, no gravel) is used to join masonry units like our concrete blocks or bricks, in a relatively thin layer. Concrete (cement, sand, gravel, water) is a structural material used in much greater volume for slabs, footings, or posts — see our concrete mix calculator for that second case.
Should the number of cement bags be rounded up?
Yes: a cement bag can't be bought as a fraction, so a result of 4.2 bags concretely means you need to buy 5. This calculator shows the exact calculated value rather than a rounded figure, so you can adjust it yourself based on your own safety margin.