How this calculation works
Calculate each coat with its own thickness, dry-volume factor and cement:sand ratio. Add coat cement masses and sand volumes only after the individual layer calculations.
Coat wet m³ = net face area × face count × thickness mm / 1,000; allocate each coat’s dry volume by its own ratioA worked example
An 18 m² net face at 12 mm base thickness needs 0.216 m³ wet mortar. A 3 mm finish adds 0.054 m³, giving 0.27 m³ across both layers before separate dry-material assumptions.
Good to know
Flat uniform layers only. No reveals, uneven substrate, mesh, primer, waste, water, strength or compatible-system recommendation. Openings and area are per face.
Reference: OpenStax: volume and dimensional geometry (opens in a new tab) ↗
Subtract openings before multiplying faces
Enter the gross area of one face and its openings. Their difference is then multiplied by one or two identical faces. Entering a two-face total and also selecting two faces would double count the area.
Reveals and soffits around an opening are separate surfaces; the calculator does not silently add them back after deducting the opening.
The nominal concrete mix material quantities calculator adds allocate an entered dry volume ratio into cement bags, sand and aggregate mass, separating bulk density and allowances from concrete strength.
Different coats can have different quantities
A finish coat may have a different thickness and material ratio from the underlying coat. Combining thicknesses before allocating ingredients would incorrectly treat both layers as one mixture.
A zero thickness omits a layer. The table still identifies its contribution so it is clear whether the base, finish or both coats are included in the total.
Distinguish wet coverage and loose materials
Thickness multiplied by area produces wet layer volume. The entered dry factor and ratio then convert that layer to a procurement scenario, rather than guaranteeing that loose ingredients will yield the exact site volume.
Cement is summed across coats before bag rounding. Site losses, unevenness, product yield and system compatibility need their own appropriate allowances and checks.
Use the result with the next measurement
The brick mortar joint volume calculator budgets joints between brick envelopes. Those hidden joints are separate from the exposed plaster layers measured here.
Flat uniform layers only. No reveals, uneven substrate, mesh, primer, waste, water, strength or compatible-system recommendation. Openings and area are per face.
A few questions, answered
Are openings entered for both faces?
Enter openings per face; the face count multiplies the net result.
Can I omit a coat?
Yes, enter zero thickness for that coat.
Are bags rounded by coat?
No, combined cement mass rounds once.