How much mortar do I need to render or plaster a wall?

Confusing interior coverings with exterior ones is a mistake that costs dearly: interior gypsum plaster doesn't stand up to damp, and using exterior mortar in a bathroom unnecessarily raises the cost of the item for no real need.

Three coverings, three different coverages

• Cement render (exterior or damp areas): 1 bag of 25 kg per 2–2.5 m² at 1 cm thick. Always use cement with water-repellent additives outdoors.
• Gypsum plaster (base coat and skim) (interior): 1 bag of 25 kg per 3–3.5 m² at 1.5 cm. Not suitable for damp areas (bathrooms, kitchens) — there you need cement mortar or specific anti-damp coverings.
• One-coat facade render: 1 bag of 25 kg per 1.5–1.7 m² at 1.5 cm thick, depending on the finish (scraped, floated...). Since the colour is built into the mix, a small future chip is much less visible than on a painted facade.

Worked example

To render 18 m² of facade: 18 ÷ 2.5 = 8 bags of 25 kg. This is the same calculation the PresupuestoYa calculator applies automatically for the Bricklayer trade.

The finishing details that stop the covering cracking

• Corner beads: a profile that protects vertical corners in plastering or rendering. Without it, corners chip over time and with wear — a cheap item that makes a big difference to the final finish, above all in corridors and entrances. Purchase margin: +5% over the length of corners.
• Fibreglass mesh: embedded in the mortar or plaster over joints between different materials (for example, a plasterboard partition against a masonry wall). It's essential at those junctions, because the two materials move differently with temperature and, without mesh, the crack reappears within a few months. Purchase margin: +10% for overlaps.

Frequently asked questions

Can I use interior gypsum plaster in a bathroom? No: gypsum doesn't stand up to damp. In bathrooms and kitchens you need cement mortar or a specific anti-damp covering.

How many bags of mortar do I need for 20 m² of facade with one-coat render? 20 ÷ 1.7 ≈ 12 bags of 25 kg, rounded up.

Why does the junction between plasterboard and traditional masonry crack? Because both materials move differently with temperature. Without fibre mesh embedded in that joint, the crack reappears within a few months even if repaired superficially.

Conclusion

Choosing the right covering for the area (dry interior, damp, or facade) and calculating the real coverage of each mortar is what avoids running short of material halfway through the job or using a product that can't stand the conditions of that wall. Not forgetting corner beads and fibre mesh at the critical points is what makes the finish last without cracking.