Construction

Rough Plaster Mortar Calculator: m³, Cement Bags & Sand

Calculate exactly how much mortar, cement bags, and sand you need for rough plaster (scratch coat). Enter wall area and thickness — get m³, bag count, and sand volume instantly. Free, no sign-up.

Data verified · June 2026
Edited by Martín Rodríguez
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Use this calculator to find the exact mortar volume (m³), cement bags, and sand needed for a rough plaster or scratch coat job. The core formula is Mortar (m³) = Area (m²) × Thickness (m), then material quantities follow from the mix ratio. For a standard 1:4 (cement:sand) mix using 50 kg bags, you need roughly 6 bags of cement and 0.9 m³ of sand per m³ of mortar. Rough plaster (also called scratch coat or base coat) is the first thick cement-sand layer applied directly to masonry before finish coats. Standard thickness is 12–25 mm for exterior and 10–15 mm for interior walls.

When to use this calculator

Estimating cement bags and sand volume before purchasing materials for a 30 m² exterior brick wall at 2 cm thickness
Calculating mortar needed to plaster interior masonry walls of a new bathroom before tiling
Determining total material cost for a contractor bidding on a multi-room rough plaster job covering 150 m²
Planning supply orders where scratch coat thickness varies between 1.5 cm and 2.5 cm across different wall zones
Verifying a subcontractor's material quote by independently computing expected mortar volume and cement bag count

Factores de mezcla y densidades de referencia para mortero de revoque (mezcla 1:4)

Parámetro	Valor	Detalle
Relación de mezcla (cemento:arena)	1:4	Por volumen; estándar para revoque grueso/base coat
Densidad aparente del cemento Portland	1.440 kg/m³	Base para cálculo de masa de cemento
Densidad aparente de arena seca	1.600 kg/m³	Base para estimación de masa de arena
Factor cemento (bolsas 50 kg por m³ de mortero)	≈ 6 bolsas	= m³ × (1/5) × 1.440 ÷ 50
Factor arena (m³ por m³ de mortero)	× 0,9 m³	Compensa el relleno de vacíos
Tamaño de bolsa estándar internacional	50 kg	(EE.UU.: 42,6 kg / 94 lb — confirmar con proveedor)
Espesor típico exterior	12–25 mm	Scratch coat / base coat en mampostería exterior
Espesor típico interior	10–15 mm	Base coat en muros interiores
Factor de desperdicio — muros lisos	+ 10 %	Sobre el volumen calculado
Factor de desperdicio — mampostería irregular	+ 15–20 %	Sobre el volumen calculado

Fuente: ASTM C926 (Application of Portland Cement-Based Plaster) y ASTM C150 (Portland Cement), astm.org

How it works

How the Rough Plaster Mortar Calculation Works

The calculation follows a three-step volumetric approach:

Step 1 — Mortar Volume
Mortar (m³) = Area (m²) × Thickness (m)
Example: 30 m² × 0.02 m = 0.60 m³

Step 2 — Cement (1:4 mix, 50 kg bags)
Cement mass = Mortar × (1/5) × 1,440 kg/m³
            = 0.60 × 0.20 × 1,440 = 173 kg
Bags (50 kg) = ⌈173 ÷ 50⌉ = 4 bags
[Factor shortcut: bags ≈ m³ × 6]

Step 3 — Sand
Sand (m³) = Mortar × 0.9  (accounts for void fill)
Example: 0.60 × 0.9 = 0.54 m³

> Key assumptions: Portland cement bulk density ≈ 1,440 kg/m³; dry sand bulk density ≈ 1,600 kg/m³; bag size = 50 kg (standard international); 1:4 cement:sand mix ratio by volume.

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Quick Reference Table — Common Plaster Jobs

Wall Area (m²)	Thickness (cm)	Mortar (m³)	Cement Bags (50 kg)	Sand (m³)
10	1.2	0.12	1	0.11
20	1.5	0.30	2	0.27
30	2.0	0.60	4	0.54
50	2.0	1.00	6	0.90
75	2.5	1.88	12	1.69
100	2.0	2.00	12	1.80
150	2.5	3.75	23	3.38

Values do not include the waste factor. Add 10% for smooth walls, 15–20% for rough or irregular masonry.

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Typical Scenarios

Case 1 — Small bathroom (interior, 1:4 mix, 1.5 cm coat)

Area: 18 m²; Thickness: 1.5 cm → 0.015 m

Mortar: 18 × 0.015 = 0.27 m³

Cement: ⌈0.27 × 6⌉ = 2 bags (50 kg)

Sand: 0.27 × 0.9 = 0.24 m³ (~384 kg)

Case 2 — Exterior facade (1:4 mix, 2 cm coat)

Area: 30 m²; Thickness: 2 cm → 0.02 m

Mortar: 0.60 m³

Cement: 4 bags (50 kg)

Sand: 0.54 m³ (~864 kg)

Case 3 — Large commercial wall (2.5 cm coat)

Area: 100 m²; Thickness: 2.5 cm → 0.025 m

Mortar: 2.50 m³

Cement: 15 bags (50 kg)

Sand: 2.25 m³ (~3,600 kg)

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Common Mistakes When Estimating Plaster Mortar

1. Forgetting to convert cm to m — Entering 2 instead of 0.02 inflates volume by 100×. The calculator handles this automatically.

2. Ignoring the waste factor — Always add 10% for smooth walls, 15–20% for rough or irregular masonry. Skipping this leads to mid-job material shortages.

3. Wrong mix ratio — 1:3 is for waterproofing; 1:6 is for lightweight interior work. Using 1:4 everywhere overspends on cement for interior jobs.

4. Not subtracting door and window openings — A 30 m² gross wall with 2 doors (2 m² each) and 2 windows (1.5 m² each) has only 23 m² of actual plaster surface.

5. Confusing bag sizes — 50 kg is standard internationally; 42.6 kg (94 lb) is the US Portland standard. Always confirm with your supplier.

Worked Example: 30 m² exterior wall at 2 cm

Mortar volume = 30 m² × 0.02 m = 0.60 m³

Cement bags (1:4 mix, 50 kg bags) = ⌈0.60 × 6⌉ = 4 bags

Sand = 0.60 × 0.9 = 0.54 m³ (~864 kg)

With 10% waste: order 5 bags cement + 0.60 m³ sand

0.60 m³ mortar — 4 cement bags — 0.54 m³ sand

Frequently asked questions

How much mortar do I need for rough plaster per square metre?

For a 2 cm (20 mm) scratch coat, you need 0.02 m³ of mortar per m² (before waste). At 1.5 cm thickness it is 0.015 m³/m², and at 2.5 cm it is 0.025 m³/m². Always add 10–15% for waste and surface irregularities.

How many cement bags do I need to plaster 30 m² at 2 cm thickness?

For 30 m² at 2 cm with a 1:4 mix and 50 kg bags: Mortar = 30 × 0.02 = 0.60 m³; Cement = ⌈0.60 × 6⌉ = 4 bags. To include a 10% waste buffer, buy 5 bags and 0.60 m³ of sand.

What is the standard thickness for a rough plaster coat?

Standard practice is 9–15 mm (about 1–1.5 cm) for interior walls and 15–25 mm (1.5–2.5 cm) for exterior or exposed masonry. Less than 9 mm risks cracking; more than 25 mm in a single pass may slump. For thick builds, apply in two passes.

What cement-to-sand ratio should I use for rough plaster?

A 1:4 (cement:sand) ratio by volume is standard for exterior rough plaster. A 1:5 ratio suits interior non-wet areas. A 1:3 ratio is used for wet zones (bathrooms, below-grade) for added waterproofing. ASTM C926 and BS EN 998-1 cover these recommendations in detail.

How much sand do I need per bag of cement for rough plaster?

In a 1:4 mix with 50 kg bags, each bag of cement corresponds to about 0.14 m³ of sand (roughly 4× the cement volume by loose bulk). For a typical 30 m² job at 2 cm, you need 0.54 m³ of sand and 4 bags of cement.

Should I add lime to the mortar mix for rough plaster?

Hydrated lime at 10–20% of cement weight improves workability and reduces shrinkage cracking. The typical Type S mortar (Portland cement + lime + sand) follows a 1:½:4.5 ratio per ASTM C270. For scratch coats on concrete block, a straight Portland-sand mix is often preferred for better mechanical bond.

Does this calculator work for both interior and exterior walls?

Yes — the volumetric formula is the same for any surface. What changes is the recommended thickness and mix ratio: exterior walls typically need a 1:4 mix at 15–20 mm; interior walls can use 1:5 at 10–15 mm. Enter the correct thickness for your application and the calculator handles the rest.

How do I calculate sand weight from the volume shown?

Multiply the sand volume (m³) by the bulk density of dry sand: approximately 1,600 kg/m³. Example: 0.54 m³ × 1,600 = 864 kg. If ordering bagged sand (typically 25–30 kg bags), divide the total kg by the bag weight.

What type of Portland cement is used for rough plaster base coats?

ASTM C150 Type I or Type II Portland cement is standard in the US. Type I is general-purpose; Type II offers moderate sulfate resistance for exterior or high-humidity applications. In the UK/EU, CEM I (BS EN 197-1) or equivalent is used. All are suitable for rough plaster scratch coats.

Sources & references

Methodology & trust

Editorial

Construction calculator with its formula verified automatically against ASTM C926 – Standard Specification for Application of Portland Cement-Based Plaster, per our editorial policy and methodology.

Updates

Updated: June 2026. Parameters are verified periodically against the cited sources.

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Limitations

Indicative results. For critical decisions, consult a professional.

📌 How to cite this calculator

Rodríguez, M. (2026). Rough Plaster Mortar Calculator: m³, Cement Bags & Sand. Hacé Cuentas. https://hacecuentas.com/en/rough-plaster-mortar-calculator

Content licensed under CC-BY 4.0 — reuse it citing the source with a link to Hacé Cuentas.

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