Plaster Mortar Mix Calculator
Enter plaster area and thickness to get mortar volume and cement/sand requirements by mix ratio — standard default thickness 24mm.
Materials needed
- Mortar volume
- 0.25 m³
- Cement weight
- 129 kg
- Sand
- 0.28 m³
- Mix ratio applied
- 1:3
Based on Korean standard estimation. A 24mm thickness is the typical total for 3-coat plastering (base + middle + finish). The ratio is cement:sand by volume — richer mixes (1:2) need more cement.
What this tool does
The plaster mortar mix calculator instantly computes mortar volume and the cement/sand needed for mix ratios of 1:2, 1:3, or 1:4, from just the plaster area and thickness. Plastering is typically applied in three coats — a base (scratch) coat, a middle (brown) coat, and a finish coat — and Korea's standard estimation guide treats the combined thickness of all three as 24mm by default. The mix ratio is cement-to-sand by volume: 1:2 is used for high-strength, waterproof finishes and needs 680kg of cement plus 0.98m³ of sand per cubic meter; the most common 1:3 needs 510kg of cement plus 1.10m³ of sand; and the leaner 1:4, used for low-strength base coats, needs 385kg of cement plus 1.10m³ of sand. Richer mixes (more cement) give higher strength and better waterproofing, but cost more in materials. The tool multiplies area by thickness to get mortar volume, adds a waste margin (default 5%) for handling loss, then calculates the material quantities and the number of 40kg cement bags for the selected ratio — helping avoid costly misorders on plastering jobs.
Who uses this
- Estimate cement and sand order quantity before plastering walls or floors
- Compare mortar volume across plaster thickness and mix ratio
- Decide between high-strength (1:2), standard (1:3), and low-strength (1:4) mixes
- Check the number of 40kg cement bags needed in advance
- Calculate material purchase quantity when preparing a DIY plastering job
How to use
- 1Enter the area to be plastered (m²).
- 2Enter the plaster thickness (mm) — 24mm is the typical default for a 3-coat (base + middle + finish) job.
- 3Select the mix ratio (1:2, 1:3, or 1:4) and waste margin (default 5%) to instantly see mortar volume, cement/sand needed, and bag count.
Mortar & material formula
Mortar volume (m³) = area (m²) × (thickness (mm)/1000) × (1 + waste rate) Cement (kg) = mortar volume × mix cement rate Sand (m³) = mortar volume × mix sand rate Cement bags = ceil(cement kg / 40) Materials per 1m³ by mix ratio (Korean standard estimation): 1:2 — 680kg cement + 0.98m³ sand 1:3 — 510kg cement + 1.10m³ sand 1:4 — 385kg cement + 1.10m³ sand
Real examples
Example 1: 10m², 24mm thick, 1:3 mix
Mortar 0.252m³ (incl. 5% waste). Cement 129kg (4 bags), sand 0.28m³.
Example 2: 20m², 24mm thick, 1:2 mix
Mortar 0.504m³ (incl. 5% waste). Cement 343kg (9 bags), sand 0.49m³.
Frequently asked questions
How is the 24mm default thickness determined?
Plastering typically has three stages — a base coat to level the substrate, a middle coat to build up thickness, and a finish coat — and Korea's standard estimation guide treats the combined thickness as 24mm. Enter a lower value if you're applying a single thin coat.
When should I use a 1:2, 1:3, or 1:4 mix?
1:2 suits bathrooms and exterior finishes needing waterproofing and high strength; the widely used 1:3 suits general wall and floor plastering; and 1:4 suits low-strength base layers like scratch coats. More cement means more strength but higher material cost.
Why add a 5% waste margin?
Some mortar always sets prematurely or falls away during mixing, transport, and application. Preparing only the exact required amount risks running short on site, so it's standard estimation practice to add a default 5% (more for larger jobs).
How is the cement bag count calculated?
The required cement weight (kg) is rounded up to the nearest 40kg bag. For example, if 129kg is needed, you'd buy 4 bags (160kg) to avoid coming up short.
What sand condition does the calculation assume?
It uses the general sand-mix baseline from Korea's standard estimation guide. Actual requirements can vary with sand grading and moisture content, so larger jobs should run a separate mix trial.
Cautions
- •Material quantities are standard-estimation baseline values — actual usage varies with applied thickness and technique.
- •The 24mm default assumes a 3-coat job; adjust it to match your actual method.
- •The 5% waste rate is a default; raise it to 10%+ for large or complex jobs.
- •Sand quantity is a standard-mix baseline — actual needs can differ with grading and moisture.
- •Confirm current cement/sand pricing with your supplier before ordering.
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Last reviewed: 2026-07-16