Render & Plaster Calculator

How to Use the Render & Plaster Calculator

Measure each wall's length and height and multiply them out; for a whole room or elevation, sum the rectangle areas first, then total the openings — every window and door, width times height — and enter that total in the openings field. Render genuinely skips openings, so deduct them properly: unlike wallpaper, where full-height strips still cross a window, there is no material in the glass. Add 10% for reveal returns if you are rendering into the window reveals, which most jobs do.

Thickness is entered per coat, because the volume formula multiplies it by the number of coats. A traditional two-coat render is an 8–10 mm scratch coat (roughly combed or scratched so the next coat keys on) plus a 4–6 mm top coat — 12–15 mm total on reasonably level backgrounds. To model that build-up in one pass, enter the average: (10 + 5) ÷ 2 = 7.5 mm with coats set to 2. Prefer to keep the coats separate? Run the calculator twice — 10 mm × 1 coat for the scratch, 5 mm × 1 coat for the top — and add the materials. The 15 mm default suits single-coat work or genuinely heavy dubbing-out on uneven walls.

Ratios for render tend to run leaner than bricklaying mortar, because a fat, strong mix shrinks and cracks as it dries. Typical practice is a scratch coat around 1:4 (strong enough to grip and level) and a top coat at 1:5 or 1:6, slightly weaker so the two shrink together rather than apart. The right mix also depends on the background — high-suction blockwork pulls water out fast, low-suction concrete and painted walls need bonding agents or spatter dash — and on exposure, which varies with climate. The mortar mix ratios guide covers the reasoning.

One boundary worth drawing clearly: this is a sand-and-cement tool. Gypsum plasters — board finish, one-coat, multi-finish and their regional equivalents — come in bags with the manufacturer's own coverage rates printed on them, coverage per bag at a stated thickness, and those rates beat any generic calculation because the product is designed around them. Check the bag. Sand-and-cement render, by contrast, is mixed on site from loose materials, which is exactly what this calculator quantifies.

Waste in rendering is real: material drops, trowel returns go off, and no wall is as flat as the thickness figure pretends. Ten percent is the sensible default for straightforward walls; push toward 15–20% for heavily uneven backgrounds, dense detail work or a first-time renderer. The droppings also need somewhere to go — sheet the floor and you can re-throw the fresh stuff.

What the numbers do not include: beading, scratch coat curing time between coats (typically a day or more), waterproofer in the mix for exposed work, and the primer or bonding coat the background may demand. For the wall underneath all this, see the brick wall quantity guide.

The Math Behind the Calculator

Area minus openings, times thickness and coat count, times waste — that is the wet volume. From there the mortar constants take over: the 1.33 dry factor, ratio parts, and the standard bulk densities.

The constants are the same standard ones used across the trade and the rest of this site's masonry tools: dry ingredients bulk about a third more than finished wet mortar (factor 1.33), loose cement weighs 1440 kg/m³, and loose dry sand 1600 kg/m³. Thickness is in millimetres, hence the division by 1000.

Worked Example: 10 × 2.5 m Wall with Openings

An exterior wall elevation 10 m long and 2.5 m high, with a door and a window totalling 6 m² to deduct. Render build-up: 10 mm scratch plus 5 mm top, modelled as 7.5 mm per coat × 2 coats. Ratio 1:5, waste 10%. Enter length 10, height 2.5, openings 6, thickness 7.5, coats 2, ratio 1:5, waste 10.

1. Net area: 10 × 2.5 − 6 = 25 − 6 = 19 m².
2. Wet volume: 19 × 0.0075 × 2 × 1.10 = 0.3135 m³ — call it 0.31 m³.
3. Dry volume: 0.3135 × 1.33 = 0.417 m³.
4. Parts: 1 + 5 = 6. Cement volume: 0.417 ÷ 6 = 0.0695 m³ → × 1440 = 100.1 kg → 100.1 ÷ 25 = 4.004 → 5 bags — barely past four, but nobody sells the hundredths.
5. Sand: 0.417 × 5 ÷ 6 = 0.347 ≈ 0.35 m³ — roughly 556 kg, a bit over half a typical 850 kg bulk bag.

Sanity-check the volume by hand: 19 m² at 15 mm total thickness is 0.285 m³ before waste, and 10% brings it to 0.31 m³ — matches. If you re-run the same wall at a single 15 mm coat (thickness 15, coats 1) the materials are identical, because 7.5 × 2 and 15 × 1 are the same total thickness. That is the arithmetic working as intended; the coat split matters for technique, not for volume.

Assumptions & Limitations

Every number above rests on these constants. If your project differs, change the matching input — or read the linked guide for the full reasoning.

• Dry factor 1.33, cement 1440 kg/m³, sand 1600 kg/m³ — standard constants as used across the site's masonry calculators
• Thickness applies per coat and multiplies by the coat count — enter the average for unequal coats
• Typical build-ups quoted (8–10 mm scratch, 4–6 mm top) are common practice, not a specification — backgrounds and exposure vary
• Sand-and-cement render only — gypsum plasters follow the coverage rates printed on the bag
• Quantities only — no beading, bonding agent, waterproofer, primer or labour