Render Calculator UK

How to Calculate How Much Render You Need

The method is the same one render contractors use on site:

1. Measure each wall: length and height in metres. A typical single-storey gable wall might be 6m long and 2.5m to the eaves.
2. Calculate the area: length × height. Our example: 6 × 2.5 = 15 m².
3. Subtract openings: windows and doors aren't rendered. A standard window is around 1.2 × 1.2m (1.44 m²).
4. Check your coverage rate: a 25kg bag of premixed sand and cement render covers roughly 1.1 m² at 15mm total (two coats).
5. Divide area by coverage: total area ÷ coverage per bag = bags needed, rounded up.
6. Add waste: add 10% for mixing losses, trowel drag and wind-blown material.

Our calculator does this automatically and adjusts for your chosen render system and thickness.

UK Render Systems and Coverage Rates

Coverage depends heavily on the render system and how thick it's applied. These are typical trade figures for a 25kg bag:

These are typical figures for standard bagged products. Always check the coverage rate printed on your specific bag, as it varies between manufacturers and can change with wall porosity and application method (hand or spray).

Traditional Two-Coat Render vs One-Coat Systems

• Traditional sand and cement (2-coat): a scratch coat (8-10mm) is applied and scored to give a key, left to firm up, then a top coat (5-7mm) is applied and finished with a texture (float, scraped or bagged finish). Total thickness is usually 12-18mm.
• One-coat monocouche/silicone render: a factory-coloured, polymer-modified render applied in a single pass, typically 15-25mm thick. Faster to apply and needs no painting, but the bagged material costs more per m² than traditional render.
• Pebbledash (roughcast): a wet bedding coat is applied and stone chippings are thrown on and lightly pressed in while it's still workable. Common on older UK properties, particularly 1930s-1960s housing stock.

Whichever system you choose, always render onto a sound, clean background. Blockwork and brick usually need a PVA or bonding treatment first to control suction. See our plaster calculator for similar guidance on bonding coats.

Weather and Curing Conditions for Rendering

Render is far more weather-sensitive than interior plaster because it's exposed as soon as it's applied:

• Temperature: apply between 5°C and 25°C. Below 5°C the cement won't cure properly and frost can blow the surface off; above 25°C or in direct sun the render dries too fast and cracks.
• Rain protection: protect fresh render from rain for at least 24-48 hours with sheeting or a scaffold debris net, especially on exposed elevations.
• Wind: strong drying wind causes the same rapid-drying cracking as direct sun. Damp down the render lightly if conditions are hot and windy.
• Curing time between coats: for a two-coat system, let the scratch coat firm up for at least 24 hours (longer in cold weather) before applying the top coat.

Frequently Asked Questions

How many bags of render do I need for a house?

It depends entirely on wall area. A typical semi-detached house elevation of around 40 m² (after subtracting windows and doors) needs roughly 36-40 bags of traditional two-coat sand and cement render at 15mm, or around 40-44 bags of one-coat monocouche at 20mm.

How much does a 25kg bag of render cover?

A 25kg bag of traditional two-coat sand and cement render covers roughly 1.1 m² at a standard 15mm total thickness. One-coat monocouche render, applied thicker at around 20mm, typically covers slightly less per bag.

How thick should render be?

Traditional two-coat sand and cement render is usually 12-18mm total. One-coat monocouche and silicone renders are typically applied thicker, at 15-25mm, since they don't have a separate top coat.

Can I render over old render or brick?

Yes, provided the background is sound, clean and not loose or hollow-sounding. Old render that's cracked or blown should be hacked off first. Bare brick or blockwork usually needs a PVA or spatterdash bonding treatment to control suction before the first coat goes on.

What's the best weather to render in?

Aim for a mild, dry, overcast day between 5°C and 25°C with light or no wind. Avoid direct sun, frost and rain, all of which cause cracking or washout before the render has cured.

Do I need scaffolding to render a house?

For anything above ground floor height, yes, proper scaffolding gives a safe, stable platform and lets you work a full elevation in one session, which matters for an even, consistent finish. Ground floor and garden walls can often be done from a stepladder or podium.