Exterior Render Coverage Calculator

It depends on the render system and thickness, not a single universal figure. As a planning guide: sand & cement uses about 1.5kg of dry mix per m² per mm of thickness (so roughly 27kg/m² at a typical 18mm two-coat depth), monocouche/one-coat render uses roughly 1.3–1.7kg/m²/mm (about 30kg/m² at 20mm), lime render uses roughly 1.6–1.7kg/m²/mm (about 33kg/m² at 20mm), and a silicone topcoat uses roughly 1.67kg/m²/mm of grain size (about 2.5kg/m² at the standard 1.5mm grain). Enter your wall size and chosen system above for an exact figure with wastage included.

Measure the total length of every wall you're rendering along the ground and add the lengths together if there's more than one wall, then measure the average height from ground/damp-proof course level to the eaves or parapet. Multiply length by height for the gross area, then this calculator deducts your windows and doors automatically to get the net renderable area.

Yes — you only render solid wall, not glazing or door leaves. This calculator deducts a standard 1.5m² per window and 2m² per door from the gross area automatically based on how many you enter. If your property has unusually large or small openings, adjust the window/door counts to approximate the correct total deduction.

A traditional two-coat sand and cement render is typically an 8–12mm scratch coat (10mm is common) followed by a 6–8mm top or float coat, for a total of roughly 16–20mm. This calculator defaults to 18mm, which you can adjust if your specification calls for a different depth.

Manufacturer datasheets for monocouche systems such as K Rend, Weber Pral M and Parex Monorex typically specify a minimum of 15mm, a typical application of 20mm, and a maximum of around 25mm. Going thinner than the minimum risks cracking and poor coverage; this calculator defaults to the typical 20mm.

Sand & cement is the traditional site-mixed two-coat system, cheapest per bag but the most labour-intensive to mix and apply correctly. Monocouche (one-coat, through-coloured) render is a factory-blended polymer-modified render applied in a single thicker coat, faster to apply and doesn't need painting since the colour runs through the material. Lime render is a traditional, breathable system generally used on older/solid-wall or listed buildings where a cement-based render would trap moisture. Silicone topcoat is a thin, flexible, dirt-resistant decorative finish applied over an existing basecoat or external wall insulation (EWI) system rather than a full-depth render in its own right.

Because it's doing a different job. Silicone topcoat is a thin (typically 1.0–2.5mm grain) decorative and weatherproofing finish applied over an existing basecoat, render, or EWI system — it is not a substitute for the structural render/basecoat itself. The comparison table shows it side-by-side with the three full-depth render systems for reference, but if you don't already have a basecoat or EWI system in place, you'd need one of the other three systems (or a dedicated basecoat product) underneath it, which this calculator does not currently price separately.

Indicative mid-2026 UK street prices for a 25kg bag/tub: sand & cement pre-mixed render roughly £9–£13, monocouche roughly £12–£20, lime render roughly £12–£20, and silicone topcoat roughly £60–£120 (silicone tubs vary widely by brand and colour). These are material-only prices — they exclude labour, scaffold hire, primer/basecoat, beading and delivery.

UK trade guidance commonly suggests 5% for a smooth, well-prepared substrate, 10% as a general standard for typical blockwork or brick, and up to 15% for a rough, uneven or older wall where more material is lost to surface irregularities. This calculator sets the wastage automatically from your substrate condition selection.

Render should not be applied below 5°C or when frost is forecast, since cold, damp conditions slow or disrupt cement hydration and can cause the render to fail. If you're planning autumn or winter work, check the forecast for the full curing period, not just application day, and budget extra time between coats since cold weather slows curing.

A scratch coat typically needs a minimum of around 48 hours' cure before the next coat goes on (longer in cold weather), and the surface is typically scored with a scarifier while still green but firm to create a key for the next coat. Full curing of the complete system takes considerably longer than the between-coat interval — follow your specific product's data sheet for exact times.

For anything beyond a single-storey wall reachable from a step ladder, yes — most UK render contractors use scaffolding for safe, consistent access across a full elevation, and it's typically priced and hired separately from the render material itself. This calculator only estimates material quantity and cost, not labour or access equipment.

Per 25kg bag, lime render (roughly £12–£20) typically costs more than standard sand & cement (roughly £9–£13), and it's usually applied as a three-coat rather than two-coat system, increasing both material use and labour time. It's chosen for older solid-wall or listed buildings where its breathability prevents trapped moisture, not as a general cost-saving option.

That means either your wall dimensions are outside the plausible domestic or light-commercial range this calculator is built for (very small or very large), or the windows and doors you entered would deduct more area than the wall actually has. Re-check your length and height are in metres (not feet), and that your window/door counts roughly match what's really on that elevation.

Yes — add together the length of every wall you're rendering (all four elevations, or just the ones you're doing) for the length field, use the average height across those walls, and enter the total window and door count across all the walls included. For a very mixed-height property, it's more accurate to calculate each roughly-uniform section separately and add the bag totals together.