Render Calculator
Last updated: 12 August 2026
Work out how much sand and cement you need to render a wall. Handles two-coat work — a scratch coat and a top coat — at the standard UK mix ratios and thicknesses.
How the calculation works
Volume = wall area × total render thickness, plus 15% for wastage (render is messy — more falls off the board than with most materials). We split that volume by the mix ratio, converting sand to tonnes at about 1.6 t/m³ and cement to 25kg bags.
Use rendering sand (a well-graded soft sand), not sharp sand or building sand. The classic UK approach is a stronger scratch coat than top coat — 4:1 under 5:1 — so the render gets progressively weaker outwards and cracks stay in the coat that can take them.
Render mix ratios by background
| Background | Scratch coat | Top coat |
|---|---|---|
| Dense concrete block, engineering brick | 3 : 1 | 4 : 1 |
| Standard brickwork | 4 : 1 | 5 : 1 |
| Lightweight/aerated block | 5 : 1 | 6 : 1 |
| Soft or old masonry | 6 : 1 (or lime) | 6 : 1 |
The golden rule: never render a strong coat over a weak background or a strong top coat over a weak undercoat — the render will crack and blow.
Rendering in UK weather
Do not render below 5°C or in direct summer heat. Frost destroys green render, and fast drying causes shrinkage cracks. Dampen suction backgrounds first, scratch the base coat for key, and let each coat cure for at least 24 hours (ideally longer) before the next.
Frequently asked questions
How much sand and cement do I need to render a wall?
For two-coat render at 20mm total in a 4:1 mix, allow roughly 0.03 tonnes of sand and 7.5kg of cement per square metre. A 20 square metre wall needs about 0.6 tonnes of sand (one bulk bag) and 6 x 25kg bags of cement, including wastage.
What is the correct mix ratio for render?
On standard brickwork, 4 parts sand to 1 cement for the scratch coat and 5:1 for the top coat. Dense backgrounds take 3:1 and 4:1; soft or lightweight blocks need weaker 5:1 and 6:1. The top coat should never be stronger than the coat beneath it.
How thick should render be?
Traditional two-coat sand and cement render is about 20mm total — an 11mm scratch coat and a 9mm top coat. Very uneven substrates may need three coats to 30mm. Thin-coat modern systems go on at 6 to 10mm over insulation boards.
What sand is used for rendering?
Rendering sand, which is a well-graded soft sand, sometimes sold as plastering or render sand. Sharp sand is too coarse for a smooth finish and building sand alone can be too fine, causing shrinkage cracking.
Can you render in cold weather?
Not below 5°C. Frost ruins uncured render, and it will need to come off and be redone. Equally, avoid direct summer sun, which dries the render too fast and causes cracking — dampen the wall and work in the shade where possible.
Related calculators
Plaster Calculator
Bags of multi-finish, bonding or hardwall plaster.
Brick Calculator
How many bricks for your wall — UK sizes, mortar bags and wastage.
Aggregate Calculator
MOT Type 1, gravel, sand or topsoil — tonnes and bulk bags.
Estimates are for guidance only. Material dimensions, densities and coverage vary by supplier — always confirm quantities before ordering, and consult a qualified professional for structural work.