Render Calculator
Render is worked out as a volume, area × thickness, and then turned into what you actually buy: bags of cement and a bulk bag of sand for a site mix, or a count of 25 kg bags for monocouche and bagged base coats. Enter the wall, pick the system, and the calculator adds waste and rounds up.
Your measurements
Check your order
Your wall and ceiling order
- Measure each surface and opening; check thickness and product coverage.
- Confirm sheet or pack sizes, cuts and the required fixing or finishing system.
- List corners, joints and finishing materials separately from the main surface quantity.
Tools for this job
Check the render system before choosing finishing tools. A steel trowel may help with application, while the final texture can require a sponge, wood or plastic float.
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- Spread and smooth compatible materials
Finishing trowel
MARSHALLTOWN MXS64D finishing trowel
A steel finishing trowel to consider for spreading and smoothing work. Follow the material instructions for tool choice and timing; some finishes need a different float or surface texture.
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How much render do I need?
Render quantity starts as a volume: the net wall area multiplied by the total thickness of all the coats. A 25 m² wall at 15 mm is 0.375 m³ of wet render. From there the route depends on what you are buying. For a site mix, the volume is bulked up by 30% for the dry materials, split into cement and sand by the mix ratio, and turned into bags of cement and bulk bags of sand. For a bagged product, the bag coverage is scaled to your thickness and the area is simply divided by it.
Measure every wall you are rendering along its length, multiply by the height, and take off the doors and windows at their full size. A 15% waste allowance is realistic: render drops off the hawk and the trowel, the scratch coat fills hollows in the brickwork, and what is left in the mixer at the end of the day goes in the skip.
The formula
Render mixes and coverage
The mix figures are what this calculator returns for the worked example below: 25.1 m² at 15 mm with 15% waste. Change the ratio in the calculator and the split moves with it.
| Render | Assumption | Material |
|---|---|---|
| 1 : 4 sand and cement | Example ratio, not a wall specification | About 162 kg cement and 720 kg sand per 0.43 m³ |
| 1 : 5 sand and cement | Example ratio, not a wall specification | About 135 kg cement and 750 kg sand per 0.43 m³ |
| 1 : 6 sand and cement | Example ratio, not a wall specification | About 116 kg cement and 770 kg sand per 0.43 m³ |
| Weber weberpral M/D, 25 kg | 18 mm applied, scraped to a 15 mm finish | Approximately 1 m² per bag before site waste |
| Cement base coat, 25 kg | Editable planning assumption at 10 mm | 1.5 m² per bag; replace with product data |
Worked example
Two walls with a total length of 12 m and a height of 2.5 m, one door and two windows, rendered at 15 mm in a 1 : 4 sand and cement mix with 15% waste.
Wall area = 12 × 2.5 = 30 m². Deduct 1.9 m² for the door and 3 m² for the windows = 25.1 m². Volume = 25.1 × 0.015 = 0.3765 m³, plus 15% = 0.43 m³ of render.
Dry materials = 0.433 × 1.30 = 0.563 m³. Cement = 0.563 ÷ 5 = 0.1126 m³ × 1440 = 162 kg, so 7 bags of 25 kg. Sand = 0.1126 × 4 = 0.450 m³ × 1600 = 720 kg, so one 850 kg bulk bag. Using the Weber scraped monocouche example of 1 m² per bag, the same wall takes 25.1 × 1.15 ÷ 1 = 28.865, rounded up to 29 bags. A 40 m² wall with the same allowance takes 46 bags (1,150 kg).
Site-mixed or bagged?
A site mix and a bagged system need different ordering information. A site mix uses the specified cement-to-sand ratio and total coat thickness. If coats have different mixes, calculate them separately. Surface preparation, sand grading, cure time, reinforcement and compatibility with the wall come from the system specification. If a paint finish is specified, estimate it separately with the exterior paint calculator.
For bagged render, use product-specific coverage at the chosen finish. Weber’s coverage guidance gives about 1 m² per 25 kg for its scraped weberpral M/D example, allowing an 18 mm application scraped to at least 15 mm, before site waste. Roughcast and greater exposure can require different quantities. The calculator’s thickness scaling is an approximation; use the product consumption calculator when the finish or specification differs.
Measuring and the mistakes that cost a bag
- Measure the wall from the DPC or the bell cast bead, not from the ground, and to the eaves or the top bead.
- Enter the total thickness of all coats. The most common error is entering the top coat only and buying half the sand.
- Uneven brickwork or a wall that has had old render hacked off eats far more scratch coat than the drawing suggests. Use 20% waste, or dub out the hollows first and measure again.
- Cement goes off in the bag within a couple of months in a damp store, so buy for the job in hand rather than in bulk.
- For the brickwork itself use the brick calculator and the mortar calculator; for inside walls, plaster rather than render: the plaster calculator.
Sources and assumptions
Site-mixed render uses area × total thickness, an allowance and approximate dry-material conversions. Bagged render uses the coverage entered at its reference thickness, scaled to the requested thickness.
- Weber: how much does a bag of render cover?: For a scraped weberpral M or D finish, Weber gives approximately 1 m² per 25 kg bag: applied at 18 mm and scraped to at least 15 mm, excluding waste and substrate irregularity.
Before using this estimate
- Published coverage must match the finish and reference thickness. Scraping losses already included in a manufacturer coverage figure should not be added a second time.
- Thickness scaling is an estimate, not permission to exceed a render system’s limits. Background, reinforcement, curing and each coat’s mix require the chosen system specification.
Source notes reviewed . See how we check calculations and handle corrections.
Planning your order
Check the inputs before buying: measure the surfaces you need, then apply an allowance and round to whole packs. The guides include worked examples you can follow alongside this calculator.
Published by OrderEnough. This is a planning estimate: check the product specification and the assumptions above before ordering. Last updated . Read how we source our figures on the methodology page.