Calculate bags of render for your external walls
External wall render protects masonry from weather damage while improving the appearance of your property. Calculating the correct quantity of render is important because running short mid-job can cause colour variation between batches, while over-ordering wastes money on heavy, bulky materials that are expensive to return. Render is typically sold in 20-25kg bags in the UK, with each bag covering a specific area depending on the render type and application thickness. The three main render types used in the UK are cement render, monocouche (through-colour render), and lime render. Traditional cement render is the most common and cheapest, applied in two coats at approximately 10mm per coat. Monocouche render is a factory-mixed product applied in a single thick coat of 15-20mm that provides colour and texture without painting. Lime render is softer and more breathable, ideal for older solid-wall buildings and listed properties where cement render could trap moisture and cause damage. This calculator computes the wall area after deducting windows and doors, determines the number of bags needed for your chosen render type and number of coats, and shows the total render thickness. It also provides a comparison chart showing how many bags each render type would need for the same wall area, helping you compare options.
To calculate render quantities: 1. Enter the wall height in metres. Measure from the damp-proof course (DPC) level to the eaves or soffit. 2. Enter the wall width in metres. For multiple walls, either enter each wall separately and add the results, or enter the total combined width. 3. Set the number of coats. Two coats is standard for cement and lime render. Monocouche is designed as a single coat but may need two on uneven surfaces. 4. Select the render type. Cement is cheapest and most common. Monocouche provides a coloured, textured finish. Lime is essential for traditional and heritage buildings. 5. Enter the total area of windows and doors to deduct from the wall area. Measure each opening (height x width) and add them together. 6. Review the net wall area, bags needed, total thickness, and comparison chart.
The render quantity calculation works as follows: Wall area = wall height x wall width - deductions for windows and doors. For a wall 3m high and 8m wide with 2 m2 of window openings: (3 x 8) - 2 = 22 m2. Each render type has a coverage rate per bag at its standard thickness. Cement render: one 20kg bag covers 2.5 m2 at 10mm thickness. Monocouche: one 25kg bag covers 1.5 m2 at 15mm. Lime render: one 25kg bag covers 3.0 m2 at 8mm. Bags per coat = wall area divided by coverage per bag, rounded up to the next whole bag. For cement: ceil(22 / 2.5) = 9 bags per coat. Total bags = bags per coat x number of coats. For 2 coats of cement: 9 x 2 = 18 bags. Total thickness = thickness per coat x number of coats. For 2 coats of cement: 10 x 2 = 20mm total. Render volume = wall area x total thickness in metres. For our example: 22 x 0.02 = 0.44 m3. The comparison chart shows that the same 22 m2 wall needs 18 bags of cement render, 30 bags of monocouche, or 16 bags of lime render at 2 coats each.
Always add 5-10% extra bags to your order for wastage, spillage, and mixing losses. Render should be applied when air temperature is between 5 and 30 degrees Celsius and not in direct strong sunlight, rain, or frost. Scaffold access is essential for working above 2 metres. For cement render, use a 4:1 sand to cement mix with plasticiser, or purchase pre-mixed bags. Monocouche render must be machine-mixed to the manufacturer's specifications. Lime render requires specialist knowledge of lime putty mixing and should ideally be applied by an experienced lime plasterer, particularly on listed buildings.