Render calculator
Estimate render quantity from wall area, openings, finished thickness, product consumption per millimetre, waste allowance and bag size.
Render quantity estimate
Enter wall area, openings, target thickness and the manufacturer’s consumption per millimetre to estimate kilograms and bags.
Total thickness across all passes. NF DTU 26.1 requires at least 10 mm at every point for a single-coat render.
See the product data sheet (often 1.3 to 1.8 kg/m² per mm).
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How does this tool work?
Facade render is bought in 25 or 30 kg bags, but its coverage is given on the data sheet in kg per m² per millimetre of thickness. This calculator bridges the two: it takes the gross wall area, subtracts the openings, multiplies the target finished thickness by the product’s consumption, applies that need per m² to the net area and adds an allowance before rounding up to whole bags.
Net area is gross area minus doors, windows and areas left unrendered: a 50 m² facade with 6 m² of openings leaves 44 m² to render.
Need per m² is finished thickness × consumption per mm: 15 mm × 1.5 kg/m²/mm gives 22.5 kg/m². The thickness entered is the total thickness of all coats, measured on a flat substrate.
Theoretical mass is net area × need per m²; the site allowance is added, then the total is divided by the bag weight and rounded up to the next bag.
Limits: consumption per mm is a data-sheet value for mortar mixed with the prescribed amount of water; an uneven substrate, local build-ups, window reveals and sills and mixing losses increase it. Scratch coat, primer, mesh and corner beads are not counted. The 10 mm minimum at every point for a one-coat render comes from NF DTU 26.1 in France; elsewhere, EN 13914-1 and the manufacturer’s instructions are the reference.
Render volume depends on surface area and applied thickness; conversion to bags should then use the yield of the chosen product. Substrate flatness and absorption, as well as the number of coats, can change real consumption.
How to use this tool
Measure the facade
Enter the gross wall area in m², gables included, then the combined area of doors and windows to deduct.
Set the finished thickness
Enter the target average total thickness in millimetres, at least 10 mm at every point for a one-coat render.
Use the product data sheet
Enter the consumption in kg/m² per mm and the bag weight stated by the manufacturer.
Adjust the allowance and read the bags
Keep 10% for a simple facade, more for an uneven substrate, then check the need per m² and the number of bags.
Render consumption
The quantity depends on net area, finished thickness and consumption per millimetre.
Consumption per m²
c = thickness (mm) × consumption (kg/m²/mm)
For example 15 × 1.5 = 22.5 kg/m².
Mass to plan
M = (A − openings) × c × (1 + allowance)
A wall area, Ao openings.
Bags
Bags = ⌈M ÷ bag weight⌉
Rounded up to whole bags.
Reveals, sills, corners and local build-ups use more than flat wall: keep a generous allowance on a façade with many details.
Use cases
Re-rendering a detached house
Estimate one-coat render bags for 100 to 200 m² of facade after deducting the windows and front door.
Comparing two renders
Enter the consumption per mm from two data sheets to compare kilos and bags at equal thickness.
Costing extra thickness
See how many bags going from 10 to 15 or 20 mm adds on an uneven block wall.
Examples
50 m² façade at 15 mm
These are the form’s default values.
Input
50 m² gross, 6 m² openings, 15 mm, 1.5 kg/m²/mm, 10% allowance, 25 kg bagsResult
44 m² net, 22.5 kg/m², 990 kg theoretical, 1,089 kg to allow for, 44 bagsSame façade at the 10 mm minimum
Aiming at exactly 10 mm leaves no tolerance: in practice the average thickness has to be higher.
Input
44 m² net, 10 mm, 1.5 kg/m²/mm, 10% allowance, 25 kg bagsResult
15 kg/m², 660 kg theoretical, 726 kg to allow for, 30 bagsTips and best practices
- Order all bags from the same production batch: a through-coloured render can vary slightly in shade from one batch to another.
- Check wall flatness with a straightedge before choosing the thickness: 5 mm of average hollows means about 7.5 kg more per m² at 1.5 kg/m²/mm.
- Include window reveals in the area: their depth, often 150 to 200 mm, adds a strip around every opening you deducted.
- For a two-pass render, enter the total thickness once: do not multiply the result by the number of passes.
Sources and references
Frequently asked questions
Why enter a thickness instead of a number of coats?
Single-coat render is applied in one or two passes, but the quantity depends on the total thickness achieved: two 8 mm passes use as much as one 16 mm pass. Enter the total finished thickness.
Should I deduct every window?
For an accurate estimate, deduct significant openings. On small openings, losses and reveals can offset part of the deduction.
Does the calculator work for interior plaster?
It can give an estimate if the data sheet states a consumption per mm of thickness, but products and thicknesses must suit the actual use.
How much render per m²?
One-coat render data sheets often state about 1.4 to 1.8 kg/m² per millimetre. At 1.5 kg/m²/mm and 15 mm, that is 22.5 kg/m² before allowance.
How many bags for 100 m² of wall?
At 15 mm and 1.5 kg/m²/mm, 100 m² net needs 2,250 kg, or 2,475 kg with a 10% allowance and 99 bags of 25 kg. Deducted openings reduce that total.
When can external render be applied?
Outside frost, rain, strong heat and high wind, within the temperature range stated by the manufacturer. A base that is too dry or hot makes the render dry too fast and encourages cracking.
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