Render Quantity Calculator
Start with net external face area, then estimate each chosen render product from its quoted coverage or explicit kg/m² consumption at your already-specified coat and thickness.
How the estimate works
A is the supplied net external face area, already excluding measured openings. E = A × allowancePercent / 100 when an extra is supplied; adjusted area B = A + E. For each separately supplied product, quoted-basis area Q = B for complete quoted build-up coverage, or Q = B × explicitly specified identical layers for per-layer coverage. Coverage mode: bags = ceil(Q / quoted m² per bag). Mass mode: kg = Q × explicitly quoted kg/m²; optional bags = ceil(kg / stated bag kg). Each product is rounded independently, only at the final whole-bag step. Thickness is a compatibility check, never a multiplier; a mismatch or unknown compatible specification prevents a product quantity.
Quantity arithmetic for already-chosen render products on measured net external faces only. No substrate, system, mesh, bead, weather, installation, engineering or permission recommendations; no Materials List output or stock, price, suitability or sufficient-order guarantee. Complete quoted coverage for one product does not include other system products unless the actual quote says so.
Worked example
One source example, not a recommendation: Selco lists Weberpral M Render Chalk 25kg. Weber quotes 1 m² per 25 kg bag at a 15 mm finished thickness; its scraped-finish basis is 18 mm scraped to 15 mm. For 12 m² net face area and an explicitly chosen 5% extra, adjusted area is 12.6 m². With that exact matching finish/thickness, select complete quoted build-up, record 1 m²/bag and calculate 13 bags. Do not multiply by application passes, reuse this figure for a different finish/thickness, or assume it covers separately supplied system products. Numeric inputs remain blank until supplied.
What to measure and check
Measure only the external faces you intend to render; linked openings are already excluded. For product quantities, add one row per separately supplied product. Use the product quote for your already-decided coat/finish and thickness. Complete quoted build-up coverage counts once even if applied in several passes. Use a layer multiplier only for an explicit per-layer quote and identical specified layers; use separate rows for different layer products or thicknesses.
Do I subtract doors and windows here?
No. The shared measurement input supplies net external face area with exclusions already linked to their owning face. This calculator uses that area unchanged and does not deduct openings a second time. Measure any intended reveals or returns as separate faces rather than assuming they are included.
Can I use a bag coverage figure for a different thickness?
No. Enter the exact quoted coat/finish and thickness basis and confirm it matches your already-specified work. If entered thicknesses differ or only one is known, product quantities are withheld. The calculator does not scale coverage or kg/m² using thickness ratios.
Should I multiply by the number of coats or passes?
Only for a quote explicitly stated for one identical specified layer, using your entered positive whole-number layer count. A quote for the complete product build-up counts once, even if the product is applied in more than one pass. Different products or differing layer thicknesses need separate rows and their own compatible quotes.
What if I know kg/m² but not the bag size?
Mass mode calculates kg from your explicit product consumption at the confirmed specification. Bags remain unavailable until the stated kg per bag is supplied. Bag mass alone cannot determine coverage or consumption.
Can I combine bags for several render products?
No. Each separately supplied product has its own quantity and final whole-bag rounding. Bag counts are not pooled across base, finish or other products, and this is not a Materials List.
Does this choose a render specification or guarantee my order?
No. It supplies measured-area and quoted-consumption arithmetic, not substrate, system, mesh, bead, weather, installation, engineering or permission advice. Extra is only what you explicitly enter; source coverage may be approximate and may exclude waste. No stock, price, suitability or sufficient-order guarantee is made.
Where the numbers come from
Formula version v1.0. Implemented and tested 10 November 2026. Product-dependent figures remain your supplied facts; a cited example is not a selected product or a universal yield.