Mould Treatment Calculator — How Much Fungicidal Wash Do I Need?

This calculator determines the quantities of fungicidal wash, steriliser and optional anti-mould paint needed for mould remediation based on the affected area and severity of contamination.

The formulas

• Fungicidal wash = (Area ÷ 5) × Severity multiplier
• Steriliser = (Area ÷ 3) × Severity multiplier
• Anti-mould paint = (Area ÷ 12) × 2 coats

Coverage rates

• Fungicidal wash: ~5 m² per litre per application (e.g. Polycell Mould Remover at 200 ml/m²)
• Surface steriliser: ~3 m² per litre per application (typical ready-to-use biocide on a porous substrate — varies by product and dilution, so always check the label; adjustable in Advanced Options)
• Anti-mould paint: ~12 m² per litre per coat, 2 coats recommended (e.g. Ronseal 6-Year Anti-Mould, Zinsser Perma-White)

Severity adjustments

• Surface mould: Standard rate (×1.0) — mould on paint surface only
• Penetrating mould: ×1.5 rate — mould has penetrated into plaster
• Deep / recurring mould: ×2.0 rate — substrate affected, multiple treatments needed

Product sizing

Fungicidal wash and steriliser are available in 500ml, 1L and 5L containers. This calculator rounds up to 5L containers (the most economical trade size). Anti-mould paint is typically sold in 2.5L and 5L tins. Results are rounded up to 2.5L tins.

Health and safety

Wear gloves, goggles and an FFP2/FFP3 mask when treating mould, and ensure adequate ventilation. Biocide handling is governed by COSHH (the HSE’s L5 Approved Code of Practice covers COSHH generally, not mould specifically). As a rough industry guide — mirroring the US EPA’s ~10 sq ft (~1 m²) figure — consider a professional assessment for larger or recurring outbreaks. In rented housing, damp and mould growth is assessed as a hazard under the Housing Health and Safety Rating System (HHSRS), administered by local authorities under the Housing Act 2004, and providers should follow the UKHSA/gov.uk 2023 damp-and-mould guidance.