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Mould Treatment Calculator — How Much Fungicidal Wash Do I Need?
Calculate fungicidal wash, steriliser and anti-mould paint quantities for mould-affected surfaces. Covers walls, ceilings, tiles and grout with severity-based treatment rates.
Total area affected by mould growth
Add anti-mould paint as a preventative barrier (2 coats at 12 m²/L)
Average cost per litre across all products for estimate
Fungicidal wash ~5 m²/L (e.g. Polycell Mould Remover at 200 ml/m²) — check your product label
Typical surface steriliser ~3 m²/L on porous substrates — check your product label, as concentrates dilute differently (ready-to-use biocides range ~3–7 m²/L)
Anti-mould paint ~12 m²/L per coat (e.g. Ronseal 6-Year, Zinsser Perma-White), 2 coats — check your product label
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How We Calculate This
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.
Frequently Asked Questions
Fungicidal mould wash covers approximately 5 m² per litre for surface mould. For penetrating mould that has gone into the plaster, apply 1.5 times the standard rate. For deep, recurring mould, apply double the standard rate (2 applications). Always allow 24 hours drying time between applications and before repainting.
The correct treatment sequence is: 1) Scrape off any loose material and wipe away surface mould. 2) Apply fungicidal wash liberally and leave for the recommended dwell time (usually 4-6 hours). 3) Apply a second coat for moderate/deep mould. 4) Once dry, apply steriliser to kill remaining spores. 5) Allow to dry fully (24-48 hours). 6) Apply anti-mould paint as a preventative barrier (2 coats). 7) Address the underlying cause (ventilation, insulation, heating).
Black mould can pose significant health risks, particularly for people with asthma, allergies, respiratory conditions and weakened immune systems. Mould spores can cause allergic reactions, respiratory irritation, coughing and, in severe cases, lung infections. As an industry rule of thumb — mirroring the US EPA guide of roughly 10 sq ft (~1 m²) — professional remediation is worth considering for larger or recurring outbreaks; note the UK authority, the UKHSA/gov.uk 2023 guidance "Understanding and addressing the health risks of damp and mould in the home", sets no fixed square-metre threshold. Children, elderly people and those with existing health conditions are most vulnerable.
Mould requires moisture, warmth and an organic food source (such as plaster, paint or wallpaper) to grow. In UK homes, the main causes are: condensation from poor ventilation (especially in bathrooms, kitchens and bedrooms), inadequate heating (cold surfaces), water leaks (from roofs, plumbing or rainwater penetration), and poor insulation (cold walls and bridges). Condensation is by far the most common cause, responsible for an estimated 80% of mould problems.
Anti-mould paint contains fungicides that inhibit mould growth on the paint surface for typically 5 years. However, it is not a substitute for addressing the underlying cause of mould. If the surface remains damp due to condensation, poor ventilation or water ingress, mould will eventually grow through or around the paint. Anti-mould paint is most effective as part of a combined strategy: treat the mould, improve ventilation and insulation, then apply anti-mould paint as a preventative layer.
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Last updated: February 2026
Verified against UK standards · estimates only, confirm with your supplier.