Timber Treatment Calculator — How Much Preservative Do I Need?

This calculator determines the volume of timber treatment product you need by dividing the total surface area by the coverage rate for your chosen treatment and application method, then multiplying by the number of coats.

The formula

Litres needed = (Surface Area ÷ Coverage Rate) × Number of Coats

Coverage rates used

• Preservative: 6 m² per litre per coat (brush, roller or spray — the same rate; spray is faster but not more material-efficient)
• Fire retardant: 4 m² per litre per coat (typical clear/solvent intumescent)
• Wood hardener: 3 m² per litre (high-absorption allowance — see note below)

These are typical coverage rates for smooth-planed softwood. Rough-sawn timber, end grain and porous hardwoods will absorb more product and reduce coverage by 20–30% — use the Rough Timber Factor in Advanced Options to allow for this. Always check the manufacturer’s product data sheet for the specific rate.

Wood hardener:the 3 m²/L used here is a deliberate high-absorption (saturation-flood) allowance, not a nominal sheet rate — degraded timber is flooded until saturated, so real usage is far higher than the product’s nominal (Ronseal Wet Rot Wood Hardener quotes up to 10 m²/L in one coat on sound timber). Hardener is normally applied in a single coat.

Fire retardant:4 m²/L is a typical figure for a clear/solvent intumescent coating, but clear and white timber coatings range from roughly 3.7 to 10 m²/L. Achieving a declared reaction-to-fire class (BS 476 or BS EN 13501-1) depends on the certified wet/dry-film loading for the specific product — always follow that product’s tested system rather than this generic rate.

Tin sizing

Results are rounded up to the nearest 5-litre tin, which is the most common trade size. Products are also available in 1L, 2.5L and 25L containers. For large projects, 25L drums offer better value.

Standards

Timber preservation should comply with BS 8417:2024 (Preservation of wood — Code of practice), which replaced BS 8417:2011+A1:2014 in August 2024. Treatment specifications are tied to the use classes defined in BS EN 335. Fire retardant treatments should be tested to BS 476 or BS EN 13501-1 for their reaction-to-fire classification.