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Timber Treatment Calculator — How Much Preservative Do I Need?
Calculate the litres of timber preservative, fire retardant or wood hardener needed for any surface area. Covers brush and spray application rates.
Total timber surface area to be treated
2 coats standard, 3 for exterior
Enter price per 5L tin for a cost estimate
Container size in litres to round up to (default 5L)
Coverage reduction multiplier for rough-sawn timber (default 1.0, use 1.2–1.3 for rough timber)
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How We Calculate This
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.
Frequently Asked Questions
Wood preservative covers roughly 6 m² per litre per coat — the same whether you brush, roller or spray it. Manufacturers quote a single coverage figure regardless of method (Barrettine Wood Preserver works out at about 5–6 m²/L; Ronseal Total Wood Preserver up to 9.6 m²/L), so spray is faster but not more material-efficient. Most makers recommend 2–3 coats for full protection. Always check the specific product data sheet as coverage varies between brands.
For general preservative treatment, 2 coats are standard for interior timbers and 3 coats for exterior or high-risk areas. Fire retardant treatments typically require 2-3 coats to achieve the required fire rating. Wood hardener is usually applied in a single coat, flooding the timber until saturated, but badly degraded timber may need 2 coats.
Wood preservative protects against fungal decay and insect attack using biocides — it is the standard treatment for structural timbers. Fire retardant treatment contains intumescent chemicals that swell when heated to form a char barrier, slowing fire spread. Wood hardener penetrates and consolidates soft, partially decayed timber by binding fibres together with a resin.
Spray is faster over large areas such as roof timbers and floor joists, but it does not stretch the product further — manufacturers quote the same coverage rate (around 6 m²/L) for brush, roller and spray, and low-pressure spray can actually use slightly more because of overspray. Brush application ensures better penetration into the grain, end grain and joints. For best results, spray the main surfaces and then brush-coat joints, end grain and hard-to-reach areas.
BS 8417:2024 (Preservation of wood — Code of practice) is the current British Standard; it replaced BS 8417:2011+A1:2014 in August 2024. It works with the use classes defined in BS EN 335: Use Class 1 covers interior timber in dry conditions, Use Class 2 covers timber under cover but with a risk of wetting, Use Class 3 covers exterior timber out of ground contact, and Use Class 4 covers timber in ground or fresh-water contact. The treatment type and preservative retention depend on the use class.
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Last updated: February 2026
Verified against UK standards · estimates only, confirm with your supplier.