Wood Stain Calculator — How Much Stain, Varnish or Oil Do I Need?

A standard UK fence panel is 6ft x 6ft (1.83m x 1.83m), giving an area of 3.35 m² per panel per side, so 10 panels on one side is 33.5 m². Ronseal Fence Life Plus covers roughly 4 m² per litre per coat and its data sheet specifies two coats on rough-sawn wood and three on smooth planed wood. For two coats over 33.5 m² that is 33.5 × 2 ÷ 4 = about 16.7 litres of treatment; adding the calculator's default 10% wastage allowance brings it to roughly 18.4 litres. Buy at least that much, for example a 9L tub plus two 5L tins (19 L), or four 5L tins (20 L). Two 9L tubs is only 18 L and a 9L tub plus a single 5L tin is only 14 L, so both would leave you slightly short once wastage is included. Add a third coat for smooth planed timber, and double the figure if you are treating both sides.

Most decking oils like Ronseal Ultimate Protection Decking Oil recommend 2 coats for new wood and 1-2 coats for maintenance. The first coat soaks into the timber and the second provides the protective finish. New, bare timber may require an additional coat. Important: grooved or profiled deck boards have far more surface area than the plan dimensions suggest. Both Ronseal and Cuprinol decking-oil data sheets advise applying liberally into the grooves, which typically needs roughly double the product of a smooth deck. This calculator works from the plan area (length × width), so allow extra for grooved boards. Allow 4-6 hours drying time between coats depending on weather conditions.

Coverage varies widely with the product and the porosity of the wood, so treat these as typical cross-brand estimates rather than exact figures. As a guide, general wood stains cover about 10 m²/L per coat (Sadolin Classic Wood Protection quotes up to 10 m²/L, more on smooth planed timber and less on sawn). Varnishes cover around 15 m²/L per coat, thanks to their thinner application. Decking oils cover about 6 m²/L per coat (a deliberately conservative figure versus the 6.4-9.6 m²/L quoted by Cuprinol UV Guard and Ronseal Ultimate). Fence treatments cover around 4 m²/L per coat (the low end of Ronseal Fence Life Plus's 4-6 m²/L range, which suits absorbent rough-sawn timber). Rough-sawn and bare timber drink up far more than previously treated wood, so always check the specific product's data sheet.

Yes, lightly sand between coats of varnish with 240-grit sandpaper and remove dust with a tack cloth. This creates a key for the next coat to adhere to and helps achieve a smooth, professional finish. Do not sand between coats of oil or fence treatment; simply apply the next coat once the first has dried.

Most wood stains and treatments should be applied when the temperature is above 5 degrees C and rain is not expected within 2-4 hours. Cold weather slows drying time significantly. Avoid application in direct strong sunlight as this can cause uneven drying and lap marks. Spring and autumn are typically the best times to treat exterior timber in the UK. If you are also painting exterior walls, try our Masonry Paint Calculator.