How much deck stain or fence stain do you need

How much stain do I need? The formula

Stain needed = area to treat × number of coats ÷ coverage per coat, plus a 10% margin. Coverage is on the can, in square feet per gallon in the US and square metres per litre in the UK. The area depends on what you treat:

1. Fence with boards or pickets. Length × height × sides you treat × the solid part. With 3 in pickets and 1 in gaps the gaps are 1 ÷ 4 = 25%, so 75% of the fence is wood. If you stain both faces, that is 2 sides.
2. Deck. Length × width, seen from above.
3. Shed. The outside walls: 2 × (length + width) × wall height, minus doors and windows.
4. Garden or patio furniture. You enter the area: measure tops, seats and backs and count both faces.
5. Shutters. Window width × height × number of shutters × 3. The factor 3 comes from Italian public works price lists: the Umbria region counts louvered shutters "three times" the opening and treats every coat as applied on both faces.

Then: area × coats ÷ coverage × 1.10. Nothing is rounded until we pick the cans.

A worked example

In feet and gallons (US). A picket fence 50 ft long and 4 ft high, 25% gaps, stained on both sides with two coats of a water-based stain at 250 sq ft per gallon, 10% margin. This is how the calculator opens in US units.

• Area: 50 × 4 × 2 × 0.75 = 300 sq ft
• Stain: 300 × 2 ÷ 250 × 1.10 = 2.64 gallons

The calculator suggests three 1-gallon cans and a stain brush. A 16 × 12 ft deck is 192 sq ft and needs 1.69 gallons: two gallons, with a deck stain pad on a pole instead of the brush.

In metres and litres (UK). A fence 10 m long and 1.50 m high, 25% gaps, both sides, water-based stain at 10 m²/L, two coats, 10% margin.

• Area: 10 × 1.50 × 2 × 0.75 = 22.5 m²
• Stain: 22.5 × 2 ÷ 10 × 1.10 = 4.95 L

With UK tin sizes that is one 5 L tin. With a decking oil at 16 m²/L the same fence needs 3.09 L: one 2.5 L tin and one 0.75 L tin.

In metric the calculator opens with this second fence. Visitors in the US see the first one; everyone else starts in metric. You can switch units at any time.

Stain coverage per gallon and per litre

Coverage is almost always a range, and the first coat on bare wood goes less far than the next, because the wood soaks it up. The calculator starts from the cautious end of each range:

• US, any type: 250 sq ft per gallon. PPG gives 250-350 sq ft per gallon both for the acrylic Olympic Quick Cover and for the oil-based Sun Proof 77-1260, depending on texture and porosity. Sherwin-Williams says one gallon covers about 250-350 sq ft of deck.
• Water-based, metric: 10 m²/L. Boero Biolegno Acqua: 10-11 m²/L for the first coat, over 12 after.
• Solvent-based, metric: 8 m²/L. Boero Biolegno: 8-10 m²/L for the first coat, over 15 after.
• Decking oil, metric: 16 m²/L. Cuprinol says a 2.5 L can covers up to 20 m² with two coats, so 20 × 2 ÷ 2.5 = 16 m²/L per coat. Osmo states about 24 m²/L, but on smooth, planed wood; Ronseal says up to 8 m²/L.

With two coats and the 10% margin at 250 sq ft per gallon:

Area	Stain	Cans
100 sq ft	0.88 gal	1 gallon
200 sq ft	1.76 gal	2 gallons
500 sq ft	4.4 gal	one 5 gallon
1,000 sq ft	8.8 gal	5 gal + 4 × 1 gal

If your can says something else, enter it in the advanced options. If it gives a range, use the lower number.

Cans and tins on sale

In the US deck and fence stains come in quarts, gallons and 5-gallon pails, as on the Olympic Maximum product page. In the UK Wickes lists fence paint in 0.75, 1, 2.5 and 5 L and Screwfix sells decking oil in 2.5 and 5 L tins; the calculator uses 0.75, 2.5 and 5 L.

Among the combinations that cover the need we pick the one with the lowest score: litres bought plus half a litre per can, and a full litre for cans under a litre, which cost more per litre. It is the same rule as our paint calculator. For 2.64 gallons, three gallons beat two gallons and three quarts: quarts cost more per litre and count a full litre each.

Common mistakes

• Counting one side of the fence. Staining both faces doubles the area. In the US example one side drops the need to 1.32 gallons: one gallon and two quarts.
• Forgetting the gaps. Without the 25% gaps the US fence becomes 400 sq ft and 3.52 gallons: four cans instead of three.
• Measuring shutters like a window. Louvers and frame have much more surface than the opening, which is why the factor is 3.
• Using the second-coat coverage for every coat. On new or stripped wood the first coat goes less far.
• Ignoring rails and posts. The calculation counts the faces of the boards. With wide rails or heavy posts, raise the margin in the advanced options.

These estimates do not replace the maker's instructions: when in doubt, follow the data sheet of the product you buy.