How the wood stain calculator works
Timber finishes are absorbed rather than laid on top, so the coverage printed on the can is only true for smooth, fresh boards. Grey decking whose surface fibres have opened up, or rough-sawn fence panels with a furry texture, can soak up a third more. The calculator therefore multiplies the label figure by a condition factor (1.0 for new planed wood, 0.75 for weathered, 0.6 for rough-sawn) before dividing the surface by it.
Stain = length × width × faces × coats ÷ (label coverage × condition factor) × 1.10For a deck, enter the length and width of the floor. For a fence, enter its length and height and choose both sides if you will also treat the neighbour's face. The 10% margin covers board edges, the ends of joists you can reach and brush loading.
Worked example
A 5 m × 4 m deck laid ten years ago has turned silver-grey. That is 20 m² of top surface. The chosen oil says 8 m² per litre, but on weathered boards the calculator uses 8 × 0.75 = 6 m² per litre. Two coats need 20 × 2 ÷ 6 = 6.67 L, and 7.3 L with the margin, so three 2.5 L cans. A 16 ft × 13 ft deck in the same state, with a label rate of 325 ft² per gallon, works out at 208 ft², a real rate of 244 ft² per gallon and 1.9 gallons: two 1-gallon cans.
Typical coverage by product
| Finish | Smooth wood | Rough or weathered | Look |
|---|---|---|---|
| Penetrating oil or saturator | 8–12 m²/L (325–490 ft²/gal) | 4–7 m²/L (160–285 ft²/gal) | Natural grain, no film |
| Semi-transparent stain | 7–10 m²/L (285–400 ft²/gal) | 4–6 m²/L (160–245 ft²/gal) | Tinted, grain visible |
| Solid-colour stain | 6–9 m²/L (245–365 ft²/gal) | 3.5–5.5 m²/L (140–225 ft²/gal) | Opaque, hides grain |
| Clear water repellent | 10–15 m²/L (400–610 ft²/gal) | 5–8 m²/L (200–325 ft²/gal) | Barely changes colour |
Cans are usually 1 L, 2.5 L and 5 L in Europe and 1 gallon or 5 gallons in the US. Railings, benches and steps are not in the deck area: add them by measuring each rail length times its height, counting both faces.
Getting a lasting finish
- Let new pressure-treated timber dry. Sprinkle water on a board: if it beads, wait; if it soaks in within ten minutes the wood is ready.
- Clean and brighten grey wood first. A deck cleaner removes the loose grey layer; staining over it gives a blotchy result that flakes early.
- Do not overload penetrating products. Oil that cannot soak in stays sticky and turns dark. Wipe off any puddles after 15–20 minutes, or apply the second coat wet-on-wet as the label directs.
- Treat end grain generously. Cut ends and board tops drink the most and rot first.
- Dispose of oily rags safely. Rags soaked in oil-based products can self-heat; lay them flat to dry outdoors or keep them in a sealed metal container with water.
A reproducible calculation with complete inputs
For a reproducible wood stain calculator example, use the metric input system and enter Deck length or fence length: 5 m; Deck width or fence height: 4 m; Faces treated: One face (deck top, or one side of a fence); Wood condition: Weathered, grey or cleaned old wood; Coats: 2 coats; Label coverage per coat: 8 m²/L. These settings describe one specific scenario, not a new default for every job. The result follows the method and allowances already explained above. Keep the entire set of values when comparing it with a hand calculation; omitting a selected material or an allowance can change the result even when the dimensions match. The following rows show the unrounded calculation values alongside the precision used by the interface.
| Result | Calculation value | Displayed value | Unit |
|---|---|---|---|
| Stain needed | 7.33333333333 | 7.3 | L |
| 2.5 L cans | 3 | 3 | |
| Surface to treat | 20 | 20.0 | m² |
| Real coverage per coat | 6 | 6.0 | m²/L |
How should I prepare these entries?
Deck length or fence length: enter the value in m. Deck width or fence height: enter the value in m. Faces treated: choose among One face (deck top, or one side of a fence), Both sides of a fence. Wood condition: choose among New planed or smooth boards, Weathered, grey or cleaned old wood, Rough-sawn boards. Coats: choose among 2 coats, 1 coat (maintenance), 3 coats. Label coverage per coat: Printed on the can for smooth wood. Oils and saturators: 8–12 m²/L; solid stains: 6–9 m²/L. Choose the unit system before transferring your measurements. A depth, length, area and coverage can have different field units even within one system. Read each label individually rather than entering the same unit convention in every box. Optional openings or allowances should describe your actual scenario instead of being silently left at a sample value. The worked inputs above identify exactly which selections produced the table.
Frequently asked questions
How much deck stain do I need for a 12x12 deck?
A 12 ft × 12 ft deck is 144 ft². Two coats of stain rated 300 ft² per gallon on new wood need about 1.1 gallons, or 1.4 gallons if the boards are weathered.
How many square metres does a litre of decking oil cover?
Most decking oils cover 8–12 m² per litre on new planed boards and 4–7 m² on old, grey or rough-sawn wood.
Does a fence need stain on both sides?
If both faces are exposed to weather, yes: an untreated back face lets moisture in and shortens the life of the whole panel. Choose two faces to double the area.
How many coats of wood stain should I apply?
Two thin coats for semi-transparent stains and oils on bare wood, one coat for yearly maintenance, and rarely more than two for solid stains.
How long should new treated wood dry before staining?
Until water no longer beads on it, which can take a few weeks to several months depending on the weather and how wet the timber was when delivered.