How to measure for fence paint
Fence: measure the length of the fence along the ground and the height of the panels, then choose one side or both. The calculator works out length × height × sides. If you are treating the posts and gravel boards too, measure the height from the ground rather than from the bottom of the panels.
Shed walls: measure the length and width of the shed, the height of the walls at the eaves and, for an apex roof, the height at the ridge. The calculator works out the four walls, 2 × (length + width) × eaves height, and adds the two triangular gables, width × (ridge − eaves). For a pent roof, follow the note by the field. The roof itself isn’t included, and doors and windows aren’t taken off.
Deck: measure the length and width. Only the top surface is counted, so allow a little more for the edges and any steps.
Any area: type an area in square metres you have already worked out, such as a gate, a trellis or a summerhouse.
Coverage by product and wood
Coverage is the area one litre covers in one coat, and it changes more with the wood than with the brand. Rough-sawn timber, the slightly furry surface of most fence panels and sheds, soaks up paint; smooth planed timber takes much less. These are the figures UK makers give on their tins and data sheets, from Cuprinol and Ronseal. For fence treatment the calculator starts from the lowest of them, Cuprinol Ducksback’s, because every maker’s figure is an ‘up to’. The buttons apply each one to the calculator.
| Surface | Product | Coverage per coat | Coats | Apply |
|---|---|---|---|---|
| Fence panelsRough-sawn | Fence treatment | 4.8–6 m² per litre | 1–2 coats | |
| Shed wallsRough-sawn | Coloured garden paint | 3–5 m² per litre | 2 coats | |
| Planed timberSmooth wood | Coloured garden paint | 10–12 m² per litre | 2 coats | |
| DeckTimber boards | Decking oil | 8 m² per litre | 2 coats |
Compare the two garden paint rows: the same paint covers two to four times as much planed wood as rough-sawn. Coverage on tins is often written as ‘up to’, which is the best case, so on a rough fence use the lower figure.
Rough-sawn or planed: the key to coverage
Most fence panels and sheds are made of rough-sawn timber, left straight from the saw with raised fibres that soak up paint. Planed timber has been smoothed, so the same litre goes much further. Cuprinol’s figures for Garden Shades show it plainly: 3–5 m² per litre on rough-sawn wood against 10–12 m² per litre on planed wood. That one choice changes the answer more than anything else you enter.
Old, dry or weathered timber tends to soak up more on the first coat. When in doubt, use the lower coverage from the tin and add a little extra allowance: a spare litre is easier to deal with than running out halfway along a fence.
One coat or two?
Most fence and shed treatments ask for two coats on rough wood: Cuprinol’s Ducksback and Ronseal’s Fence Life Plus both do. Some are made to go on in one, such as Ronseal’s One Coat Shed & Fence Protector; for those, set Coats to 1. Two coats need twice the paint, so after the coverage this is the setting that matters most.
Coverage on a tin is usually given per coat. If yours gives one figure for two coats together, set Coats to 1 and type that figure into Coverage per coat.
Sheds, decks and other jobs
The same sum works for anything made of timber: the area × the number of coats ÷ the coverage per coat, plus any extra you add. Only the measuring changes.
Shed paint calculator
Pick Shed walls and the calculator becomes a shed paint calculator: it works out the four walls up to the eaves and adds the two gable triangles from the height at the ridge. The Shed button starts with a coloured garden paint on rough-sawn wood at 3–5 m² per litre, because most sheds are clad in rough-sawn boards. If your shed’s cladding is smooth, planed timber, choose the planed product and the litres drop sharply.
Decking oil coverage
Decking oil goes further than fence treatment. Ronseal gives up to 8 m² per litre for its decking oil, and the calculator uses that per coat with two coats, the cautious reading. Cuprinol gives the coverage of its UV Guard oil for both coats together, which works out at more, so check how your tin puts it. Only the top of the deck is counted; add the edges and steps if you are oiling them too.
Which tins to buy
Fence and shed treatments usually come in tins of 5 litres and 9 litres; garden paints and decking oils come in smaller tins as well. The calculator tries every mix of the sizes your chosen product is sold in and picks the fewest tins that cover the litres you need without leaving a whole small tin’s worth over. Any spare shows under the result as paint for touching up.
Buy it all at once if you can. A tin bought later for the last few panels may come from a different batch, and the colour can differ slightly.
Worked example: both sides of a fence
Here is the full calculation for treating both sides of a straight run of 1.83 m high panels with a fence treatment for rough-sawn wood, at the coverage and number of coats the Fence button sets. Every line comes from the same code as the calculator above.
- Surface 12 m × 1.83 m × 2 sides = 43.92 m²
- Paint 43.92 m² × 2 coats ÷ 4.8 m² per litre + 0% = 18.30 litres
- Tins the fewest tins that cover 18.3 litres, with less than the smallest tin left over = 2 × 5 litre tins + 1 × 9 litre tin
On the list:
- 25 litre tins of fence treatment, rough-sawn wood18.3 litres needed
- 19 litre tin of fence treatment, rough-sawn wood
Notice how quickly the litres add up: painting both sides doubles the area, and two coats double it again, so you are covering four times the face of the fence. Notice too which tins are chosen: the calculator compares every mix of the sizes this treatment comes in and goes for the fewest tins that don’t leave a lot over.
UK paint and treatment tin sizes
Fence treatments, garden paints and decking oils are all sold by the litre in the UK, in a handful of tin sizes. Each product comes in only some of them, and the calculator only offers the sizes your chosen product is sold in.
| Product | Size | Good for |
|---|---|---|
| Fence treatment, rough-sawn wood | about 4.8 m² per litre per coat | Shed and fence treatments in 5 and 9 litre tins. |
| Garden paint, rough-sawn wood | about 4 m² per litre per coat | Coloured wood paint on fences and sheds. |
| Garden paint, smooth planed wood | about 11 m² per litre per coat | Coloured wood paint on planed timber. |
| Decking oil | about 8 m² per litre per coat | Oil for timber decks. |
| 1 litre tin | 1 litre | |
| 2.5 litre tin | 2.5 litres | |
| 5 litre tin | 5 litres | |
| 9 litre tin | 9 litres |
The coverage in this table is the typical figure the calculator starts from. Brands differ, and the figure on your own tin should always win: type it into Coverage per coat and the litres and tins update straight away.
Common mistakes when buying fence paint
- Using the smooth-wood figure on a rough fence. Rough-sawn panels can take two to four times as much paint as planed timber. Use the rough-sawn coverage.
- Counting one side when you’ll paint two. Choose Both sides if you’re treating the back of the fence as well.
- One coat when the tin says two. Most treatments need two coats on rough wood. One-coat products say so clearly on the tin.
- Measuring the shed to the ridge all round. Only the gable ends rise to the ridge; the long walls stop at the eaves. Enter both heights and the calculator does the rest.
- Mixing up per-coat and two-coat coverage. Some decking oils give the coverage for both coats together. Check before you type it in.
- Trusting ‘up to’. The coverage on the front of a tin is the best case. Rough, dry or old wood gets less.
What else you'll need
- A new fence first? The fence calculator counts the panels, posts and post-fixing concrete.
- A new deck to oil? The decking calculator works out the boards, joists and screws.
- A base for a new shed: the shed base calculator works out the materials for it.
- A stiff brush and dust sheets: one to clean dirt and moss off the wood before you start, the other to keep treatment off plants and paving.
- Brushes, a paint pad or a sprayer, whichever suits the job and the product.
Last checked 28 October 2026