What this calculator works out
This calculator multiplies deck area by the number of coats and divides by the coverage rate, giving the litres of oil needed.
Decking coverage figures are optimistic for the first coat on bare or weathered timber, which drinks considerably more than the tin suggests.
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Before you order
- Measure the deck area including the visible edges of the boards. A deck with many narrow boards has more surface than its footprint.
- Coverage figures assume smooth prepared timber. Weathered, rough-sawn or grooved boards take noticeably more on the first coat.
- Two coats is normal. Bare new timber often needs three, with the first acting as a primer.
- Do not oil a damp deck. Timber needs to be dry throughout, which after rain means several dry days.
Decking oil penetrates the timber, feeds it and slows water ingress while letting the grain show. Stain adds pigment and sits closer to the surface. Paint forms a film that can peel and trap moisture underneath. For decking that gets wet and walked on, a penetrating oil is generally the most forgiving and the easiest to refresh.
How this calculator works
Area times coats, divided by coverage:
Litres = deck area × number of coats ÷ coverage in m² per litreThe calculation applies the same coverage to every coat. In practice the first coat on bare timber may use half again as much as later coats, so ordering slightly over the calculated figure is sensible.
Worked example: a 25 m² deck
Using the default figures — 25 m², two coats, 12 m² per litre:
- Litres: 25 × 2 ÷ 12 = 4.17 litres
Five litres is the practical order, and on bare timber it may still be tight — a first coat on new decking can absorb 8 m² per litre rather than 12. Grooved boards, which are common and which many people lay grooves-up, have noticeably more surface area than smooth ones and consume more again.
Common mistakes
- Oiling damp timber. The oil cannot penetrate and the finish fails.
- Using tin coverage for bare timber. The first coat absorbs considerably more.
- Skipping preparation. Cleaning and, where needed, sanding matters more than the product.
- Applying in direct hot sun. It dries before it soaks in and leaves a patchy surface.
- Laying grooved boards grooves-up. They hold water and dirt and are more slippery, not less.
Frequently asked questions
How often should decking be oiled?
Typically once a year, and twice in the first year on new timber. A deck in full sun and heavy use may need more; a sheltered one less. The test is water: if it beads on the surface the finish is still working, and if it soaks straight in the deck is due.
Should grooved boards face up or down?
Down, in most cases, despite the widespread belief that grooves provide grip. They hold water, dirt and algae, which makes them more slippery rather than less, and they are harder to clean. Laying the smooth face up gives a surface that sheds water and is easier to maintain.
How do I stop decking being slippery?
Keep it clean — the slipperiness is algae rather than the timber. Clear leaves and debris, improve airflow underneath, cut back overhanging planting, and clean annually. Anti-slip strips or inserts are available for steps and shaded areas where growth is persistent.
Can I paint decking instead?
You can, and it generally performs worse. Paint forms a film that traps moisture in the timber and peels where boards flex and get wet, which is everywhere on a deck. Stripping failed paint from decking is a substantial job. Penetrating oils and stains are more forgiving and easier to refresh.
Is what I enter stored?
No. Measurements are processed in your browser and never transmitted or retained.
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References
- Health and Safety Executive — safe use of solvent-based products and slip hazards
- Royal Horticultural Society — garden structures and timber maintenance
Sources are checked at publication and can change — how I choose and check references.