Deck coating Calculator: measuring notes and method
Measure all finished faces
Include the deck surface and any additional steps, edges or rails receiving the same product. Add the underside only if it will be coated. A footprint measures only the top face.
Use one coating system per item
Oil, stain and other coatings can have different application rates. Use the product coverage per coat and required coat count; do not count coats twice if a label gives yield for a complete system. Keep differing products, timber species or surface conditions as separate estimates. Cleaning and preparation products are separate quantities.
CHECK A PRACTICAL EXAMPLE
Keep the deck and extra timber faces visible
Suppose the same finish covers a 20 m² deck plus 8 m² of measured rail and step faces. Both options use two coats at an illustrative 8 m²/L.
Deck surface only
- Finish required
- 5 L
- Containers to buy
- 2
Deck plus 8 m² extras
- Finish required
- 7 L
- Containers to buy
- 2
Only combine surfaces using the same finish and coverage basis. If rails use a different product or rate, save them as a separate item instead.
Fixed comparison in metric units. Values are illustrative unless linked to a recorded project; changing the calculator above does not change this example.
CHECK A PRACTICAL EXAMPLE
Top face only, or top and underside?
An illustrative deck has 20 m² on each broad face. Both options use one application at 8 m²/L, in 4 L containers.
20 m² top face
- Finish required
- 2.5 L
- Containers to buy
- 1
40 m² both faces
- Finish required
- 5 L
- Containers to buy
- 2
Add the underside only if it receives this product. Neither option includes board edges, rails or steps. Product instructions determine the actual preparation, coverage and applications.
Fixed comparison in metric units. Values are illustrative unless linked to a recorded project; changing the calculator above does not change this example.
Formula
Coating = (area − excluded area) × coats ÷ coverage per coat × (1 + waste ÷ 100). Containers are rounded up.
Product reference: Sherwin-Williams coverage guidance
Worked examples in metric and imperial units
Illustrative values match Reset example and stay fixed when you edit the calculator. Both columns describe the same calculation; converted inputs are rounded.
Metric worked example
- Total surface area
- 24 m²
- Openings / excluded area
- 0 m²
- Number of coats
- 2 coats
- Coverage per coat
- 8 m²/L
- Container size
- 4 L
- Waste allowance
- 5 %
Finish required: 6.3 L
Imperial worked example
- Total surface area
- 258.3339 ft²
- Openings / excluded area
- 0 ft²
- Number of coats
- 2 coats
- Coverage per coat
- 325.9667 ft²/US gal
- Container size
- 1.0567 US gal
- Waste allowance
- 5 %
Finish required: 1.6643 US gal
Follow the calculation
- Net area = 24 − 0 = 24 m².
- Coat area = 24 × 2 = 48 m².
- Quantity = 48 ÷ 8 × 1.05 = 6.3 L. Divide by 4 L and round up for containers.
Unit conversions
Liquid conversions use US gallons (3.785411784 L), not UK imperial gallons (4.54609 L).
Reviewed 19 October 2026. How calculations are checked · Report a correction