How much deck stain do I need per square metre?

Most oil-based deck stains cover 8 to 12 square metres per litre per coat on smooth, previously coated timber. Rough sawn or new untreated timber is more absorbent and can drop coverage to 4 to 6 square metres per litre. Always check the coverage rate printed on the tin you plan to use, since products vary. Two coats are standard for new or bare timber, and one maintenance coat is often enough for a deck that already has a sound coating.

Do I need to allow extra for wastage when calculating deck stain?

Yes. A waste allowance of around 10 percent is standard practice to cover uneven application, spillage, absorbent end grain, and touch-ups. Deck boards also have far more surface area than a flat floor plan suggests once you account for the gaps between boards and the edges, so builders often add a further allowance for board edges on gap-decking.

How many coats of stain does a deck need?

New or bare timber typically needs two coats to get adequate film build and UV protection. A deck that is being recoated as maintenance, and still has most of its existing coating intact, often only needs one coat. Manufacturers such as Resene, Cabot's and Intergrain publish specific recoat guidance for their products, so check the data sheet for your chosen stain.

Deck Stain Calculator NZ

This calculator works out how many litres of deck stain or oil you need to cover your deck, plus the total cost, so you buy the right amount before you start. You enter your deck's length and width, the timber surface type (existing smooth coating, standard gap decking, or rough sawn and untreated timber), the coverage rate from your chosen product's tin in square metres per litre per coat, the number of coats you plan to apply, a waste allowance percentage, and the price per litre. It multiplies your deck's footprint by a surface adjustment factor that accounts for the extra area created by board gaps and porous timber, factors in your coats and coverage rate to work out the litres required, then adds your waste allowance for spillage and touch-ups. You get back your deck area, the effective area to coat, the total litres of stain needed, and the estimated total cost, along with a suggested combination of tin sizes to buy and a full calculation breakdown. Use it before heading to the paint shop so you buy enough stain from one batch to finish the whole job, since colour can vary slightly between batches. Coverage rates vary by brand, product type and timber condition, so always check the label on your chosen stain or oil. This is an indicative estimate only, meant to guide your budget and purchase.

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Updated Coverage rates based on typical NZ deck stain and oil product data sheets.

1. Deck Size

Deck length

Deck width

Timber surface (gap decking adds edge area)

2. Stain and Coats

Coverage rate (check your tin's label)

m²/L per coat

Number of coats

Waste allowance

Price per litre

Your Deck Stain Results

Deck Area

24.00 m2

Length x width

Effective Area to Coat

27.60 m2

Adjusted for surface type

Stain Needed
6.75 L
Including waste allowance

Estimated Cost

$303.60

At your price per litre

Calculation Breakdown

Deck footprint area6.00m x 4.00m = 24.00 m2

Surface adjustment factorx1.15

Area per coat27.60 m2

Number of coats2

Total area to cover55.20 m2

Coverage rate9.0 m2/L per coat

Litres before waste allowance6.13 L

Litres needed (incl. waste)6.75 L

Cost and Buying Guide

Price per litre$45.00 per litre

Litres needed6.75 L

Estimated total cost$303.60

Suggested tins to buy1 x 10L tin (or 4L tins as needed)

Summary: Enter your deck dimensions above.

How the Deck Stain Calculator Works

This calculator multiplies your deck's length by its width to get the footprint area, then applies a surface adjustment factor to account for the extra area created by gaps between deck boards and rougher, more absorbent timber. It multiplies that effective area by the number of coats you plan to apply, then divides by your product's coverage rate (square metres per litre per coat) to work out the litres of stain required. Finally, it adds your chosen waste allowance to cover spillage, uneven application and touch-ups, and multiplies by your price per litre to estimate total cost.

Typical Deck Stain Coverage Rates

Product Type	Coverage per Coat	Typical Coats Needed
Oil-based deck stain (smooth timber)	8 to 12 m²/L	2 (new), 1 (maintenance)
Water-based deck stain	6 to 10 m²/L	2
Rough sawn or new untreated timber	4 to 6 m²/L	2 to 3
Solid colour deck coating	6 to 8 m²/L	2

Coverage always varies by brand and product, so check the technical data sheet for the specific stain or oil you plan to use. New Zealand brands such as Resene, Cabot's, Intergrain and Dulux publish their own coverage figures.

Why Gap Decking Needs More Stain Than the Footprint Suggests

A deck measured as 6m by 4m has a footprint of 24 square metres, but the actual surface area you paint is larger once you account for the top edges of each board either side of the gaps. Standard 5mm gaps between boards typically add 10 to 15 percent extra surface area compared with a solid, flat area of the same footprint. Rough sawn or new untreated timber is also more porous and soaks up more product per coat, which is why this calculator lets you choose a surface adjustment factor.

Tips for a Good Result

Clean and lightly sand the deck before staining, especially if recoating over an old finish
Apply stain along the length of the boards, not across them, for an even finish
Check the weather forecast. Most stains need at least 24 hours dry weather to cure properly
Buy slightly more than the calculated amount so you have leftover product for future touch-ups from the same batch
Store leftover stain sealed and out of direct sunlight for future maintenance coats

Related Calculators

Sources: Typical coverage rates published by New Zealand deck stain and oil manufacturers (Resene, Cabot's, Intergrain, Dulux) on product data sheets, available from resene.co.nz, cabots.co.nz and dulux.co.nz.

This calculator provides an indicative estimate only. Actual coverage varies by product, application method, timber condition and porosity, and weather. Always check the coverage rate stated on the tin of the product you buy, and round up to the nearest full tin size when purchasing.