Wood Stain or Varnish Quantity Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/9/2026
The amount of stain or varnish needed is calculated with (surface ÷ product coverage) × number of coats. For 20 m² to treat with a product covering 10 m²/L, in 2 coats, 4 liters are needed.
Explanation
Wood stain and varnish protect and maintain outdoor wood (decking, cladding, shutters, gates) or indoor wood, but their actual coverage — the surface covered per liter — varies greatly from one product to another: a thick varnish generally covers less surface than a thin, penetrating stain, and the same product covers less on raw or already-greyed wood (highly absorbent) than on wood that's already been treated. That's why this calculator, like our paint quantity calculator, leaves the coverage as a free input rather than fixing it to a single value: the figure shown on the product's packaging always remains the most reliable reference. The number of coats needed also depends on the surface's condition and intended use: a first application on new or highly absorbent wood often needs 2 to 3 coats for lasting protection, such as on a wood deck like the one covered by our deck board calculator, while a maintenance coat on already-treated wood can be enough with a single coat. Between coats, you generally need to respect a drying time stated by the manufacturer (often several hours), or risk a less durable final result. For the amount of paint for an interior wall rather than an outdoor stain, see our paint quantity calculator.
Example: 20 m² to treat, coverage 10 m²/L, 2 coats
Inputs
Surface: 20 m². Product coverage: 10 m²/L. Number of coats: 2.
Calculation
Quantity = (20 ÷ 10) × 2 = 2 × 2 = 4 liters.
Result
You need to plan for 4 liters of stain or varnish for this job.
Frequently asked questions
How do I find my product's actual coverage?
Coverage (in m²/L) is stated on the packaging or the manufacturer's technical sheet, generally given for one coat on a standard surface. On very absorbent wood (new, raw, or already greyed by weather), the actual coverage can be noticeably lower than the stated value from the first coat, which also acts as a primer: allow a margin if you're not sure of the surface's exact condition.
How many coats should I plan for new outdoor wood?
For new or raw wood, 2 to 3 coats are generally recommended to ensure lasting protection against moisture and UV, since the first coat is often largely absorbed by the wood. For simple annual maintenance of already-treated wood, a single coat is generally enough.
What is the difference between stain and varnish for this calculation?
The calculation itself is identical for both: only the product's coverage (shown on the packaging) and the recommended number of coats change. Stain, generally micro-porous, lets the wood breathe and protects it in depth; varnish forms a protective film on the surface, more waterproof but which can flake over time if not properly maintained.