Wood Stain Calculator

Step 1: Choose Imperial or Metric.

Step 2: Select the surface type for each section of your project. Deck means the horizontal top surface only. Fence is one face of a fence; if you are staining both sides, add the fence twice or double the length. Railing counts both sides automatically. Stairs uses step width × tread depth × number of steps. Custom is for any surface you can describe as length × width.

Step 3: Enter dimensions. For decks and custom surfaces: length × width. For fence: length × height. For railing: rail length × rail height. For stairs: step width × tread depth × step count.

Step 4: Click "+ Add Surface" to add additional sections (deck + railing + stairs is a typical combination for a deck staining project).

Step 5: Set coats. Two coats is the standard recommendation — first coat penetrates and seals, second coat builds color and protection. One coat works for transparent or quick-touch-up stains.

Step 6: Adjust the coverage rate. Default is 175 sqft per gallon (6 sqm/L), which is mid-range. Oil-based stains typically cover 150–250 sqft/gal (4–6 sqm/L); water-based cover 250–350 sqft/gal (6–8 sqm/L); semi-transparent stains cover more area than solid stains. Rough or weathered wood absorbs significantly more — reduce coverage by 20–30 percent.

Step 7: Optionally enter price per gallon for total cost.

Step 8: Click Calculate. Results show gallons (or quarts for smaller jobs) needed, with surface-by-surface breakdown and step-by-step math.

Wood stain is sold in gallons and quarts like paint, but the math runs differently because stain absorbs into wood rather than building a film on top. This affects coverage in three important ways.

Coverage varies dramatically by wood condition. Manufacturer-stated coverage (typically 175 sqft per gallon for mid-range deck stain) assumes smooth, previously-finished wood. Rough or weathered wood absorbs 25 to 50 percent more stain because the open grain takes more product to saturate. Pressure-treated lumber that has weathered for a few months can drop coverage to 100 sqft per gallon for the first coat.

Two coats is standard, but the second coat covers more area. The first coat soaks deeply into the wood (lower coverage). The second coat sits more on the surface (higher coverage). Manufacturers usually quote a combined-coats figure, but if you are buying stain for a known job, plan on the first coat using slightly more than half the total stain.

Oil-based vs water-based. Oil-based stains penetrate deeper and last longer (3–5 years on a deck) but cover less area per gallon — typically 150–250 sqft/gal. They take longer to dry (8–24 hours between coats) and have stronger fumes. Water-based stains cover 250–350 sqft/gal, dry faster (2–4 hours), have low odor, and clean up with water — but typically need re-application every 1–3 years.

Stain coverage and paint coverage are not interchangeable. A gallon of exterior paint covers about 350 sqft on smooth siding; a gallon of deck stain covers about 175 sqft on the same area — half. Always read the can's coverage statement, not the paint-can rule of thumb.

Calculate by surface, not by gallon. Buy stain based on the total square footage of all surfaces (deck top + railing both sides + stairs treads), apply the coverage rate of your specific stain product, multiply by the number of coats, and round up to whole gallons. Buying 10–15 percent extra is recommended for touch-ups and uneven absorption.

For each surface:

Deck or custom: Area = Length × Width
Fence (one side): Area = Length × Height
Railing (both sides): Area = Length × Height × 2
Stairs: Area = Step width × Tread depth × Number of steps

Across all surfaces:

Total area = sum of all surface areas

Gallons needed = ⌈(Total area × Number of coats) ÷ Coverage rate⌉

Example: 16 × 12 ft deck with 40 ft of 3 ft railing and 10 stairs (3 ft wide × 1 ft tread).
- Deck: 192 sqft
- Railing (both sides): 40 × 3 × 2 = 240 sqft
- Stairs: 3 × 1 × 10 = 30 sqft
- Total: 462 sqft
- Two coats at 175 sqft/gal: ⌈462 × 2 / 175⌉ = ⌈5.28⌉ = 6 gallons

Example 1: Single deck. 16 × 12 ft deck (192 sqft), two coats, 175 sqft/gal stain. ⌈192 × 2 / 175⌉ = ⌈2.19⌉ = 3 gallons.

Example 2: Deck + railing. 16 × 12 ft deck (192 sqft) + 40 ft × 3 ft railing both sides (240 sqft) = 432 sqft. Two coats: ⌈864 / 175⌉ = ⌈4.94⌉ = 5 gallons.

Example 3: Privacy fence (one side). 100 ft long, 6 ft tall = 600 sqft. Two coats, semi-transparent oil at 250 sqft/gal: ⌈1,200 / 250⌉ = ⌈4.8⌉ = 5 gallons. For both sides, double the area to 1,200 sqft → 10 gallons.

Two thin coats beat one thick coat. Stain penetrates wood; piling it on causes it to sit on the surface, peel, and flake. Apply the first coat, let it absorb and dry overnight, then apply the second coat thinly.

Wood condition matters more than the product label. Fresh lumber absorbs more than the stated coverage. Old weathered wood absorbs even more. Smooth, previously-stained wood absorbs less. Always have an extra quart in reserve for the first coat.

Strip and clean first. Stain applied over old finish, dirt, or mildew adheres poorly and fails within a year. Power-wash and let dry 48 hours before staining. For old finishes, use a wood stripper or sand the surface first.

Match wood condition to product. New pressure-treated lumber needs to weather for 1–3 months before staining. Manufacturers print this on the lumber tag. Cedar, redwood, and hardwoods can be stained immediately.

Don't stain in direct sun. Hot wood causes stain to dry before it can absorb properly, leading to lap marks and uneven color. Stain in shade or on overcast days, with surface temperatures between 50 and 90°F (10–32°C).

Test color on a hidden spot first. Wood absorbs stain differently than the can swatch suggests, especially on weathered or pressure-treated lumber. Apply a test patch and let it fully dry before committing to gallons of product.