Stain Calculator
Estimate gallons of stain for decks, fences, and wood surfaces - by surface type, coats, and railing.
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For a fence, enter total length × height. For an L-shape, split and add.
Your Stain Estimate
- Enter the surface size - deck length × width, or fence length × height.
- Pick the surface type - rough, new, and weathered wood soak up more stain than smooth or previously stained wood.
- Choose coats - one for a maintenance re-stain, two for bare or weathered wood.
- Add railing and stairs - these add a lot of surface area and are easy to forget.
- Buy the gallons shown - all from the same batch for an even color.
A 200 ft² deck (10 × 20) needs roughly 1 gallon of stain for one coat on average wood, which covers about 200 ft² per gallon. Bare or rough wood drops to 150 ft²/gal and usually wants two coats; add 25% for railings, 40% with stairs. Round up to the nearest quart.
What Counts as Stainable Area
Coverage by Surface
A gallon of stain covers very different areas depending on the wood:
| Surface | Coverage per gallon | Typical case |
|---|---|---|
| Smooth / previously stained | 250 ft² | Maintenance re-coat on sealed wood |
| Average wood | 200 ft² | Most decks in fair condition |
| Rough / new / weathered | 150 ft² | Bare lumber, old gray decks, sawn faces |
How Much Stain Do I Need?
Stain quantity comes down to three things: the surface area, how absorbent the wood is, and how many coats you apply. Multiply the area by the number of coats, divide by the coverage rate for your surface, and round up. This calculator does all three at once - including the coats, which the simpler area-only stain calculators make you work out by hand even though bare wood almost always needs two.
Coverage varies more for stain than for paint because wood porosity differs so much. Smooth or previously stained boards can reach 250 square feet per gallon, while rough, new, or weathered wood may only get 150. Pick the surface type that matches your deck and the calculator uses the right rate.
Worked Examples: Stain by the Gallon
A 12 × 16 maintenance re-coat - 192 ft² of smooth, previously stained boards at 250 ft²/gal - needs 192 ÷ 250 = 0.77 gal for one coat, which rounds up to 1 gallon. Sealed wood is the easy case.
A bare, weathered 16 × 20 deck (320 ft²) with a railing, two coats on rough wood at 150 ft²/gal: 320 × 1.25 for the railing = 400 ft², then 400 × 2 ÷ 150 = 5.3 gal - round up to 5.5 gallons, a 5-gallon pail plus a quart or two.
A 20 × 20 deck with railing and stairs (400 ft², +40%) on average wood at 200 ft²/gal, one coat: 400 × 1.4 = 560 ft², then 560 ÷ 200 = 2.8 gal, rounding to 3 gallons. The railing and stairs alone account for nearly a full gallon here.
Do Not Forget Railings and Stairs
The flat deck boards are only part of the job. Railings, balusters, posts, stair stringers, and trim add a surprising amount of surface area - balusters in particular are coated on all four sides. Forgetting them is the most common reason people run out of stain mid-project.
As a planning shortcut, add about 25% to the flat-board area for a deck with a standard railing, or 40% if it also has stairs. This calculator applies that for you when you choose the railing option. If you want to be precise, measure the railing length times height and the steps separately and fold them into the surface area instead.
Quick Reference: Gallons by Deck Size (1 coat, average wood)
| Deck | Flat area | Flat only | With railing (+25%) |
|---|---|---|---|
| 10 × 12 | 120 ft² | 0.75 gal | 0.75 gal |
| 12 × 16 | 192 ft² | 1 gal | 1.25 gal |
| 16 × 20 | 320 ft² | 1.75 gal | 2 gal |
| 20 × 20 | 400 ft² | 2 gal | 2.5 gal |
At 200 ft²/gal, rounded up to the nearest quart. Double for two coats; bump down a tier for smooth wood, up for rough.
Stain Formulas
Area, adjusted for railing and waste, times coats, over coverage:
Gallons = area × coats ÷ coverage rate
Buy = round up to the nearest quart (stain comes in gallons and quarts)
Rough, weathered, or freshly stripped wood absorbs more - if your deck is in rough shape, choose the lower coverage rate or add 10–15% waste.
The Rest of the Deck or Fence Project
Staining is usually one task in a bigger job. Common companions:
Related Calculators
See the full surface calculator collection for paint, siding, and more.
Frequently Asked Questions
How much stain do I need for a 20x20 deck?
A 20 × 20 deck is 400 square feet. On average wood at 200 square feet per gallon, that is about 2 gallons for one coat, or roughly 2.5 gallons if it has a railing. For two coats on bare or weathered wood, plan on about 4 to 4.5 gallons.
How many coats of stain does a deck need?
Bare, new, or heavily weathered wood usually needs two coats; a maintenance re-stain over a sound finish often needs only one. Always follow the product label - some penetrating stains specifically say not to apply a second coat until the first has dried, which can take 24 to 48 hours.
Does rough or old wood need more stain?
Yes. Rough-sawn, new bare, and old weathered wood are more porous and soak up more stain, dropping coverage toward 150 square feet per gallon. Smooth or previously sealed wood resists absorption and can reach 250. Choose the surface type that matches yours so the estimate is realistic.
Should I include the railing in my measurement?
Yes - railings, balusters, posts, and stairs add significant area, and balusters get coated on four sides. Add about 25% for a railing or 40% for a railing plus stairs, or measure them directly. This calculator includes that adjustment when you select the railing option.
What is the difference between deck stain and fence stain?
Deck stains carry more resin and UV blockers to handle foot traffic and standing water, while fence stains are thinner for vertical surfaces. You can use deck stain on a fence, but fence stain on a deck tends to wear through within a season. Coverage rates are similar, so this calculator works for both.