A 16 × 12 ft deck with 40 ft of railing and four steps has about 491 sq ft to stain, not 192: the railing alone adds 281 sq ft. Two coats of Behr Premium Semi-Transparent stain need about 3 gallons; one coat of Cabot Australian Timber Oil on smooth boards needs 2.
How much stain a deck needs
Stain goes on every surface you can see, so the calculator adds three areas: the deck floor, the railing counted face by face, and the stair treads and risers. Gallons are that area divided by the spread rate on the manufacturer's data sheet, coat by coat.
Gallons = area ÷ first-coat spread rate + (coats − 1) × area ÷ later-coat spread rate
Data sheets give a range (250 to 300 sq ft per gallon, for example) because thirsty wood takes more. The calculator buys for the low end of the range, so you only return an unopened can.
Worked example
- Floor: 16 × 12 ft = 192 sq ft
- Railing: 40 ft × 7.02 sq ft per foot = 281 sq ft
- Stairs: 4 steps × 3 ft × (11¼ in tread + 7½ in riser) = 18.75 sq ft
- Total: 491 sq ft
Behr's sheet for its semi-transparent waterproofing stain (No. 5077) asks for two thin coats: the first at 250 to 300 sq ft per gallon, the second at 500 to 600. That is 491 ÷ 250 + 491 ÷ 500 = 2.95 gallons at most, 2.46 at best: buy 3 gallons.
Spread rates from the data sheets
| Stain | Type | Spread rate per gallon | Coats on the sheet | 491 sq ft needs |
|---|---|---|---|---|
| Cabot Australian Timber Oil (3400) | transparent, oil | 400 to 600 smooth, 250 to 350 rough | 1 only | 2 gal |
| Sherwin-Williams SuperDeck semi-transparent | semi, oil | 150 to 400 | 1 only | 4 gal |
| Behr Premium semi-transparent (5077) | semi, water | 250 to 300, then 500 to 600 | 2 | 3 gal |
| Thompson's WaterSeal semi-transparent | semi, water | 150 to 400 | not stated | 4 gal |
| Behr Premium solid color (5011) | solid, water | 200 to 400 | not stated | 3 gal a coat |
Two of the oil stains say plainly to apply one coat: Cabot writes "Requires a 1-coat application" and Sherwin-Williams "do not apply more than one coat". The calculator holds those products to one coat.
Where the railing number comes from
A railing has far more surface than its outline suggests. For a 36 in railing (the minimum height for a deck guard in IRC R312.1.2), the calculator counts four faces of 2×2 balusters set 5 in apart so the gap stays under the 4 in sphere of R312.1.3, two 2×4 rails, a flat 2×6 cap and a 4×4 post every 6 ft. That comes to 7.02 sq ft per foot of railing, and it grows with height. A metal or glass infill railing with only a wood cap has much less; switch the railing off and add its cap to the floor area instead.
Transparent, semi-transparent or solid
Transparent and semi-transparent stains soak in and show the grain, so they spread by how much the wood absorbs: rough-sawn and weathered boards take more, which is why Cabot gives separate smooth and rough rates. Solid stains cover the grain. Both Behr stains on the list are 100% acrylic and clean up with water. On new wood, Behr's sheets ask you to clean the boards first to remove mill glaze and open the pores.
Buying pails or gallons
Deck stain comes in 1 gallon cans and 5 gallon pails. The calculator splits the order into pails and single gallons: 3 gallons stays as three cans, 8 gallons becomes one pail and three cans. Keep one can of the same batch for touch-ups.
Next steps
A new deck is planned from footings to boards in the deck calculator, and the board count on the deck board calculator. For painted house siding use the exterior paint calculator, and for inside rooms the paint calculator. To price the deck itself in your state, use the deck cost calculator.