Deck Stain Calculator

board area = deck area × board width ÷ ( board width + gap width ) then stain = board area × coats ÷ coverage

L

Deck length, in feet

W

Deck width, in feet

w

Width of one deck board, in inches (5/4×6 decking measures 5.5 in)

g

Gap between two boards, in inches (1/4 in is the usual drainage gap; 0 for butted or grooved boards)

c

Coverage on the can, in square feet per gallon per coat (300 for a transparent oiling stain, 150 for a solid colour on old wood)

n

Number of coats — two is the usual job on new decking

Measure the deck once, then read the can before you type anything: the coverage figure is a field, not a constant, because the same stain spreads twice as far on new cedar as it does on a deck that has been grey for two years. Multiply nothing by hand — enter the deck's length and width, the actual width of one deck board, the gap you are leaving, and the number of coats. If your boards are butted, grooved or are composite with no visible gap, set the gap to zero and the board area becomes the deck area. If you are staining both sides of removable boards, or doing a railing and stairs at the same time, run them as separate areas and add the gallons up — the page answers one flat surface per run.

1. A 16 × 12 ft deck, 5/4×6 decking, 1/4 in gap, two coats

   1. Deck area: 16 × 12 = 192 sq ft
   2. Board share: 5.5 ÷ (5.5 + 0.25) = 0.9565, so the decking is 192 × 0.9565 = 183.65 sq ft (17.06 m²)
   3. Stain: 183.65 × 2 coats ÷ 300 = 1.22 gallons, rounded up to 2
   4. In litres: 1.22 × 3.785 = 4.63 L, rounded up to 5

   These are the page defaults. The gap costs about 8 sq ft of coating surface — 4% of the deck — and it is the only thing here that a plain area calculation would miss. Note the two pack sizes at the end: 2 gallons is 7.57 litres and 5 litres is 1.32 gallons, so the page reports both and neither is a conversion of the other. You buy whichever size your supplier stocks.

2. The same deck with a wider gap and a semi-transparent water-based stain

   1. Deck area: 192 sq ft
   2. Board share: 5.5 ÷ 6 = 0.9167, so the decking is 176 sq ft (16.35 m²)
   3. Stain: 176 × 2 ÷ 250 = 1.41 gallons, rounded up to 2
   4. In litres: 1.41 × 3.785 = 5.33 L, rounded up to 6

   Opening the gap from 1/4 in to 1/2 in removes another 8 sq ft of decking, and the semi-transparent stain covers less per gallon than the transparent one, so this deck needs a bigger margin — 1.41 gallons against 1.22 — yet still rounds to the same 2 gallon cans. The litres are where the difference shows: 5 against 6. That is why the page reports both, and why you should read the row you actually buy in.

3. A 24 × 16 ft deck, three coats, solid oil stain on old wood

   1. Deck area: 24 × 16 = 384 sq ft
   2. Board share: 5.5 ÷ 5.75 = 0.9565, so the decking is 367.30 sq ft (34.12 m²)
   3. Stain: 367.30 × 3 ÷ 150 = 7.35 gallons, rounded up to 8
   4. In litres: 7.35 × 3.785 = 27.81 L, rounded up to 28

   A big deck, the thirstiest product on the list and a third coat: 7.35 gallons of arithmetic becomes 8 gallons of shopping, and 8 gallons is 30 litres, so buying by the litre instead gets you close to the same money. On weathered boards the real answer is usually larger than this rather than smaller — a deck that has been grey for years takes the first coat like a sponge, and the figure on the can assumes wood that is already sealed.

This page multiplies areas and divides by a coverage figure. It does not know your stain: the coverage number is an input, the four rows in the reference table are typical values rather than guarantees, and the spread between a new cedar deck and a weathered one is roughly two to one on the same product. It measures flat decking only — it does not count the railings, the stairs, the fascia or the posts around the deck, which on a 16 × 12 ft deck can easily add 30% to the total surface. It assumes the boards are uniform, that the gap is the same everywhere and that nothing is double-coated at the ends, and it does not subtract for a stair opening or any other hole cut in the deck. It says nothing about preparation, which decides how the job turns out far more than the arithmetic does: a deck must be cleaned, stripped and dry before staining, and stain applied over a previous film-forming finish can fail within a season. Finally the two outputs are pack sizes, not a volume: gallons and litres round up separately to whole containers, so 2 gallons and 5 litres on the same result are both correct and are not the same amount of liquid.

How much stain do I need for a 16 × 12 ft deck?

Two gallons of a transparent oiling stain at 300 sq ft per gallon, for two coats. The deck is 192 sq ft, the decking itself is 183.65 sq ft once the 1/4 in gaps are taken out, and 183.65 × 2 ÷ 300 = 1.22 gallons, which rounds up to the 2 gallon can. In litres the same job is 4.63 L, which rounds up to 5 — how much stain you actually buy depends on which pack size your supplier sells.

Do I count the gaps between the deck boards?

Take them out, yes. The gaps between deck boards are not coated, so the decking area is the deck area multiplied by board width ÷ (board width + gap). A 1/4 in gap between 5-1/2 in boards removes 4.3% of the surface, and a 1/2 in gap removes 8.3%. It is a small correction, but it is the only reason this page is not a plain area calculation, and it is why the board width and gap are both fields.

Why does the page say 2 gallons and 5 litres for the same job?

Because they are two different containers, each rounded up on its own. 1.22 gallons is 4.63 litres, so the gallon figure becomes 2 and the litre figure becomes 5 — and 5 litres is only 1.32 gallons, while 2 gallons is 7.57 litres. Neither number is a conversion of the other; they are two shopping lists for two markets. Buy in whichever unit your supplier stocks and read that row.

One coat or two?

Two is the usual job on new decking, and it is what the default assumes. A transparent oiling stain on new wood is often given two coats with the second applied while the first is still slightly tacky; a solid colour on a deck that is already coated may only need one refresher. The page does not decide this for you — change the coats field and both the gallons and the litres move with it, so 1.22 gallons becomes 0.61 for a single coat.

How much does the coverage figure matter?

More than anything else on the page. The four typical values span 150 to 300 sq ft per gallon, which is a factor of two, and the wood itself can move the real number by as much again: new cedar holds stain differently from a deck that has been grey for two years. Use the number printed on the can you bought, and if you are between two rows, take the lower coverage — running out mid-deck and starting again with a fresh can is how you get a visible lap line.

Does the page cover the railings and stairs too?

No. It measures the flat decking between the boards, using the length and width you enter. Railings, stair treads, the fascia board around the rim and the posts are all coated surfaces that this page never sees, and on a 16 × 12 ft deck they commonly add 30% or more. Estimate them as separate areas with this same coverage figure and add the gallons together — but only if you are buying for the whole job in one go.

• Approximate Conversions from U.S. Customary Measures to Metric — the square-foot to square-metre figures reported on this page — National Institute of Standards and Technology
• Lumber — nominal and actual dimensions, including the 5/4×6 decking board this page defaults to — Wikipedia