How to use the deck stain calculator
- Choose the unit you measured in, then add each surface as a rectangle, triangle or circle: deck floor, steps, fence runs or siding panels. Subtract anything you will not stain.
- Set whether to coat one side or both sides, and enter the railing length and height if the deck has a railing.
- Pick smooth or rough wood and enter the coverage per gallon from your stain can. Set the number of coats and keep or change the 5% extra.
- Read the gallons to buy, the exact gallons and quarts, and copy the shopping list or add a price per gallon.
Formula
Surface area = net shape area x sides coated (1 or 2)
Railing area = railing length x railing height x 2 faces
Total area = surface area + railing area
Stain (gal) = total area x coats / coverage per gallon
With extra = stain x (1 + extra percent / 100)
Gallons to buy = with extra, rounded up to a whole gallon
One liter is 0.264172 US gallons, and one gallon is 3.785411784 liters. The railing is counted as two faces of its height, which is a simple stand-in for rails, balusters and posts.
Worked example: a 20 by 16 foot deck with 40 feet of railing
The deck floor is 20 x 16 = 320 sq ft. The railing is 40 ft long and 3 ft high, counted on two faces, so it adds 40 x 3 x 2 = 240 sq ft. The total is 560 sq ft.
Two coats at an example coverage of 250 sq ft per gallon need 560 x 2 / 250 = 4.48 gallons.
With 5% extra that becomes 4.48 x 1.05 = 4.70 gallons, which is 4 gallons and 3 quarts, or 17.8 liters. You would buy 5 gallons.
If the wood is rough or weathered and the can says 150 sq ft per gallon, the same job needs 7.84 gallons with extra, so you would buy 8.
Why coverage changes so much
Coverage per gallon printed on a stain can is measured on a standard surface. Fresh, planed lumber takes stain slowly and holds it near the surface. Rough sawn boards, old weathered decking and porous woods soak it in, so a gallon covers less. Because the real number depends on both the product and the wood, this tool keeps two separate coverage boxes and lets you edit the one for your surface.
Many stains are meant to be applied in a single coat and wiped or brushed to avoid a sticky film. If your product says one coat, set the number of coats to 1 instead of doubling the quantity.
What counts as surface area
Deck floors are simple, but the surfaces people forget are stair risers and stringers, post faces, rail tops and the edges of deck boards. Add them as extra rectangles. The railing option covers the common case of two vertical faces. For a fence stained on both sides, switch the sides option to both so every area row is doubled.
The underside of boards, joists and any hidden framing are only included if you add them. Many people leave the underside unfinished, but you should follow the product instructions for your wood.
Preparation affects the order
Stain should go on clean, dry wood. Cleaner, brightener, sanding and stripping change how well it soaks in, and a poorly prepared surface can make the first coat disappear. Plan for prep products separately, since their coverage is listed on their own labels.
Assumptions and limits
- Coverage figures of 250 and 150 square feet per gallon are example values. Use the number on your product label.
- Railing area uses a flat two-face estimate. Balusters, posts and decorative parts can take more or less stain than that.
- Every coat is assumed to use the same coverage. Some stains cover more on the second coat, and some are one-coat products.
- The extra percentage is yours to choose. Spills, brush loading and overspray are not predicted.
- Cost uses the price you enter for whole gallons and ignores tax, sales, brushes and prep products.
Frequently asked questions
How many gallons of deck stain do I need?
Divide the total surface area by the coverage on your stain can, then multiply by the number of coats. Round up to a whole gallon, because stain is sold in standard container sizes.
How much does a gallon of deck stain cover?
It depends on the product and the wood. The tool uses 250 square feet for smooth wood and 150 for rough wood as examples, and lets you replace them with your label value.
Do I need to stain both sides of a fence?
Not necessarily, but many people finish both faces for a uniform look and weather protection. Choose both sides and every surface row is counted twice.
Should the railing be included in the deck area?
Yes, if you plan to stain it. Enter its length and height and the tool adds two faces of that area. Complex railings with many balusters may need extra.
Can I use this for a fence or siding instead of a deck?
Yes. Enter each fence run or wall panel as a rectangle, subtract doors and windows, and use the both sides option when both faces are stained.