Two variables, not one
Almost every deck stain calculator asks for an area and divides by a
single coverage number. The manufacturers publish three coverage ranges,
not one figure — and every data sheet adds that weathered wood needs more
than new. Between them those two facts move the answer by more than
double.
transparent 200–400 ft²/gal max 1 coat
semi-transparent 150–350 ft²/gal 1 coat
solid colour 200–400 ft²/gal max 2 coats
new smooth wood → the top of the range
weathered or rough → the bottom of the range
gallons = ceil(area × coats ÷ coverage)
The coat limit is not advice
Transparent oil stain says do not apply more than one coat. It is
a penetrating finish: the wood absorbs what it can and a second coat has
nowhere to go, so it sits on the surface and stays tacky for weeks. This
calculator will not quote you a second coat of it. Solid colour stain is
a film former and allows two.
Common mistakes
- Measuring the deck surface only. Railings and spindles can add half as much area again.
- Using one coverage number for every product. The three types genuinely differ.
- Treating weathered boards as new. The same tin covers roughly half as much ground.
- Buying a second tin later. Stain is tinted per batch and a later tin can shade differently.
The rest of the deck
If you are still building it, the
deck board calculator
counts the boards and the
deck footing calculator
sizes the concrete underneath.
Stain needed for a 200 ft² deck, one coat
Deck surface only. Add railings, stairs and fascia separately — they are not in this 200 ft².
Common questions
How much stain do I need for a deck?
For a 200 square foot deck in semi-transparent stain, one to two gallons — and which of those it is depends entirely on the wood. New smooth boards take a gallon; weathered boards can take two, because old wood is far more porous and drinks the first coat. That is not a hedge: it is the note printed on the manufacturer data sheet for every one of these products.
Does the type of stain change how much I need?
Substantially. Semi-transparent is published at 150 to 350 square feet per gallon; transparent and solid both at 200 to 400. Most calculators quote a single figure and are therefore wrong for two of the three products. Pick your type above and the number changes with it.
How many coats of deck stain should I apply?
Usually one, and for some products one is the maximum. Transparent oil stain says plainly: do not apply more than one coat. Semi-transparent needs only one. Only solid colour stain allows two. This calculator refuses to compute more coats than the product permits, because a second coat of a penetrating stain has nowhere to go and stays tacky on the surface.
Why does weathered wood need more stain?
Because it is more porous. A new board has a relatively closed surface and the stain sits in the top layer. Weathering opens the grain, and the first coat is partly absorbed rather than being available to cover. The published coverage ranges exist to capture exactly this, and the low end of each is the weathered case.
Do I need to include the railings?
Yes, and on a deck with a lot of balustrade they can add half as much area again. Spindles have four faces each and almost no footprint, so they punish an area-only estimate badly. Measure the railing, stairs and fascia and enter the total in the field above — the deck surface alone will leave you short.
Is this figure the same for every brand?
No. These are the published rates for one manufacturer’s three product lines, cited below with their data sheets. Another brand will be in the same territory but not identical, and the coverage printed on the tin you actually buy always wins over any calculator, including this one.
Should I buy to the high or low end of the range?
Low, which is what this does when you say the wood is weathered. Coverage is quoted in square feet per gallon, so the smaller number is the one that needs more product. Running out of stain halfway across a deck leaves a lap mark you cannot sand out without stripping the whole thing.
Sources
- Default waste allowance
- 0%
Coverage is a published range with no typical value, so none is stored: a midpoint would be a number the manufacturer never printed. Every one of these three data sheets carries the same note — new wood normally requires less product than old, weathered wood, because older wood is more porous — so the top of each range is smooth new timber and the bottom is rough or weathered timber. That is the manufacturer's own statement, not an interpolation. Maximum coat counts are published per product and differ: transparent is one coat only, semi-transparent needs only one, solid allows two. A calculator that offers three coats of transparent stain is offering something the tin forbids.
Unit conversions are exact by definition and need no source.
How we choose and cite these figures