How Much Stain for My Deck?

A 16 × 12 deck is 376 sq ft once you count the railing — and 49% of that isn't the floor. Old weathered wood drinks more stain than new wood, not less.

The floor is the easy half

A 16 × 12 deck is 192 sq ft. That's the number everyone measures, and it's the number that leaves people one tin short on a Sunday afternoon.

Add 40 ft of baluster railing and four steps and the real total is 376 sq ft.

The railing and the steps are 49% of the job.

At 250 sq ft per gallon over two coats that's an exact gallons needed of 3.01 — so you buy 3.25 and have a little spare for the cut ends.

Your details

Deck length(ft)

Deck width(ft)

Railing length(ft)

Total run of railing. Measure it — it is usually more than you think.

Railing type

Surface area per linear foot. The baluster figure is derived from 2x2 balusters at code spacing.

Number of steps

Each step counts its tread and its riser.

Stair width(ft)

Coats

Two. The data sheets cap it there — a third coat cannot penetrate and will peel.

Coverage(sq ft/gal)

READ YOUR TIN. Semi-transparent is 100-350, solid 200-400. Use the LOW end on old, weathered wood — it drinks.

Result

Stain to buy

3.25

Gallons, rounded up to the next quarter.

Exact gallons neededBefore rounding.: 3.01
Surface to stainSquare feet — floor, railing and steps together.: 376
Deck floorSquare feet. The part everybody measures.: 192
RailingSquare feet. The part almost nobody measures.: 160
StepsSquare feet — tread plus riser, about 1.5 ft of surface per foot of width.: 24
Railing and steps, as a shareOf the whole job. This is why buying for the floor alone leaves you short.: 49%
Per coatGallons for one coat, if you are staging the work.: 1.5

Open the Deck Stain Calculator on its own page to bookmark or share it.

Why a railing is that big

Because a railing has far more surface than its length suggests, and you can work out exactly how much.

A standard 36" baluster railing comes to about 4 sq ft of stainable surface per linear foot. Here's where that comes from, rather than asking you to take it on trust:

2×2 balusters are 1.5" actual
Building code (IRC R312.1.3) says a 4" sphere must not pass through a guard, so balusters sit around 5" on centre — about 2.4 per foot
Each is roughly 29" tall between the rails, with four faces of 1.5" → 1.21 sq ft each
Balusters alone: 2.4 × 1.21 = 2.9 sq ft per foot
Top and bottom rails add about 0.54 each

≈ 4.0 sq ft per linear foot.

Forty feet of that is 160 sq ft — most of a deck floor, standing vertically where nobody thinks to measure it.

A solid panel railing is simpler and bigger: 3 ft tall × both faces = 6 sq ft per foot. Cable and glass are the opposite — you're only coating posts and rails, so about 1.2.

Steps: each one is its width × about 1.5 ft, being an 11" tread plus a 7" riser. Four steps on a 4 ft staircase is 24 sq ft.

The stain type isn't what decides coverage

This is the most useful thing on this page, and it contradicts most of what's written about deck stain.

From Sherwin-Williams' own SuperDeck data sheets:

Stain	Coverage
Waterborne semi-transparent	100–350 sq ft/gal
Waterborne solid colour	200–400 sq ft/gal

Those ranges overlap almost completely.

The type of stain is not what decides how far a gallon goes. The wood is.

So pick your stain for the look — how much grain you want to see — and then set coverage from the tin in your hand and the state of your timber. That second part is where the real variation lives.

Old wood drinks more. Not less.

Nearly everyone guesses this backwards, including me before I read the data sheet.

The intuition is that a tired grey deck is smooth and closed-up, so the stain will sit on top and go further. The SuperDeck sheet says the opposite, in plain language:

New wood normally requires less product than old, weathered wood. This is due to older wood being more porous than newer wood.

Weathering opens the timber up. A deck that's been bare for a few seasons is thirsty.

If that's your deck, work from the low end of the range — 100–150 for semi-transparent — not the middle.

Two coats. Never three.

This is the biggest single difference between stain and paint, and it runs against the instinct that more protection is better protection.

The solid-colour data sheet says it flatly: do not apply more than two coats. The semi-transparent sheet requires two for proper colour development, with the second applied within 7 days of the first.

The reason is mechanical. Stain works by penetrating the wood. Once the timber is saturated, a third coat has nowhere to go — it cures as a film sitting on the surface, and a film on a deck peels. You end up with something markedly worse than two coats would have given you, and a stripping job to fix it.

The calculator caps coats at two for exactly this reason.

Check the overnight low, not the afternoon

The temperature that matters is the one after you've finished.

The semi-transparent data sheet requires that air and surface temperature not drop below 50°F for 24 hours after application.

A pleasant October afternoon that falls to 45°F at midnight will spoil the cure — and you won't find out for weeks. It surfaces later as poor adhesion and early wear, and by then it looks like the stain was bad rather than the timing.

Semi-transparent needs bare wood

The data sheet specifies bare wood only, and it isn't being fussy.

Semi-transparent stain has to get into the timber. It cannot penetrate a surface already sealed by a film-forming coat, so over an old solid stain it sits on top, goes blotchy, and wears off in patches.

If your deck currently carries solid stain or paint, your honest options are:

Another solid stain over it, or
Strip and sand back to bare wood

There isn't a third one, whatever the tin promises.

What about the undersides?

Skip them, and this calculator does.

The underside sees no sun and no foot traffic, which is what actually destroys a deck finish, and access is normally miserable.

The exception worth taking seriously: new boards you can coat before they go down, especially the cut ends. End grain is the thirstiest part of any board and it's where rot starts. Ten minutes with a brush on a stack of fresh boards buys years.

The short version

376 sq ft, not 192 — measure the railing
49% of the job is railing and steps
Coverage is decided by the wood, not the stain type
Old weathered wood drinks more
Two coats maximum — a third one peels
Stay above 50°F for 24 hours afterwards