Deck Stain Calculator
The floor is the easy half. Railings and steps are nearly 49% of a typical deck — and old, weathered wood drinks more stain than new wood, not less.
Measure the deck floor, then the railing separately — it is the part that gets left out, and on a typical deck it is almost as much surface as the boards. Set coverage from your tin, and be honest about how weathered the wood is.
Your details
Total run of railing. Measure it — it is usually more than you think.
Surface area per linear foot. The baluster figure is derived from 2x2 balusters at code spacing.
Each step counts its tread and its riser.
Two. The data sheets cap it there — a third coat cannot penetrate and will peel.
READ YOUR TIN. Semi-transparent is 100-350, solid 200-400. Use the LOW end on old, weathered wood — it drinks.
Result
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
About this tool
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.
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Deck Staining PlanMeasure the railing separately — it is nearly half the job. Two coats maximum, and check the overnight low.
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How this is calculated
THE FLOOR IS THE EASY HALF. Length times width gives the deck boards, and that is the number everyone measures. On a 16 by 12 deck that is 192 square feet. The railing and the steps add another 184 - almost half the job, and the half that gets forgotten at the shop. WHERE THE RAILING FIGURE COMES FROM. A standard 36 inch baluster railing works out at about 4 square feet of surface per linear foot, and that is derived rather than borrowed: 2x2 balusters are 1.5 inches actual, and building code (IRC R312.1.3) requires that a 4 inch sphere cannot pass through a guard, so balusters sit around 5 inches on centre - about 2.4 per foot. Each one is roughly 29 inches tall between the rails with four faces of 1.5 inches, which is 1.21 square feet, so the balusters alone are 2.9 square feet per foot. The top and bottom rails add about 0.54 each. Total, near enough 4. A SOLID PANEL RAILING is simpler and bigger: 3 feet tall, both faces, is 6 square feet per foot. Cable and glass railings are the opposite - you only stain the posts and rails, so about 1.2. STAIRS: each step is its own width times about 1.5 feet - an 11 inch tread plus a 7 inch riser, which is the usual deck step. Four steps on a 4 foot wide staircase is 24 square feet. THE ARITHMETIC: (deck length x width) + (railing length x factor) + (steps x stair width x 1.5), times coats, divided by coverage. ⚠️ COVERAGE DEPENDS ON THE WOOD, NOT THE STAIN TYPE. This is the part that surprises people, and it comes straight off Sherwin-Williams' own SuperDeck data sheets: waterborne SEMI-TRANSPARENT is 100-350 square feet per gallon, and waterborne SOLID colour is 200-400. Those ranges overlap almost completely. The type of stain is not what decides how far a gallon goes - the wood is. ⚠️ AND IT RUNS THE OPPOSITE WAY TO INTUITION. The data sheet is explicit: NEW WOOD NORMALLY REQUIRES LESS PRODUCT THAN OLD, WEATHERED WOOD, because older wood is more porous. Everyone assumes a tired grey deck will take less because it is smooth-ish and the stain will just sit on it. The reverse is true - it drinks. If your deck has been bare for a few seasons, work from the LOW end of the range. ⚠️ MORE THAN TWO COATS IS A DEFECT, NOT INSURANCE. This is the biggest single difference between stain and paint. The solid-colour data sheet says plainly: DO NOT APPLY MORE THAN TWO COATS. The semi-transparent sheet says two coats ARE required for proper colour development, with the second applied within 7 days of the first. Stain works by penetrating; once the wood is saturated, an extra coat has nowhere to go and sits on the surface as a film that will peel. That is why this calculator will not let you choose three. TEMPERATURE MATTERS AFTER YOU FINISH, NOT JUST DURING. The semi-transparent sheet requires that air and surface temperature not drop below 50F for 24 HOURS after application. An autumn afternoon that is warm at 2pm and 45F at midnight is a failed finish, and you will not know for weeks. SEMI-TRANSPARENT IS FOR BARE WOOD. The data sheet says so directly - it needs to penetrate, and it cannot get into a surface that is already sealed by a previous film-forming coat. If your deck currently has a solid stain or paint on it, your realistic options are another solid stain or a full strip.
Common questions
- How much stain do I need for my deck?
- For a 16 by 12 deck with 40 ft of baluster railing and four 4 ft wide steps, the total is 376 square feet - 192 of floor, 160 of railing and 24 of steps. At 250 square feet per gallon and two coats that is 3.01 gallons, so you buy 3.25. The figure that catches people is not the floor, it is that the railing and steps come to 184 square feet - 49% of the job.
- Why is the railing such a big part of it?
- Because a railing has far more surface than its length suggests. A standard 36 inch baluster railing is about 4 square feet of stainable surface per linear foot: 2x2 balusters are 1.5 inches actual and, since building code requires that a 4 inch sphere cannot pass through a guard, they sit around 5 inches on centre - about 2.4 per foot. Each has four faces. The balusters alone come to 2.9 square feet per foot, and the top and bottom rails add roughly another square foot. Forty feet of that is 160 square feet, which is most of a deck floor.
- Does semi-transparent stain go further than solid?
- Not reliably, and this is the most useful thing on this page. Sherwin-Williams' own SuperDeck data sheets put waterborne semi-transparent at 100-350 square feet per gallon and waterborne solid colour at 200-400. Those ranges overlap almost entirely. The type of stain is not what decides how far a gallon goes - the wood is. Pick the stain for the look and the amount of grain you want to see, then set coverage from the tin and the state of the timber.
- Does old weathered wood need more stain or less?
- More, and it is worth being clear about because almost everyone guesses the other way. The SuperDeck data sheet states it plainly: new wood normally requires LESS product than old, weathered wood, because older wood is more porous. A tired grey deck that has been bare for a few seasons will drink stain. If that is your deck, work from the low end of the coverage range - 100-150 for semi-transparent - rather than the middle.
- Should I do a third coat for extra protection?
- No. This is the biggest single difference between stain and paint, and it is the opposite of the instinct. The solid-colour data sheet says directly: do not apply more than two coats. The semi-transparent sheet requires two for proper colour development, with the second within 7 days of the first. Stain works by penetrating the timber; once the wood is saturated, a further coat has nowhere to go and cures as a surface film that will peel and look far worse than two coats would have. The calculator caps coats at two for that reason.
- How cold is too cold to stain a deck?
- The thing to watch is not the temperature while you work, it is the temperature afterwards. The semi-transparent data sheet requires that air and surface temperature do not drop below 50F for 24 HOURS after application. A pleasant autumn afternoon that falls to 45F overnight will spoil the cure, and you will not find out for weeks - it shows up later as poor adhesion and early wear. Check the overnight low before you open the tin.
- Can I put semi-transparent stain over an old solid stain?
- No. The data sheet specifies bare wood only, and the reason is mechanical rather than fussy: semi-transparent stain has to penetrate the timber to work, and it cannot get into a surface already sealed by a film-forming coat. It will sit on top, look blotchy and wear off in patches. If your deck currently carries a solid stain or paint, your realistic choices are another solid stain over it, or stripping and sanding back to bare wood first.
- Do I need to stain the underside of the boards?
- Not usually, and this calculator does not include it. The underside is not exposed to sun or foot traffic, which is what actually destroys a deck finish, and access is normally awful. It is worth doing on boards you are installing new and can coat before they go down - especially the cut ends, which are the thirstiest part of any board and the place rot starts. Once the deck is built, concentrate on the walking surface and the railing.
Take it further with AI
Copy this into ChatGPT or Claude with your own numbers filled in. It hands over the figures this calculator worked out, so the answer is built on real arithmetic instead of a guess.
I used the Deck Stain Calculator at https://www.bfcbrilliance.com/tools/deck-stain-calculator.
What I entered:
- Deck length (ft): ___
- Deck width (ft): ___
- Railing length (ft): ___
- Railing type: ___
- Number of steps: ___
- Stair width (ft): ___
- Coats: ___
- Coverage (sq ft/gal): ___
What it calculated:
- Stain to buy: ___
- Exact gallons needed: ___
- Surface to stain: ___
- Deck floor: ___
- Railing: ___
Use those figures as given — they are already worked out, so please don't recalculate or estimate your own. Help me turn them into a plan: what to buy or do, in what order, roughly what it should cost, and the mistakes people most often make with this job.
Where the job touches structure, gas, or electrics, tell me to check local building code and whether a permit or a licensed trade is required.Last updated
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Part of a bigger job
How to Plan a Deck or FenceSpans before boards, posts before panels — the order that stops a deck being rebuilt, and the half of a stain job everyone underestimates.
Walks through all 5 decking & fencing tools in order.