How the gallons are worked out
The calculator adds up the area to stain: the deck, the stairs, and both faces of the railing. Each coat takes that area divided by the label’s coverage per gallon. Labels give coverage as a range, so the result shows the label’s best and worst case, and an expected figure: the middle of each range on new or smooth wood, the low end on weathered or rough wood.
Worked example: a 16 × 12 ft deck with Behr No. 5077
- Area: 16 × 12 = 192 sq ft.
- Behr’s label: 2 coats, the first at 250–300 sq ft per gallon and the second at 500–600.
- Best case: 192 ÷ 300 + 192 ÷ 600 = 0.64 + 0.32 = 0.96 gal.
- Worst case: 192 ÷ 250 + 192 ÷ 500 = 0.768 + 0.384 = 1.15 gal.
- New wood, the middle of each range (275 and 550): 0.70 + 0.35 = 1.05 gal. Buy two one-gallon cans.
- Weathered wood, the low end of each range: 1.15 gal, also two cans.
The gallons are rounded up to whole cans only at the end, after the coats are added together.
Coverage by product
Each maker publishes coverage on its product page or data sheet. The figures are square feet per gallon; where only one is given, it’s for every coat. Cabot gives separate figures for smooth and rough wood.
| Product | First coat | Second coat | Coats |
|---|---|---|---|
| Behr Premium Transparent No. 500 | 300–350 | 600–700 | 2 thin coats required |
| Behr Premium Semi-Transparent No. 5077 | 250–300 | 500–600 | 2 required |
| Behr Premium Solid Color No. 5011 | 200–400 | 400–800 | 2 recommended |
| Thompson’s WaterSeal Clear Wood Sealer | 200–400 | — | 1 is sufficient |
| Thompson’s WaterSeal Semi-Transparent | 150–400 | — | 1; a 2nd within 4 hours |
| Thompson’s WaterSeal Solid Color | 150–400 | — | 1; a 2nd is optional |
| Cabot Semi-Transparent Deck & Siding | 450–650 smooth, 200–300 rough | — | 1 |
| Cabot Solid Color Acrylic Deck & Siding | 400–500 smooth, 200–300 rough | — | 1 on new wood, 2 on weathered |
| Cabot Australian Timber Oil | 400–600 smooth, 250–350 rough | — | 1 only |
| Sherwin-Williams SuperDeck Waterborne Semi-Transparent | 100–350 | — | 2 required, the 2nd within 7 days |
| Sherwin-Williams SuperDeck Oil-Based Semi-Transparent | 150–400 | — | No more than 1 |
| Ready Seal Exterior Wood Stain and Sealer | 125–175 | — | 1 (data sheet) or 2 (2026 guide) |
Ready Seal’s data sheet says its 125–175 sq ft per gallon allows for 15% material loss, and puts decks at about 150. Brand names identify products only.
Why weathered wood takes more
Sherwin-Williams’ SuperDeck data sheet says new wood normally needs less product than old, weathered wood. Cabot puts numbers on the difference: its semi-transparent stain covers 450–650 sq ft per gallon on smooth wood but only 200–300 on rough wood. At the middle of each range that’s 550 against 250 sq ft, so rough wood takes more than twice as much.
That’s why the calculator uses the low end of every range when you choose weathered or rough wood, and the middle for new or smooth wood. For Cabot’s stains it also switches to the rough-wood figures. On a 192 sq ft deck, one coat of Cabot’s semi-transparent stain takes 192 ÷ 550 = 0.35 gal on smooth wood and 192 ÷ 200 = 0.96 gal on rough wood.
One coat or two
Behr calls for 2 coats of its transparent and semi-transparent finishes and recommends 2 of its solid color stain. Sherwin-Williams calls for 2 coats of SuperDeck Waterborne, the second within 7 days. Cabot’s solid color stain takes one coat on new wood and two on weathered wood. The calculator starts from the label’s number, and you can change it.
Some products allow only one. Cabot says to apply its Australian Timber Oil in one coat only, and Sherwin-Williams says not to apply more than one coat of oil-based SuperDeck, so the calculator won’t count a second. Thompson’s calls for one coat of its sealers: one coat of the clear is sufficient, and an optional second coat of the semi-transparent must go on within 4 hours of the first. The data sheets for those two also ask for air and surface temperatures above 50 °F for 48 hours.
Ready Seal disagrees with itself: its data sheet calls for one coat, but its 2026 application guide calls it a two-coat application. The calculator starts at one; choose 2 coats to follow the guide.
Behr rates each second coat at twice the first coat’s coverage, 500–600 sq ft per gallon against 250–300 for No. 5077. Where a label gives one figure, the calculator uses it for both coats.
Stairs and railings
Each step counts a tread and a riser. The calculator takes a tread of two 5½ in boards with a ¼ in gap, 11¼ in deep, and a 7½ in riser: 18¾ in, or 1.5625 sq ft for each foot of stair width. Three steps 4 ft wide come to 3 × 4 × 1.5625 = 18.75 sq ft. The sides of the stringers aren’t counted.
A railing counts as a solid panel stained on both faces: length × height × 2. An open railing comes out about the same once every part is counted. Take a 36 in railing of 2×2 balusters (1½ in square) with 4 in gaps, two 2×4 rails and a 2×6 cap, stained on every face. Per foot of railing:
- Balusters: one every 5½ in is 2.18 per foot. Each is 6 in around; counted the full 36 in tall, that’s 216 in², or 1.5 sq ft. 2.18 × 1.5 = 3.27 sq ft.
- Rails: a 2×4 (1½ × 3½ in) is 10 in around, so a foot of rail is 120 in², or 0.83 sq ft. Two rails: 1.67 sq ft.
- Cap: a 2×6 (1½ × 5½ in) is 14 in around, so a foot of cap is 168 in², or 1.17 sq ft.
- Total: 3.27 + 1.67 + 1.17 = 6.1 sq ft, against 3 ft × 2 faces = 6 sq ft for the solid panel.
So 40 ft of 36 in railing is about 240 sq ft to stain, more than the 192 sq ft floor of a 16 × 12 ft deck.
Buying: gallons and pails
The calculator rounds the expected gallons up to whole one-gallon cans. Over 3 gallons it also gives the number of five-gallon pails, rounded up the same way: 12 gal is twelve cans or three pails. Buy enough stain for each surface at once, so the color is the same from board to board.
Olympic’s guide gives a quick check: a deck of up to 175 sq ft needs one gallon. Olympic ELITE covers 250–350 sq ft per gallon, so 175 sq ft takes 0.5 to 0.7 gal: one can either way.
Questions people ask
How much stain do I need for a 12×12 deck?
About a gallon. A 12 × 12 ft deck is 144 sq ft. Two coats of Behr’s semi-transparent No. 5077 take 144 ÷ 275 + 144 ÷ 550 = 0.79 gal at the middle of its ranges, and between 0.72 and 0.86 gal across them. One coat of Cabot’s semi-transparent stain on rough wood takes 144 ÷ 200 = 0.72 gal. Olympic’s guide says a deck of up to 175 sq ft needs one gallon.
How much stain do I need for a 16×12 deck?
For two coats of Behr’s semi-transparent No. 5077, two one-gallon cans. A 16 × 12 ft deck is 192 sq ft. The stain covers 250–300 sq ft per gallon on the first coat and 500–600 on the second, so two coats take 0.96 to 1.15 gal, about 1.05 gal on new wood. One gallon is enough only if you get close to the label’s best coverage.
How many coats of deck stain do I need?
As many as the label says. Behr calls for 2 coats of its transparent No. 500 and semi-transparent No. 5077, and Sherwin-Williams for 2 coats of SuperDeck Waterborne, the second within 7 days. Thompson’s says one coat of its Clear Wood Sealer is sufficient. Cabot’s Australian Timber Oil and Sherwin-Williams’ oil-based SuperDeck take one coat only. Cabot’s solid color stain takes one coat on new wood and two on weathered wood.
Does the second coat take as much stain as the first?
Not by Behr’s figures: its second coat covers twice the area of the first. No. 5077 covers 250–300 sq ft per gallon on the first coat and 500–600 on the second, so on a 192 sq ft deck the second coat takes 0.32 to 0.38 gal against 0.64 to 0.77 gal for the first. The other labels here give one figure, which the calculator uses for both coats.
How much stain do deck railings need?
More than their length suggests. A 36 in railing has 6 sq ft to cover for each foot of its length, counting both faces, so 40 ft of railing is 240 sq ft: more than the 192 sq ft floor of a 16 × 12 deck. Two coats of Behr No. 5077 on 240 sq ft take about 1.31 gal on new wood.
How much stain do I need for a fence?
Count both faces: length × height × 2. A fence 120 ft long and 6 ft tall is 120 × 6 × 2 = 1,440 sq ft. Two coats of Behr No. 5077 take 1,440 ÷ 275 + 1,440 ÷ 550 = 7.85 gal on new wood, and between 7.2 and 8.64 gal across the label’s ranges: eight one-gallon cans or two five-gallon pails. The fence calculator counts the pickets, posts and rails.
Sources for the numbers on this page
- Behr — Premium Transparent Waterproofing Wood Finish No. 500 (coverage per coat) · checked October 3, 2026
- Behr — Premium Semi-Transparent Waterproofing Stain & Sealer No. 5077 (coverage per coat) · checked October 3, 2026
- Behr — Premium Solid Color Waterproofing Stain & Sealer No. 5011 (coverage per coat) · checked October 3, 2026
- Thompson’s WaterSeal — Clear Wood Sealer — product data sheet, April 2022 · checked October 3, 2026
- Thompson’s WaterSeal — Semi-Transparent Wood Sealer — product data sheet, April 2022 · checked October 3, 2026
- Thompson’s WaterSeal — Solid Color Wood Sealer (coverage) · checked October 3, 2026
- Cabot — Semi-Transparent Deck & Siding Stain — product data sheet, March 2023 · checked October 3, 2026
- Cabot — Solid Color Acrylic Deck & Siding Stain — product data sheet, March 2023 · checked October 3, 2026
- Cabot — Australian Timber Oil — product data sheet, April 2023 · checked October 3, 2026
- Sherwin-Williams — SuperDeck Waterborne Semi-Transparent Stain — product data sheet, December 2019 · checked October 3, 2026
- Sherwin-Williams — SuperDeck Oil-Based Semi-Transparent Stain — product data sheet, September 2020 · checked October 3, 2026
- Ready Seal — Exterior Wood Stain and Sealer — technical data sheet, May 2026 (PDF) · checked October 3, 2026
- Ready Seal — Initial application instructions, 2026 (PDF) · checked October 3, 2026
- Olympic — ELITE Advanced Stain + Sealant in One, transparent (coverage) · checked October 3, 2026
- Olympic — How much stain do I need for a deck or patio? · checked October 3, 2026
- Southern Forest Products Association — Southern Pine standard sizes (nominal and actual dimensions) · checked October 3, 2026
Corrections and updates are logged on the sources page.