Deck Paint Calculator

Painting a 16×20 ft deck starts with one honest number: how much paint you actually need. This page works that number out for a 16×20 ft deck, using the typical coverage ranges in our paint data — and it starts with a warning worth taking seriously. The result here is an estimate, not a guarantee: coverage and prices vary by brand, finish, surface and region, and the product label on the can you buy governs. A satin paint that covers well on wood can cover far less on a thirsty or textured surface, and a colour change can add a whole coat. That is why every figure below is shown as a range, and why the calculator on this page asks for your brand, finish and surface instead of handing you a single magic number. Expect about 2.1–2.8 gallons, so buy 3 gallons here; the rest of this guide shows exactly how that range is worked out, what moves it up or down, and how to measure your own space so you buy the right amount the first time.

Most people planning this job are working on a surface they already live with, which means the measurements are real and the condition is what it is. The scenario below matches that case: a straightforward job on wood with satin finish, using Behr as the reference brand. Change any one of those — a darker colour, a rougher surface, a cheaper line — and the quantity changes with it. A calculator that spits out one exact number is misleading; the honest answer is always a band, and this page works in bands from start to finish.

How the calculation works

The calculator needs five inputs, and each one changes the answer. First, the area to cover: The deck floor is 16 ft by 20 ft, which is 320 sq ft, and the rails and balusters add about another 100 sq ft of painted surface, for 420 sq ft in total. Decks are measured by floor area plus the rails, so do not forget the underside of the handrail and the posts — they take paint even though they barely add floor area. That gives about 420 sq ft (39.1 sq m) of paintable surface for this job. Second, the number of coats: 2 coats means the area is covered 2 times, so 420 sq ft becomes about 840 sq ft of coverage to buy. Third, the finish: satin paint typically covers 300–400 sq ft per gallon (≈27.9–37.2 sq m per 3.79 L) in our data, labelled typical because the can label on your exact product is the authority. Fourth, the surface: wood typically covers 250–350 sq ft per gallon, and the lower of the finish and surface ranges is what limits you in practice. Fifth, the brand: Behr is the reference here, and its own coverage and price ranges are listed in the data file.

The formula is short: gallons = (paintable area × coats) ÷ coverage, run twice — once with the high coverage figure for the low gallon number, and once with the low coverage figure for the high gallon number. That gives you the band. Always round up — paint is sold by the gallon and the quart, not by the tenth — and buy a little extra inside that band for touch-ups and the last stretch that always seems to appear. One US gallon is 3.79 litres, and one square foot is about 0.093 square metres, so the metric equivalents in parentheses are approximate by design. Note what the formula leaves out on purpose: it does not assume a single coverage rate. If a retailer quotes you one number for coverage, treat it as a starting point and check the can — the range is the honest figure.

Worked example: a 16×20 ft deck

Here is the full arithmetic for a 16×20 ft deck.

Step 1 — paintable area. The deck floor is 16 ft by 20 ft, which is 320 sq ft, and the rails and balusters add about another 100 sq ft of painted surface, for 420 sq ft in total. Decks are measured by floor area plus the rails, so do not forget the underside of the handrail and the posts — they take paint even though they barely add floor area. Net paintable area = 420 sq ft (39.1 sq m).

Step 2 — multiply by coats. 2 coats × 420 sq ft = 840 sq ft of coverage to buy.

Step 3 — divide by the coverage range. satin paint on wood typically covers 300–400 sq ft per gallon. At the generous end, 840 ÷ 400 = 2.10 gallons; at the thirsty end, 840 ÷ 300 = 2.80 gallons. That is 2.1–2.8 gallons, so buy 3 gallons — buy the high end to be safe, and keep the receipt for the unopened spare.

Step 4 — the sanity check. If the surface is a deep colour going over white, or white going over a deep colour, expect to land at the top of the range or add another coat; a same-colour refresh in good light often lands near the bottom. Either way, the range is the plan: buy the low end and you risk a mid-job trip to the store, buy the high end and you have touch-up paint for the next few years. Measure twice before you trust the maths: measure each run separately, because older homes are rarely square, and use the larger figure.

Cost guidance

Costs on this page are ranges, labelled typical retail range — check the current shelf price before budgeting. No single national price is quoted, because paint prices vary by brand, product line, region and retailer, and a price that is right in one store can be wrong in the next. For Behr, typical retail prices per gallon run about $30–$60 in the Northeast, $30–$55 in the Midwest, $25–$55 in the South and $30–$60 in the West — typical retail range, check current shelf price. Premium lines sit at the top of the range and contractor or mid-tier lines lower down. For this job's 2.1–2.8 gallons, paint alone works out to roughly $50–$170 before rollers, tape, drop cloths and primer, which are separate. That spread is wide on purpose: region, product line and promotions move the shelf price more than any national average could capture, so check the price on the day you buy and keep the receipt. Buying all gallons from the same tint run avoids faint shade lines between batches, and comparing brands only makes sense tier to tier — a premium line against a contractor line is not a fair comparison.

Two cost traps catch first-time painters. The first is ordering the low end of the range and then paying a second trip — or a higher per-gallon price — for the paint that finishes the job. The second is forgetting the consumables: brushes, painter's tape, drop cloths and primer together are a meaningful share of a small job's material cost, and a colour change usually needs primer as well. Compare the total, not the per-gallon price, and keep a little paint for future touch-ups; a matching batch is worth far more than the shelf price of that spare gallon later. If you are hiring the work out, labour is usually quoted separately per job or per sq ft and varies by region and by the condition of the surface — a surface that needs washing, sanding and priming costs more to paint than one in good condition, so get the quote against the same job you used for the material count.

What changes the number for a 16×20 ft deck

Decks add weather and wear to the maths: foot traffic, sun and rain all shorten a paint job's life, and horizontal boards hold water longer than vertical ones. Wash the deck, let it dry fully, and prime bare patches first. A deck stain or solid-colour deck paint is usually a better choice than ordinary wall paint outdoors, and the coverage range on this page assumes a deck-rated product. Colour is the biggest single lever after surface: a dark colour over a light surface, or a light colour over a dark one, almost always needs an extra coat, and that extra coat is not a rounding error — it is roughly half the paint again. Surface condition is the second lever: weathered, patched or bare areas drink paint, so prime them first rather than paying for topcoat to do a primer's job. Finish is the third: flat and matte finishes hide imperfections but typically cover at the lower end of the range, while glossier finishes cover a little further per gallon but show every bump. Finally, application matters — a brush loads paint differently from a roller or a sprayer, and a first coat on thirsty wood always goes further into the surface than the second coat does. None of these change the formula; they change where inside the range you land, which is why the range, not a single number, is the answer.

Step by step: measure, calculate, order

1. Measure twice. Measure a 16×20 ft deck with a tape measure, in feet and inches, and write every figure down before you do any maths. Convert inches to decimal feet (6 in = 0.5 ft) before multiplying.

2. Work out the area. Use the area steps in the worked example above, and do not skip the deductions — forgetting them is the most common reason people overbuy.

3. Choose the finish and the coats. Pick satin finish, then decide the coats: one coat only works for a same-colour refresh in good condition; two coats is the normal plan, and a colour change or bare surface usually needs a primer coat plus two topcoats.

4. Use the calculator on this page. Enter the area, the finish (satin), the surface (wood) and the brand (Behr). The tool returns a range — about 2.1–2.8 gallons, so buy 3 gallons for the example above — and you should plan on the high end.

5. Check the can label in the store. Confirm the coverage range printed on the exact product matches the range you planned with, and adjust if it does not. Coverage varies — the can label governs.

6. Order and check the delivery. Order enough for the high end of the example range, all from the same tint run, and check the batch numbers before you start. Keep primer, brushes and tape dry and within their shelf life.

7. Before you start painting. Clean the surface, sand any rough or glossy spots, mask what you are not painting, and stir — do not shake — the paint. Work in manageable sections and keep a wet edge to avoid lap marks.

Mistakes that waste paint (and money)

  • Skipping the measurements: guessing the area is the fastest way to buy the wrong amount of paint for a 16×20 ft deck.
  • Trusting a single coverage number: coverage is a range, and planning on the best-case figure is how you end up back at the store mid-job.
  • One coat over a colour change: dark-to-light almost always needs two coats plus primer, and budgeting for one is the classic underestimate.
  • Buying paint from different batches: tint can drift between runs, so a surface finished with two batches can show a faint line in daylight.
  • Painting unprepared wood: bare, dusty or glossy surfaces drink paint or reject it, and a wash, sand or primer coat is cheaper than the extra paint.
  • Skipping primer on bare spots: bare spots flash through the topcoat, and the fix is a third coat over the whole surface instead of one primer coat over the spot.

Frequently asked questions

How many gallons of paint do I need for a 16×20 ft deck?

For the worked example — about 420 sq ft, two coats of satin paint on wood — the answer is 2.1–2.8 gallons. The range exists because coverage varies: satin paint typically covers 300–400 sq ft per gallon, and weathered wood drinks more paint than new boards. Measure your own deck and run the same steps to get your range.

Does weathered wood need more paint?

Yes — weathered wood is thirstier than new boards, and the worked example already assumes a typical used surface rather than brand-new stock. Wash the surface, let it dry fully, and prime bare or greyed patches first; a primer coat is cheaper than the extra topcoat that weathered wood would otherwise absorb.

What finish is best for a 16×20 ft deck?

Satin is a good default outdoors: it sheds water reasonably well and does not show every board edge. Flat finishes hide imperfections but weather faster, while gloss shows every bump. Pick the finish for exposure and cleaning, then use its own coverage range — 300–400 sq ft per gallon for satin — not a generic one.

When should I paint a 16×20 ft deck?

Paint on a dry day with mild temperatures and no rain forecast for at least a day after, following the can label's temperature range. Morning is often best: the surface is dry from overnight, and the paint has the day to cure. Never paint a surface that is wet, hot to the touch in direct sun, or about to be rained on.

How much will paint cost for a 16×20 ft deck?

Paint alone for the example job runs from roughly the low end of the Behr range times 2.1 gallons to the high end times 2.8 gallons — a wide band, because typical retail prices per gallon for Behr run about $25–$60 depending on region and product line. That is a typical retail range, so check the current shelf price before budgeting, and remember brushes, sprayers, tape and drop cloths are on top.

Related calculators

Results are estimates — verify with the product label. Coverage, pack sizes, yields and prices vary by brand, product line, surface and region; the label on the product you buy governs. Cost figures are typical retail ranges — check the current shelf price before budgeting. Coverage varies — the can label governs.