DIYCalcLab

Topic · Paint estimating

Paint coverage: coats, waste and tin sizes

By DIYCalcLab Editorial Team · Updated

This page is the method, not the tool list. Paint volume is paintable area times coats, divided by the coverage on the tin. Worked examples below open in the paint calculator with the same inputs. The room-by-room walkthrough stays on the paint guide.

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Core calculators

Start with the tool that matches your job. Each page shows the formula, defaults, shopping list and sources.

Coverage formulas

  • Wall area = 2 × (length + width) × height − door openings − window openings
  • Paint = paintable area × coats ÷ coverage per coat
  • Primer = the same area × 1 coat ÷ the primer’s own coverage
  • Purchase = round that volume up to the tin sizes for the unit system and region

Variables

Coverage
Default in the paint calculator is 350 sq ft per gallon, shown in metric as 8.6 m²/L. The tin label replaces the default
Openings
The calculator deducts 20 sq ft (1.86 m²) per door and 15 sq ft (1.39 m²) per window
Tins
Imperial: gallons and quarts. Metric, Australia: 10 / 4 / 2 / 1 L. Metric, every other region including the UK and Canada: 5 / 2.5 / 1 L
Unit factors the paint calculator already uses
FromTo
1 US gallon3.785 L, and 4 quarts (src/lib/units.ts)
Default coverage350 sq ft/gal = 8.6 m²/L
UK metric tins5 L, 2.5 L, 1 L
AU metric tins10 L, 4 L, 2 L, 1 L

Worked examples

Each example uses the same inputs as the linked calculator. The link loads those inputs; the page URL without parameters stays the canonical.

12 × 12 ft room, 8 ft walls, one door, two windows, two coats

Imperial · length 12 ft · width 12 ft · height 8 ft · 1 door · 2 windows · 2 coats · 350 sq ft/gal · no ceiling · no primer

  1. Wall area = 2 × (12 + 12) × 8 − 20 − 2 × 15 = 334 sq ft
  2. Paint = 334 × 2 ÷ 350 = 1.91 gal (7.2 L)
  3. Purchase in gallons and quarts: 2 × 1 gal

Result: 1.91 gal for 334 sq ft. Purchase: 2 × 1 gal.

Open this example in the calculator →

4 × 3.5 m room, 2.4 m walls, one door, two windows, two coats

Metric · length 4 m · width 3.5 m · height 2.4 m · 1 door · 2 windows · 2 coats · 8.6 m²/L · no ceiling · no primer

  1. Wall area = 31.4 m² after the calculator’s door and window deductions
  2. Paint = 31.4 × 2 ÷ 8.6 = 7.3 L (1.93 US gal)
  3. Non-AU metric tins (UK list, also used for Canada in metric): 1 × 5 L + 1 × 2.5 L
  4. Australia metric tins for the same 7.3 L: 2 × 4 L

Result: 7.3 L for 31.4 m². Non-AU metric purchase: 1 × 5 L + 1 × 2.5 L. AU purchase: 2 × 4 L.

Open this example in the calculator →

What changes the amount — and what does not

  • Cited coverage, not a site waste percent: Sherwin-Williams states a gallon of paint typically covers about 350–400 sq ft, and a gallon of primer about 200–300 sq ft. Dulux UK quotes up to 13 m² per litre on one retail page; Dulux Australia’s Wash&Wear sheet quotes a theoretical 16 m²/L. Those are product figures. This site’s calculator default stays at 350 sq ft/gal (8.6 m²/L), which is the low end of the Sherwin-Williams band.
  • Extra coats, primer and porous surfaces are how this category accounts for more paint. Enter the coverage printed on the tin you will buy.
  • Stain and garage-floor epoxy are separate calculators. Their coverage is not this wall-paint default.

Limits

  • Results are purchase quantities only. They do not specify a coating system for fire, wet areas or exterior substrates.
  • Imperial mode always buys gallons and quarts, including when the region is Canada. Metric mode buys AU tins only when the region is Australia. Every other metric region, including the UK and Canada, buys the 5 / 2.5 / 1 L list.
  • Wallpaper rolls, deck stain and epoxy kits have their own calculators and their own head terms.

Related calculators

Guides

  • GuideHow Much Paint Do I Need for a Room?Calculate wall area, deduct doors and windows, choose coverage and coats, and buy the right mix of gallons, quarts or liter tins — with US and UK examples.

Quick questions

Why does the paint calculator default to 350 sq ft per gallon?

Sherwin-Williams states a gallon of paint typically covers about 350–400 sq ft. This site’s default is 350 sq ft/gal, the low end of that band, shown in metric as 8.6 m²/L. Dulux quotes are product figures (up to 13 m²/L on one UK page; a theoretical 16 m²/L on the Australia Wash&Wear sheet). Enter the coverage printed on the tin you buy.

Which tin sizes does Canada use?

Imperial mode buys gallons and quarts, including when the region is Canada. Metric mode buys the 10 / 4 / 2 / 1 L list only when the region is Australia. Every other metric region, including the UK and Canada, buys 5 / 2.5 / 1 L. This page does not add a separate Canadian tin size.

Is primer included in the wall-paint volume?

No. Primer is the same area times one coat, divided by the primer’s own coverage. Extra coats and porous surfaces are entered as coats or as the coverage on the tin, not as a concrete-style waste percent. This page is that method. The room walkthrough is the paint guide. The paint category hub is the tool list. Stain and garage epoxy stay on their own calculators.

Sources

  1. Sherwin-Williams – Paint Calculator FAQs — accessed : “A gallon of paint typically covers about 350-400 square feet … A gallon of primer covers about 200-300 square feet”
  2. Dulux UK – Easycare Washable & Tough Matt — accessed : “Size 2.5L 5L … Coverage Up to 13m2/litre”
  3. Dulux Australia – Wash&Wear Low Sheen datasheet (AUDU06253, PDF) — accessed : “Theoretical Spread Rate (m²/L) 16 … Size: 15L 10L 4L 2L 1L 0.5L”

Prefer manufacturer data sheets and published factors. Coverage, yield and waste vary by product — confirm the label you buy.

Looking for every paint tool in one list? See the Paint calculators category hub · Methodology · Updated