Paint Calculator by Square Footage: How Much Paint

This paint calculator works out how much paint you need for a room or wall based on the square footage you enter, the number of coats, and the paint’s coverage rate. You input the total paintable area, choose the number of coats, and set the coverage per gallon (or litre); the calculator returns the total paint required in gallons and litres. The result is an estimate, not an exact order, because real-world factors like surface porosity and application method change coverage.

Formula

The basic formula is straightforward:

Paint needed (gallons) = (Area to paint × Number of coats) ÷ Coverage per gallon

Where:

• Area to paint – the total surface area you will cover, in square feet (or square metres).
• Number of coats – how many layers of paint you plan to apply. Usually 2 for a colour change, 1 for touching up.
• Coverage per gallon – the manufacturer’s stated spread rate, typically 250–400 sq ft per gallon for most interior paints.

For metric users, the same logic works with square metres and litres: Paint (litres) = (Area × Coats) ÷ Coverage (m² per litre). A typical latex paint covers about 10–12 m² per litre per coat.

Worked Example

Using the calculator’s defaults: area = 400 sq ft, coats = 2, coverage = 350 sq ft per gallon.

1. Multiply area by coats: 400 × 2 = 800 sq ft of total surface to cover.
2. Divide by coverage: 800 ÷ 350 = 2.2857 gallons.
3. Round to two decimals: 2.29 gallons.
4. Convert to litres: 2.29 × 3.78541 = 8.67 litres.

So for a 400 sq ft room with two coats, you need about 2.3 gallons – meaning you should buy 3 gallons to have a little extra for touch-ups.

How to Measure the Square Footage of a Room

You do not need to paint the floor, ceiling, doors, or windows, so measure only the surfaces you will actually cover.

• Walls: Measure the length of each wall and multiply by the wall height (usually 8–9 ft). Add all wall areas together.
• Subtract openings: For each door (about 21 sq ft) and each window (about 12–15 sq ft), subtract that area from the total.
• Ceiling: Multiply the room length by the room width to get the ceiling area, if you plan to paint it.
• Trim and baseboards: Measure linear feet of trim and multiply by the trim width (often 3–6 inches). This is usually a small addition.

Example: A 12 ft × 15 ft room with 8 ft ceilings has two walls of 12×8 = 96 sq ft each and two walls of 15×8 = 120 sq ft each, for a total of 432 sq ft. Subtract one door (21) and two windows (30) = 381 sq ft of wall area. Add the ceiling (12×15 = 180 sq ft) if painting it, giving 561 sq ft total.

Why Coverage Rates Vary

Manufacturers list a nominal coverage rate, but the actual spread rate depends on several factors:

• Surface porosity: Fresh drywall, bare wood, and old plaster soak up more paint, reducing coverage by 20–30%.
• Texture: Stucco, brick, and textured wallpaper have far more surface area than smooth drywall, so they need more paint.
• Colour change: Going from dark to light (or vice versa) may require an extra coat or a primer.
• Application method: Spraying wastes more paint than rolling, and thick rollers use more than thin ones.
• Paint quality: Higher-quality paints often have more solids and better hiding, so they may cover more area per gallon.

A safe rule is to assume 300 sq ft per gallon for interior walls on smooth, primed surfaces, and 250 sq ft per gallon for unprimed drywall or textured surfaces.

Common Mistakes When Estimating Paint

Even with a calculator, people often order too little or too much. Here are the most frequent errors:

• Forgetting to multiply by coats. Most jobs need two coats; a single-coat estimate will leave you short.
• Not subtracting doors and windows. This overestimates by 10–15% – not a disaster, but it can mean buying an extra gallon you do not need.
• Ignoring the ceiling and trim. If you are painting those too, include them in the area.
• Using the wrong unit. Coverage is often given in square feet per gallon, but some labels use square metres per litre. Mixing them gives wildly wrong results.
• Rounding down. Always round up to the nearest quart or gallon. Leftover paint is useful for touch-ups.

For example, if the calculator says 2.1 gallons, buy 3 gallons, not 2. The extra half-gallon is cheap insurance against running out mid-project.

How to Read the Result and Adjust for Your Project

The calculator gives you a number, but you should interpret it with context:

• If the result is 0.5–1 gallon: You will likely buy a single gallon, but check if a quart would suffice for a small wall or touch-up.
• If the result is 2.3 gallons: Buy 3 gallons. Most paint is sold in full gallons, and the extra will be used for a second coat or future repairs.
• If the result is 5.1 gallons: Buy 5 gallons plus a quart, or 6 gallons if you want a reserve. Many stores allow returns of unopened cans.

For large projects, it is wise to buy all paint from the same batch to avoid slight colour differences between production runs. Note the batch number on the cans.

Limits of the Square Footage Method

The square footage formula is a good starting point, but it is not exact. It does not account for:

• Wastage: Paint left on rollers, brushes, or in the tray; spills; and overspray. Add 10% for waste on a typical job.
• Primer: If you need a separate primer coat, that is an additional product and should be calculated separately using the same formula.
• Multiple colours: If you are painting an accent wall a different colour, you need to calculate that wall separately.
• Repainting vs. new work: Repainting a previously painted, sound surface covers better than painting bare drywall.

For most DIY projects, adding 10–15% to the calculated amount is a sensible buffer. Professional painters often use a similar rule of thumb.

Frequently Asked Questions

How many square feet does a gallon of paint cover?

A typical gallon of interior latex paint covers about 350–400 square feet per coat on smooth, primed surfaces. On rough or porous surfaces, coverage can drop to 250–300 square feet per gallon. Always check the label for the specific product’s spread rate.

Do I need two coats of paint?

In most cases, yes. Two coats provide even colour, better durability, and easier cleaning. One coat is only enough when you are painting the same colour over a similar shade on a well-prepared surface. If you are changing colour significantly, you may even need three coats or a tinted primer.

Should I include windows and doors in the square footage?

No. Subtract the area of windows and doors from the total wall area because you will not paint those surfaces. A standard door is about 21 square feet, and a typical window is about 12–15 square feet. If you are painting the door or window trim, add that as a separate trim area.

How do I calculate paint for a ceiling?

Measure the length and width of the room and multiply them to get the ceiling area in square feet. Then use the same formula: (area × coats) ÷ coverage. Ceilings often need only one coat if you are using a ceiling paint, but two coats are better for a flawless finish over stains or patches.

What if my walls are textured or made of brick?

Textured surfaces like knockdown, orange peel, or brick have more surface area than smooth walls, so they require significantly more paint. Reduce the coverage rate to about 200–250 square feet per gallon for such surfaces. You may also need a thicker roller or a sprayer to apply the paint properly.

Can I use the same calculator for exterior paint?

Yes, the formula is the same, but exterior surfaces are often more porous and weather-exposed, so coverage is typically lower – around 250–300 square feet per gallon for siding. Also, exterior jobs usually require a primer and at least two top coats, so factor that into your inputs.

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in area | Area to paint | sq ft | 400 | 0
in coats | Number of coats | - | 2 | 1
in coverage | Paint coverage | sq ft per gallon | 350 | 1
out paint | Paint needed | gallons | area * coats / coverage | 2
out litres | Paint needed (litres) | L | (area * coats / coverage) * 3.78541 | 2