Paint Calculator: How Much Paint Do I Need?

This paint calculator tells you how many gallons or liters of paint you need for a room or a set of walls. You enter the wall dimensions, the number of coats, and the paint coverage per gallon or liter, and it returns the total paint volume required. It also shows the area to be painted so you can double-check the math.

Formula

The basic formula is simple: Paint needed = (Total wall area × Number of coats) ÷ Coverage per gallon

Where:

• Total wall area = width of one wall × height of that wall × number of walls (in square feet or square meters)
• Number of coats = how many layers of paint you plan to apply (usually 2 for a color change, 1 for a repaint of the same color)
• Coverage per gallon = the area one gallon covers on the surface you are painting (typically 250–400 sq ft per gallon for latex paint on smooth drywall)

If you are using liters and square meters, the same formula works: coverage per liter is usually 8–12 m² per liter. The calculator above uses feet and gallons, but you can convert: 1 gallon ≈ 3.785 liters, 1 sq ft ≈ 0.0929 m².

Worked Example

Using the default values: a wall 12 ft wide and 8 ft high, 4 walls, 2 coats, and a coverage of 350 sq ft per gallon.

1. Area of one wall = 12 × 8 = 96 sq ft
2. Total wall area = 96 × 4 = 384 sq ft
3. Multiply by coats: 384 × 2 = 768 sq ft of painted surface
4. Divide by coverage: 768 ÷ 350 = 2.194 gallons

Rounded to one decimal, you need 2.2 gallons. Since paint is sold in whole gallons (or 5-gallon buckets), you would buy 3 gallons to have a little extra for touch-ups.

When to Use This Calculator

Use this calculator before buying paint for any interior wall project, whether you are painting a single bedroom, a whole apartment, or just an accent wall. It is also useful for exterior walls if you enter the total area directly. The calculator works for any surface that has a roughly rectangular shape—drywall, plaster, wood paneling, or previously painted walls.

For irregular walls (with windows, doors, or sloped ceilings), measure the width and height of the wall as if it were a full rectangle, then subtract the area of openings. You can do that by entering a reduced width or height, or by simply rounding down the total area. Most paint calculators include this step mentally, but this one keeps it simple: it gives you the raw area, and you adjust the input if needed.

How to Choose Your Inputs

• Wall width and height: Measure in feet (or meters) from floor to ceiling and from corner to corner. For multiple walls of the same size, multiply the number of walls. If walls differ, calculate each separately and add the results.
• Number of coats: If you are painting over a similar color with good primer, 1 coat may be enough. For a drastic color change or bare drywall, use 2 coats. For staining or very porous surfaces, use 3.
• Coverage per gallon: Check the paint can label. Most premium latex paints list 350–400 sq ft per gallon, but rough surfaces (stucco, brick, unprimed drywall) may only get 200–250 sq ft per gallon. When in doubt, use a lower number.

Common Mistakes and How to Avoid Them

The most common mistake is forgetting to subtract windows and doors. A standard door is about 20 sq ft, and a typical window is 12–15 sq ft. For a 12×8 wall with one window, you would actually paint about 84 sq ft, not 96. The calculator gives the gross area; you need to adjust the width or height to account for openings, or simply round down the final amount.

Another mistake is mixing units. If you measure in meters, do not use a coverage per gallon that is in square feet. Convert everything to the same system first. Also, remember that the calculator output is the theoretical minimum—paint is wasted on roller trays, brush cleanup, and uneven application. Always add 10–15% extra, especially for textured walls.

Limits of the Formula

This formula assumes a flat, smooth, non-absorbent surface. In reality, paint coverage varies with:

• Surface porosity: Bare drywall, wood, and masonry absorb more paint than primed or previously painted surfaces.
• Application method: Spraying uses more paint (overspray), rolling uses a medium amount, and brushing uses the least but takes longer.
• Paint quality: Higher-quality paints often have more solids and better coverage, so you may need less paint even if the label says the same coverage.
• Color change: Going from dark to light, or vice versa, often requires an extra coat or a tinted primer.

For these reasons, treat the calculator result as a starting point, not a guarantee. It is always safer to buy one extra quart (or liter) than to run out mid-project.

Frequently Asked Questions

How do I measure a wall with windows and doors?

Measure the full width and height of the wall, then subtract the area of each window or door. For example, a 12×8 wall (96 sq ft) with a 3×4 window (12 sq ft) and a 3×7 door (21 sq ft) gives 96 − 12 − 21 = 63 sq ft of paintable area. You can enter 63 as the width (if height is 1) or adjust the width to 7.875 ft (63 ÷ 8) to keep the calculator simple.

How much paint do I need for a 12×12 room?

A 12×12 room with 8-ft ceilings has four walls, each 12×8 = 96 sq ft, for a total of 384 sq ft. With two coats and 350 sq ft/gallon coverage, you need about 2.2 gallons, so buy 3 gallons. If you are also painting the ceiling, add that area (12×12 = 144 sq ft) and recalculate.

What if the paint coverage on the can is in square meters per liter?

Convert to the same units as your measurements. If your wall area is in square feet and the coverage is in m²/L, first convert the coverage to sq ft per gallon: 1 m²/L ≈ 40.7 sq ft per gallon. Or convert your wall area to square meters (divide by 10.764) and use the coverage directly.

Should I buy more paint than the calculator says?

Yes, it is wise to buy 10–15% extra. This covers waste, uneven surfaces, and future touch-ups. For the example above, 2.2 gallons becomes about 2.5 gallons, so you would buy 3 gallons. If you are painting a large area, consider buying a 5-gallon bucket, which is usually cheaper per gallon.

Does primer count as a coat?

Primer is a separate product and should be calculated separately. If you are priming bare drywall, use the same formula with the primer’s coverage rate (usually 300–400 sq ft per gallon). Then apply the paint coats on top. Many painters use one coat of primer and two coats of paint for the best result.

Can I use this calculator for exterior painting?

Yes, but exterior surfaces are often more porous and may have more texture, so reduce the coverage per gallon by 20–30%. Also, measure the actual wall area, not the footprint. For a two-story house, multiply the perimeter by the height, then subtract large windows and doors.

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in w | Wall width | ft | 12 | 0
in h | Wall height | ft | 8 | 0
in n | Number of walls | - | 4 | 1
in c | Coats | - | 2 | 1
in cov | Coverage per gallon | sq ft/gal | 350 | 50
out total_area | Total wall area | sq ft | w * h * n | 0
out paint_gallons | Paint needed | gallons | (total_area * c) / cov | 1