Paint Calculator by Room: How Much Paint Do You Need?

This paint calculator by room tells you how many gallons or liters of paint to buy for the walls and ceiling of a single rectangular room. You enter the room’s length, width, and height, plus the number of doors and windows, and it returns the total paintable area and the paint volume needed for one or two coats. The result is an estimate, not a guarantee, but it’s the same math professional painters use to avoid buying too much or too little.

Formula

The calculator uses two simple formulas. First, the paintable wall area:

Wall Area = (2 × (Length + Width) × Height) − (Doors × 21) − (Windows × 15)

Then the paint volume:

Paint Needed (gallons) = (Wall Area ÷ 350) × Number of Coats

Where:

• Length, Width, Height are in feet (or meters – see below).
• Doors subtract 21 square feet each (a typical 3′ × 7′ door).
• Windows subtract 15 square feet each (a typical 3′ × 5′ window).
• 350 square feet per gallon is the standard coverage for one coat of interior latex paint on primed drywall.

If you measure in meters, use 33 square meters per liter (the metric equivalent of 350 sq ft/gal). The calculator above works in feet and gallons; for metric, simply enter meters and divide the final gallons by 3.785 to get liters.

Worked Example

Using the default values: a room 12 ft long, 10 ft wide, 8 ft high, with 1 door and 1 window, and 2 coats of paint.

1. Perimeter: 2 × (12 + 10) = 44 ft
2. Gross wall area: 44 × 8 = 352 sq ft
3. Subtract door: 352 − 21 = 331 sq ft
4. Subtract window: 331 − 15 = 316 sq ft
5. Paint for one coat: 316 ÷ 350 = 0.90 gallons
6. For two coats: 0.90 × 2 = 1.80 gallons

The calculator shows 316 sq ft and 1.80 gallons. In practice, you’d buy 2 gallons – paint is sold in whole gallons, and it’s always smart to have a little extra for touch-ups.

Why 350 Square Feet per Gallon?

Paint manufacturers list coverage on the can, usually 350–400 square feet per gallon for a single coat. That number assumes a smooth, primed surface and normal application with a roller. Rough surfaces like stucco or unpainted drywall absorb more – sometimes as little as 250 sq ft per gallon. The 350 figure is a safe middle ground for most interior rooms. If you’re painting over a dark color with a light one, or over bare drywall, expect to need more paint, and consider using a primer first.

Also note that the coverage on the can is for one coat. If you’re applying two coats (which is standard for a uniform finish), you double the paint needed. The calculator does that automatically with the “Number of coats� input.

How to Measure Your Room Correctly

Accurate measurements are the key to a useful estimate. Here’s what to do:

• Measure the length and width of the room from wall to wall, in feet, to the nearest 0.1 ft.
• Measure the height from floor to ceiling. If the ceiling is sloped, measure the tallest point and the shortest point, and use the average.
• Count all doors and windows that are fully inside the room. For a door, use the actual size if it’s larger than standard (e.g., double doors). For a window, measure the rough opening if it’s much larger than 3×5 ft.
• If you’re painting the ceiling, add the ceiling area (Length × Width) separately – the calculator above does not include it.

For a room with a cathedral ceiling or lots of trim, the simple formula will be less accurate. In those cases, measure each wall individually and add them up.

Common Mistakes and How to Avoid Them

• Forgetting to subtract doors and windows: This can overestimate by 10–15%, leading to leftover paint.
• Using the wrong coverage number: High-end paints often cover better (400 sq ft/gal), while cheap paints or rough surfaces cover less. Adjust the 350 if you know better.
• Mixing units: Don’t enter feet and then read the result as meters. The formula is linear, so if you enter meters, the area is in square meters – but the paint output is still in gallons. Convert to liters separately.
• Rounding too early: Always round up to the nearest whole gallon when buying. Paint is not sold in fractions of a gallon (except in quarts, which are 1/4 gallon).
• Ignoring trim and baseboards: If you’re painting woodwork, that’s separate paint and coverage. The calculator only covers walls.

When to Use This Calculator (and When Not To)

Use this calculator for a standard bedroom, living room, hallway, or office with four rectangular walls and a flat ceiling. It’s perfect for planning a DIY paint job and for comparing paint costs (multiply gallons by price per gallon).

Do not use it for:

• Rooms with vaulted ceilings, arches, or non-rectangular shapes – measure wall by wall.
• Exterior painting – different coverage and more variables (siding, trim, etc.).
• Painting only one accent wall – just measure that wall’s area and divide by 350.
• Primer – primer has different coverage (usually 300 sq ft/gal) and is often a separate product.

For a single accent wall, the formula is simply: Wall width × Height ÷ 350 × Coats. For example, a 12 ft wide wall, 8 ft high, 2 coats: 12×8=96 sq ft, 96÷350×2=0.55 gallons – buy 1 gallon.

Frequently Asked Questions

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

Using the calculator: 12×12 room, 8 ft ceiling, 1 door, 1 window, 2 coats = about 2 gallons. The exact amount is 1.8 gallons, so you’d buy 2 gallons. If you’re also painting the ceiling, add about 1 gallon for the ceiling (144 sq ft ÷ 350 × 2 = 0.82 gallons, so 1 gallon).

Should I always buy extra paint?

Yes, it’s wise to buy 10–20% more than the calculated amount. You’ll need extra for touch-ups after the job, and paint can be hard to color-match later if you run out. For a 2-gallon job, buying 2 gallons is usually enough; for a 5-gallon job, consider 6.

Does paint coverage vary by brand?

Yes, but not by much. Most interior latex paints list 350–400 sq ft per gallon. Premium paints with more titanium dioxide (the pigment) tend to cover better. Always check the can – the coverage is printed on the label. If it says 400, you can use 400 in the formula instead of 350.

What if my room has no windows or doors?

Then enter 0 for both. The calculator will subtract nothing, and you’ll get the full wall area. That’s correct for a closet or a room with a door but no window – just count the door if it’s there.

How do I convert the result to liters?

One US gallon equals 3.785 liters. So multiply the gallons by 3.785. For example, 2 gallons = 7.57 liters. In metric countries, paint is sold in liters, and coverage is typically 10–12 m² per liter per coat (about 33 m² per gallon).

Does the calculator include the ceiling?

No, it only covers walls. To include the ceiling, add Length × Width to the wall area, then divide by 350 and multiply by coats. For a 12×10 room, that’s 120 sq ft extra – about 0.7 gallons for two coats, so you’d add one gallon to your order.

Related

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• Gravel Volume Calculator: How Much Do You Need?

in l | Room length | ft | 12 | 0
in w | Room width | ft | 10 | 0
in h | Room height | ft | 8 | 0
in d | Number of doors | - | 1 | 0
in win | Number of windows | - | 1 | 0
in coats | Number of coats | - | 2 | 1
out area | Wall area | sq ft | (2 * (l + w) * h) - (d * 21) - (win * 15) | 0
out paint | Paint needed | gallons | area / 350 * coats | 2