Paint Calculator by Room Size: How Much Paint

This paint calculator works out how much paint you need for a room based on its dimensions. You enter the room’s length, width, and height, plus the number of doors and windows, and it returns the total wall area in square meters and the estimated liters of paint for one coat. The result helps you buy the right amount of paint for your project without overbuying or running short.

Formula

The calculator uses a standard wall-painting formula. First, it finds the total wall area by multiplying the room’s perimeter by the wall height, then subtracts the areas of doors and windows. The paint volume is then the wall area multiplied by the number of coats, divided by the paint’s coverage rate (typically 10 m² per liter per coat).

In symbols: Wall Area = (2 × (Length + Width) × Height) − (Doors × 1.9) − (Windows × 1.5), and Paint (L) = Wall Area × Coats ÷ 10.

• Length, Width, Height – room dimensions in meters (m).
• Doors – each standard door subtracts about 1.9 m² (roughly 0.9 m × 2.1 m).
• Windows – each standard window subtracts about 1.5 m² (roughly 1.2 m × 1.2 m).
• Coats – number of coats you plan to apply (usually 2 for a color change).
• Coverage – 10 m² per liter is a common average for interior latex paint; check your paint can for the exact figure.

Worked Example

Using the default values: length = 4 m, width = 3 m, height = 2.5 m, doors = 1, windows = 1, coats = 2.

1. Perimeter = 2 × (4 + 3) = 14 m
2. Gross wall area = 14 × 2.5 = 35 m²
3. Subtract door (1.9) and window (1.5): 35 − 1.9 − 1.5 = 31.6 m²
4. Paint for one coat = 31.6 ÷ 10 = 3.16 L
5. For two coats = 3.16 × 2 = 6.32 L

The calculator rounds to 6.3 L. In practice, you’d buy a 7.5 L can or two 4 L cans, depending on what the store offers.

How to Measure Your Room Accurately

Accurate measurements are the foundation of a good estimate. Use a laser distance measurer or a steel tape, and measure at floor level, because walls can be slightly taller or shorter than they appear. Measure the length and width of the room from wall to wall, not including baseboards. For height, measure from the floor to the ceiling, again excluding trim. If the room is not a perfect rectangle, break it into rectangular sections and add their areas.

For doors and windows, count each standard opening as one unit. If you have unusually large windows (e.g., a picture window), measure them and subtract the actual area instead of using the 1.5 m² default. The same goes for sliding glass doors, which are much larger than a standard door.

Common Mistakes and How to Avoid Them

• Forgetting to subtract openings – this can overestimate paint by 10–15%.
• Mixing units – if you measure in feet, convert to meters or use a feet-based formula. One meter = 3.28 feet; one square meter = 10.76 square feet.
• Ignoring the number of coats – a dark-to-light color change may need three coats, while a same-color touch-up may need only one.
• Rounding up too little – paint coverage varies with surface texture; rough plaster or brick absorbs more paint than smooth drywall.
• Not accounting for waste – spillage, drips, and touch-ups can add 5–10% to your needs.

Coverage Rates: Why They Vary

The 10 m² per liter figure is a common average, but actual coverage depends on several factors:

• Surface porosity – new drywall or bare wood soaks up more paint than previously painted, sealed walls.
• Paint quality – higher-quality paints often have more solids and better hiding power, so they may cover more area per liter.
• Application method – rollers and brushes leave different film thicknesses; sprayers can waste more paint due to overspray.
• Color change – going from dark to light or vice versa usually requires extra coats, effectively reducing coverage per coat.

Always check the label on the paint can – it will state the specific coverage rate for that product. If it says 12 m²/L, use 12 instead of 10 in the formula.

How to Use the Result

The result is a net estimate for the wall area only – it does not include the ceiling, baseboards, or trim. If you plan to paint the ceiling, measure its area (length × width) and add that separately. For trim, estimate about 0.5–1 L per standard room for baseboards and door frames.

When buying paint, round up to the nearest can size. Paint is sold in various volumes – 1 L, 2.5 L, 4 L, 5 L, and 10 L are common. For the example above (6.3 L), you might buy a 7.5 L can or a 5 L plus a 2.5 L. It’s often better to have a little extra than to run out mid-project, since color matching between batches can be imperfect.

Limitations of the Formula

This calculator gives a good starting point, but it cannot account for every real-world variable. It assumes rectangular rooms with flat, smooth walls. If you have textured walls, wainscoting, or built-in shelving, the actual surface area will differ. It also assumes you’re painting the entire wall surface uniformly – if you’re doing an accent wall or a two-tone design, you’ll need to adjust the inputs accordingly.

For rooms with cathedral ceilings, bay windows, or other architectural features, the formula becomes less accurate. In those cases, measure each wall individually and add them up manually, then subtract openings. The calculator is best used for standard, rectangular rooms – which covers the majority of bedrooms, living rooms, and hallways.

Frequently Asked Questions

How many square meters does 1 liter of paint cover?

On average, 1 liter of interior paint covers about 10 square meters per coat. However, this varies by paint type, surface texture, and application method. Check the paint can label for the manufacturer’s specific coverage rate – it may be anywhere from 8 to 14 m²/L.

Do I need to prime before painting?

Primer is not always necessary, but it’s recommended for new drywall, bare wood, or when making a dramatic color change. Primer seals the surface and provides a uniform base, which can reduce the number of paint coats needed. If you use primer, you don’t count it as a paint coat – apply it first, then paint over it.

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

A 12×12 foot room (about 3.7 m × 3.7 m) with 8-foot (2.4 m) ceilings and one door and one window has roughly 110 square feet (about 10 m²) of wall area after subtracting openings. For two coats, you’d need about 2 liters of paint, or about 0.5 gallons. In practice, you’d buy a gallon can for convenience.

Should I include the ceiling in the paint calculator?

No, this calculator is for walls only. To estimate ceiling paint, multiply the room’s length by its width to get the ceiling area, then divide by the paint’s coverage rate. For the example room (4 m × 3 m), the ceiling is 12 m², needing about 1.2 L per coat.

What if my room has large windows or sliding doors?

Measure the actual height and width of each opening and subtract the total area from the wall area. For a sliding glass door that’s 2 m wide and 2.1 m tall, subtract 4.2 m² instead of the default 1.5 m². Adjust the calculator by reducing the number of windows to keep the math simple, or do the calculation manually.

How many coats of paint should I use?

Most interior walls need two coats for even, durable coverage. One coat may be enough if you’re painting the same color with high-quality paint, but two coats are safer for color changes or over porous surfaces. Dark colors often require three coats to achieve full richness.

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in length | Room length | m | 4 | 0
in width | Room width | m | 3 | 0
in height | Room height | m | 2.5 | 0
in doors | Number of doors | - | 1 | 0
in windows | Number of windows | - | 1 | 0
in coats | Number of coats | - | 2 | 1
out wall_area | Wall area | m² | (2 * (length + width) * height) - (doors * 1.9) - (windows * 1.5) | 1
out paint_liters | Paint needed | L | wall_area * coats / 10 | 1