Paint Calculator by Square Footage: How Much Paint You Need

This paint calculator works out how many gallons (or liters) of paint you need for a room or surface based on the square footage you enter. You provide the total area to be painted and the number of coats, and it returns the required paint volume, accounting for standard coverage rates and typical waste. The result is an estimate that helps you buy the right amount of paint without excessive leftovers or a second trip to the store.

Formula

The basic paint calculation uses a simple formula: Paint needed (gallons) = (Total area × Number of coats) ÷ Coverage per gallon. In the calculator above, we add a 10% safety margin to account for spillage, touch-ups, and uneven surfaces, so the full expression is: Paint = (area × coats) / coverage × 1.1.

Variables:

• Total area (sq ft) – the sum of all wall (and ceiling) surfaces you plan to paint, measured in square feet.
• Number of coats – how many layers of paint you intend to apply (usually 2 for a color change, 1 for a similar shade).
• Coverage per gallon (sq ft/gal) – the manufacturer’s stated spread rate, typically 250–400 sq ft per gallon for most interior paints.
• Paint needed (gallons) – the final volume of paint, rounded to one decimal place in the calculator.

For metric users, the same logic applies: Paint (liters) = (Area in m² × Coats) ÷ Coverage in m² per liter. Most paints cover about 10–12 m² per liter.

Worked Example

Using the default values from the calculator: area = 400 sq ft, coats = 2, coverage = 350 sq ft/gal.

1. Multiply area by coats: 400 × 2 = 800 sq ft (total area to cover).
2. Divide by coverage: 800 ÷ 350 = 2.2857 gallons.
3. Add 10% safety margin: 2.2857 × 1.1 = 2.5143 gallons.
4. Round to one decimal: 2.5 gallons.

So for a 400 sq ft room with two coats, you’d need about 2.5 gallons of paint. In practice, you’d likely buy 3 gallons (since paint is sold in whole gallons) to have a little extra for touch-ups.

How to Measure a Room for Painting

To use this calculator accurately, you first need the square footage of the surfaces you’ll paint. Here’s the standard method:

• Walls: Measure the length of each wall and multiply by the ceiling height. Add all wall areas together. For example, a 10×12 ft room with 8 ft ceilings: (10+12+10+12) × 8 = 44 × 8 = 352 sq ft.
• Subtract openings: Measure each door (about 21 sq ft) and window (about 12–15 sq ft) and subtract them from the total. For a typical room with two windows and one door, subtract roughly 45–50 sq ft.
• Ceilings: Multiply the room length by width (10 × 12 = 120 sq ft) if you’re painting the ceiling.
• Trim and baseboards: Measure linear feet and multiply by the width of the trim (e.g., 50 linear ft of 3-inch baseboard = 50 × 0.25 = 12.5 sq ft).

If you’re painting a complex shape, break it into rectangles and add them up. For exterior painting, measure the outside walls (height × width) and subtract windows and doors.

Understanding Paint Coverage Rates

Coverage per gallon varies by paint type and surface. Here are typical ranges:

• Interior flat/eggshell latex: 300–400 sq ft per gallon
• Interior satin/semi-gloss: 250–350 sq ft per gallon
• Primer: 200–300 sq ft per gallon (often lower because it seals porous surfaces)
• Exterior latex: 250–350 sq ft per gallon
• Heavy texture (stucco, brick): 100–200 sq ft per gallon (much less due to surface roughness)

Always check the can label for the exact coverage. The number is based on a smooth, non-porous surface; rough or highly porous surfaces will absorb more paint, reducing coverage. If you’re painting over a dark color with a light one, you may need an extra coat or a primer, which increases the total paint needed.

Common Mistakes and How to Avoid Them

• Forgetting to subtract windows and doors: This can overestimate by 10–15%. Measure carefully.
• Ignoring the number of coats: A color change usually needs two coats; a same-color touch-up might need only one. Always multiply by coats.
• Using the wrong coverage number: Don’t assume 400 sq ft/gal for every paint. Check the label; some high-hide paints cover more, but most are in the 300–350 range.
• Not adding a waste factor: Paint spills, drips, and uneven application can waste 5–10%. The calculator adds 10% automatically, but if you’re a beginner, consider 15–20%.
• Mixing units: If you measure in feet, use square feet; if in meters, use square meters. Never mix feet with liters or meters with gallons.
• Rounding too early: Always do the full calculation, then round up to the nearest gallon (or quart) when buying. For example, 2.1 gallons means you need 3 gallons, not 2.

How to Read the Result and Buy Paint

The calculator gives you a decimal number (e.g., 2.5 gallons). Paint is typically sold in gallons (US) or liters (metric). Here’s how to convert to purchase quantities:

• If result is 2.5 gallons: Buy 3 gallons (you’ll have half a gallon left for touch-ups).
• If result is 0.8 gallons: Buy 1 gallon (you’ll have a bit extra, but that’s fine).
• For quarts: 1 gallon = 4 quarts. If you need 0.3 gallons, you could buy 1 quart (0.25 gal) plus a little more, but it’s often simpler to buy a gallon.
• Metric: Paint is sold in 1L, 2.5L, 5L, and 10L cans. Round up to the nearest available size.

If you’re painting multiple rooms with the same color, add the areas together and then calculate. If you’re using different colors, calculate each separately. Also consider primer: if you’re painting bare drywall or a dramatic color change, you’ll need a separate primer coat, which is not included in this calculator’s default (you can add it by increasing the coat count or doing a separate calculation).

Limitations of This Calculator

This calculator provides a solid estimate, but real-world conditions can change the number. Surface porosity, application method (brush, roller, spray), and painter skill all affect paint consumption. Spraying uses more paint (up to 30% more) because of overspray. Textured surfaces (like popcorn ceilings) absorb more. Also, the coverage rate on the can is measured on a smooth, primed surface; if you’re painting over bare drywall or old, porous wood, expect lower coverage.

For very large projects (whole house exterior), you may want to use a professional estimator or consult the paint manufacturer’s guidelines. But for a typical room, this formula is accurate within 10–15%.

Frequently Asked Questions

How many square feet does a gallon of paint cover?

Most interior latex paints cover about 300–400 square feet per gallon on a smooth, primed surface. The exact number is printed on the can. For rough or unprimed surfaces, coverage can drop to 200–250 sq ft per gallon.

Do I need two coats of paint always?

Not always. If you’re painting the same color over a similar shade, one coat may suffice. But for a significant color change, covering a dark color with a light one, or painting bare drywall, two coats are usually necessary for even, durable coverage. Some high-quality paints are self-priming and cover well in one coat, but two coats always last longer.

Should I include the ceiling in the square footage?

Only if you’re painting the ceiling. If you’re painting only walls, measure the wall area and subtract openings. If you’re painting both, add the ceiling area (length × width) to the wall area. The calculator works for any surface; just input the total area you plan to cover.

How do I account for windows and doors?

Measure each window and door and subtract their area from the wall total. A standard door is about 21 sq ft (3 ft × 7 ft), and a typical window is about 12–15 sq ft. If you’re painting the trim around them, you can leave that area in or add it back as trim, but for simplicity, subtract the openings and paint the trim separately.

What if my room is not a perfect rectangle?

Break the walls into rectangles and triangles. For triangles (e.g., a vaulted ceiling), multiply base × height ÷ 2. Add all areas together. The calculator doesn’t care about shape; it only needs the total square footage.

Is it better to buy more paint than the calculator says?

Yes, it’s wise to round up to the next whole gallon (or liter) because you’ll likely need touch-ups later. The 10% margin in the calculator helps, but having a little extra paint stored for future repairs is practical. Just make sure to store it properly (sealed, in a cool place) to keep it usable.

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in area | Total area to paint | sq ft | 400 | 0
in coats | Number of coats | - | 2 | 1
in coverage | Coverage per gallon | sq ft/gal | 350 | 50
out paint | Paint needed | gallons | (area * coats) / coverage * 1.1 | 1
out area_per_gal | Area covered per gallon | sq ft/gal | coverage | 0