Paint Calculator by Square Feet: How Much Paint

This paint calculator works out how many gallons (or liters) of paint you need for a room or surface when you enter its total square footage (or square meters), the number of coats, and the paint’s coverage rate. You get back the exact amount of paint required, plus a recommended purchase quantity so you buy enough without wasting money. The default values assume a typical 10×10-foot room with 8-foot ceilings and two coats, which is the standard for most interior wall paints.

Formula

The core formula is simple: Paint needed (gallons) = (Total area in square feet × Number of coats) ÷ Coverage per gallon. Here, total area is the sum of all wall and ceiling surfaces you plan to paint, measured in square feet; coats is how many layers you’ll apply (usually 2 for a color change, 1 for a repaint of the same color); coverage per gallon is the manufacturer’s stated spread rate, typically 250–400 sq ft per gallon for most interior latex paints. The result is in gallons, and the calculator also rounds up to the nearest whole gallon, since paint is sold in whole or half-gallon containers.

If you measure in square meters and liters, the same relationship holds: Paint needed (liters) = (Area in m² × Coats) ÷ Coverage in m² per liter. Typical coverage is about 10–12 m² per liter for one coat.

Worked Example

Using the default values from the calculator: a room with 320 sq ft of paintable wall area, 2 coats, and a paint that covers 350 sq ft per gallon.

1. Multiply area by coats: 320 × 2 = 640 sq ft of total coverage needed.
2. Divide by coverage per gallon: 640 ÷ 350 = 1.8286 gallons.
3. Round up to the nearest whole gallon: you need to buy 2 gallons.

The calculator shows 1.83 gallons as the exact amount and 2 gallons as the amount to buy. That extra 0.17 gallons is your safety margin for touch-ups and waste.

How to Measure Your Square Footage

Getting the area right is the most important step. For walls, measure the length and height of each wall in feet, multiply them together, and add up all the walls. For a rectangular room, you can also use the perimeter: (2 × length + 2 × width) × height. Subtract the area of doors and windows (roughly 20 sq ft per door, 15 sq ft per window) if you’re not painting those. For ceilings, multiply length × width. For trim, measure the linear feet and multiply by the width of the trim (usually 2–4 inches, converted to feet).

Here’s a quick checklist:

• Measure each wall separately – rooms are rarely perfectly square.
• Subtract large openings (doors, windows, built-ins) from the total.
• Add back any extra surfaces like alcoves, columns, or wainscoting.
• If painting the ceiling, measure it separately; it’s usually 20–25% of the total wall area.

Choosing the Right Coverage Rate

Most paint cans list a coverage rate, but that number assumes a smooth, primed, matte surface. Real-world coverage varies significantly:

• 350–400 sq ft/gal – smooth, primed drywall or previously painted walls with a flat finish.
• 250–300 sq ft/gal – textured surfaces (stucco, popcorn ceilings, brick), unprimed drywall, or glossy paints that go on thinner.
• 150–250 sq ft/gal – highly porous surfaces like raw wood, concrete, or old, chalky paint that absorbs more.

If you’re unsure, use the lower end of the range (250 sq ft/gal) to avoid running short. It’s always easier to have a little extra than to make a second trip to the store for a matching batch.

Common Mistakes and How to Avoid Them

The biggest error is forgetting to multiply by the number of coats. A single gallon covers roughly 350 sq ft, but two coats means you need double that. Another frequent mistake is mixing units: if you measure the room in feet but the paint coverage is in square meters per liter, you’ll be off by a factor of 10. Always convert everything to the same system before calculating.

Also, don’t subtract too much for windows and doors. A typical window is about 15 sq ft, but if you’re painting the trim around it, you still need paint for that trim. A good rule of thumb is to subtract 50% of the window/door area, not 100%, unless you’re leaving the trim unpainted.

Finally, remember that the first coat always absorbs more paint than the second. If you’re painting bare drywall or a dark color over light, the first coat may use 20–30% more paint than the coverage rate suggests. In that case, add 10–15% to your final number.

Cost Considerations (Without Specific Prices)

Paint prices vary widely by quality and brand, but a simple rule is: better paint covers better and lasts longer, so you may need fewer coats. A high-quality paint might cost 50% more per gallon but cover in one coat what a budget paint needs two for. In the long run, the higher-priced paint can be cheaper because you buy less of it and it lasts years longer before needing a repaint.

Beyond the paint itself, factor in primer (usually 1 gallon per 300–400 sq ft), rollers, brushes, tape, and drop cloths. If you’re painting a room that’s 320 sq ft, the paint might be the largest cost, but supplies can add 20–30% on top. The calculator’s “buy” amount gives you the minimum; adding one extra quart is often worth it for touch-ups later.

Using the Calculator for Different Surfaces

The same formula works for any surface, but you must adjust the coverage rate. For a deck or fence, use a coverage rate of 200–300 sq ft per gallon for stain, and 300–400 for solid paint. For exterior walls, coverage drops to 200–300 sq ft per gallon because of rough siding and weather exposure. For metal or radiators, use 300–400 sq ft per gallon for enamel. The calculator doesn’t know what surface you’re painting – you tell it the coverage rate, so it’s your job to pick the right one for your material.

For a single wall accent, you can still use the calculator: just enter the wall’s area and 1 coat. For a ceiling, measure the floor area (they’re the same) and use the same coverage rate. For trim, measure linear feet and multiply by the trim width in feet (e.g., 100 linear ft × 0.25 ft = 25 sq ft).

Frequently Asked Questions

How many square feet does a gallon of paint cover?

Most interior latex paints cover 350–400 square feet per gallon for one coat on a smooth, primed surface. For textured or porous surfaces, expect 250–300 square feet per gallon. Always check the can for the exact number, since high-hide paints may cover more and budget paints may cover less.

Do I really need two coats of paint?

Generally yes, especially if you’re changing colors or painting over bare drywall. Two coats give a uniform finish and better durability. If you’re repainting the same color over a well-sealed wall, one coat might suffice, but it’s risky – touch-ups will be noticeable later. The calculator defaults to 2 coats because that’s the safe standard.

How do I calculate paint for a room with 8-foot ceilings?

Measure the perimeter of the room (sum of all wall lengths) and multiply by 8 feet. For a 10×10 room, that’s 40 × 8 = 320 sq ft. Subtract doors and windows (about 20 sq ft each) if you’re not painting them. Then use the calculator with that area and your chosen coats.

Should I round up or down when buying paint?

Always round up to the nearest whole gallon. Paint is sold in gallons and quarts, so if you need 1.8 gallons, buy 2 gallons. The extra paint is useful for touch-ups and future repairs. If you’re on a tight budget, you can buy 1 gallon plus 1 quart (1.25 gallons), but you risk running short if the coverage is lower than expected.

Does primer count as a coat?

No. Primer is a separate product and should be calculated separately. Use the same formula with the primer’s coverage rate (usually 300–400 sq ft per gallon) and 1 coat. Primer is essential for bare drywall, stained surfaces, or drastic color changes – it seals the surface so the paint goes on more evenly and covers better.

What if my room isn’t a perfect rectangle?

Break the room into rectangles, calculate each one’s area, and add them together. For angled ceilings or alcoves, measure the longest point and multiply by the width – it’s better to overestimate slightly than to run short. The calculator doesn’t care about the shape, only the total square footage you enter.

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in area | Total area to paint | sq ft | 320 | 0
in coats | Number of coats | - | 2 | 1
in coverage | Coverage per gallon | sq ft/gal | 350 | 50
out paint_needed | Paint needed | gallons | area * coats / coverage | 2
out buy | Buy this much | gallons | ceil(area * coats / coverage) | 0