Paint Calculator Sq Ft: How Much Paint Do You Need?

This paint calculator tells you how many gallons (or liters) of paint you need for a room or surface. You enter the total square footage to be painted, the number of coats, and the paint coverage rate (usually listed on the can), and the calculator returns the exact amount of paint to buy. It also shows the total area in square meters if you work in metric.

Formula

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

• Total area – the sum of all wall and ceiling surfaces you plan to paint, in square feet (or square meters).
• Number of coats – how many layers of paint you will apply. Usually 2 for a color change, 1 for a same-color touch-up.
• Coverage per gallon – the manufacturer’s stated spreading rate, typically 250–400 sq ft per gallon for most interior paints.

If you measure in meters, use the same formula with square meters and liters: Paint needed (liters) = (Area in m² × Coats) ÷ Coverage per liter. Most paint labels list coverage in both sq ft/gal and m²/L.

Worked Example

Using the default values from the calculator above:

1. Total area = 400 sq ft
2. Coats = 2
3. Coverage = 350 sq ft per gallon

Step 1: Multiply area by coats: 400 × 2 = 800 sq ft of total surface to cover.

Step 2: Divide by coverage: 800 ÷ 350 = 2.2857 gallons.

Step 3: Round up to the nearest quart or gallon. You need 2.29 gallons, so buy 2 gallons plus 1 quart (or 3 gallons if you prefer a margin for waste).

The calculator gives 2.29 gallons, which matches.

How to Measure Your Room’s Square Footage

To get an accurate estimate, you need the actual paintable area, not just the floor area. Here’s a simple method:

• Measure the length and height of each wall. Multiply length × height for each wall to get its square footage.
• Add all wall areas together.
• Subtract areas that won’t be painted: doors (about 20 sq ft each), windows (about 15 sq ft each), and built-in cabinets.
• If painting the ceiling, measure floor length × width.

For example, a 10×12 ft room with 8-ft ceilings has walls totaling (10+12+10+12) × 8 = 352 sq ft. After subtracting one door (20) and two windows (30), you get 302 sq ft of wall area. Add the ceiling (120 sq ft) if needed.

Factors That Affect Paint Coverage

Coverage rates on the can are based on smooth, primed, drywall in perfect conditions. Real-world factors can reduce actual coverage by 10–20%:

• Surface texture: Stucco, brick, or textured walls absorb more paint than smooth drywall.
• Porosity: New drywall, bare wood, or unprimed surfaces soak up the first coat quickly.
• Color change: Going from dark to light (or vice versa) often requires an extra coat or a tinted primer.
• Application method: Spraying wastes more paint than rolling; thick-nap rollers use more than thin ones.
• Temperature and humidity: Very hot or dry conditions cause paint to dry faster, which can reduce leveling and coverage.

As a rule of thumb, add 10% to your final number to account for touch-ups and waste. For textured surfaces, add 20%.

Common Mistakes When Estimating Paint

Even with a calculator, people often misjudge their needs. Watch out for these pitfalls:

• Using floor area instead of wall area: A 200 sq ft room does not mean 200 sq ft of walls—walls are usually 2–3 times that.
• Forgetting the ceiling: If you’re painting the ceiling, include it separately; it’s often a different color and finish.
• Ignoring the number of coats: A single coat covers less than two; always multiply by coats.
• Rounding down: Paint is sold in gallons and quarts. If you need 1.2 gallons, you must buy 1 gallon + 1 quart (1.25 gal) or 2 gallons—you can’t buy 0.2 gallons.
• Not accounting for primer: Primer is a separate product with its own coverage, often lower than paint.

The biggest mistake is underestimating area. Measure twice, buy once—or buy an extra quart, which is cheaper than a second trip to the store.

Paint Calculator vs. Paint Can Labels

Most paint cans list coverage as “350 sq ft per gallon� or “400 sq ft per gallon.� This is the theoretical maximum on a smooth, sealed surface. The calculator uses that number, but you should adjust it based on your surface. If you’re painting over a dark color with a light one, expect to need 3 coats—or use a primer first. If you’re painting new drywall, the first coat will cover less because the drywall absorbs moisture. In those cases, reduce the coverage per gallon in the calculator (e.g., use 300 instead of 350) to get a more realistic estimate.

Cost Considerations

Once you know the gallons needed, you can estimate cost by multiplying by the price per gallon. Paint prices vary widely, but a mid-range interior paint typically costs $30–$60 per gallon. For a 400 sq ft room with 2 coats (2.29 gallons), you’d need 3 gallons, so paint alone would cost roughly $90–$180. Add primer if needed (about $20–$40 per gallon). This doesn’t include brushes, rollers, tape, or drop cloths, which add another $20–$50. For a whole house, costs scale quickly—a 2,000 sq ft interior might need 10–15 gallons for walls alone.

Frequently Asked Questions

How many square feet does a gallon of paint cover?

Most interior paints cover 250–400 sq ft per gallon, with 350 sq ft being a common average. Exterior paints and primers often cover less, around 200–300 sq ft per gallon, because surfaces are rougher.

Do I need to prime before painting?

Primer is recommended for new drywall, bare wood, stained surfaces, or when making a drastic color change (e.g., dark red to white). It seals the surface and improves adhesion, reducing the number of topcoats needed. For repainting a similar color over already painted walls, primer is usually unnecessary.

How do I calculate paint for a room with many windows and doors?

Measure each wall’s total area, then subtract the square footage of each window (about 15 sq ft) and door (about 20 sq ft). For accuracy, measure the actual window and door frames—don’t rely on averages if they’re unusually large or small.

Is it better to buy more paint than I need?

Yes, buying 10–15% extra is wise. You’ll need it for touch-ups after the job, and paint can be color-matched later, but it’s easier to have the same can. Many stores allow returns of unopened cans within 30 days, so you can return unused paint if you buy too much.

Does the type of paint (matte, eggshell, satin) affect coverage?

Generally, flat and matte finishes cover slightly better than glossy ones because they scatter light and hide imperfections. However, the difference is small—coverage is more influenced by surface texture and porosity than by sheen. Always check the label for the specific coverage rate of the paint you choose.

Can I use the same calculator for exterior paint?

Yes, but adjust the coverage rate. Exterior surfaces like wood siding, stucco, or brick are more porous and uneven, so coverage drops to 200–300 sq ft per gallon. Also, exterior paint often requires two coats, and you may need to account for trim and eaves separately.

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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 | 1
out paint | Paint needed | gallons | area * coats / coverage | 2
out area_m2 | Area in square meters | m² | area * 0.092903 | 1