Paint Calculator: How Much Paint Do I Need?

This paint calculator works out how many gallons or liters of paint you need for a room or wall. You enter the total wall area (or room dimensions), the number of coats, and the paint coverage rate, and it returns the exact amount of paint required. It also gives you a practical estimate in whole cans so you know what to buy.

Formula

The basic formula is simple: multiply the total wall area by the number of coats, then divide by the coverage rate of your paint.

Paint needed (gallons) = (Wall area × Number of coats) ÷ Coverage per gallon

• Wall area – the total surface you plan to paint, in square feet (or square meters). For a room, sum the area of all walls, minus doors and windows.
• Number of coats – usually 2 for a color change, 1 if repainting the same color with a quality paint.
• Coverage per gallon – the manufacturer’s stated spread rate, typically 250–400 sq ft per gallon (about 6–10 m² per liter).

For metric, the same formula works if you use liters and square meters: Paint (liters) = (Area in m² × Coats) ÷ Coverage in m² per liter. A common metric coverage is 10–12 m² per liter per coat.

Worked Example

Using the default values from the calculator above:

• Wall area = 400 sq ft
• Coats = 2
• 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 whole gallons: you need 3 gallons.

The calculator shows 2.29 gallons and 3 whole gallons. Always buy the rounded-up number, because paint is sold in whole gallons and you’ll want a little extra for touch-ups.

How to Measure Your Wall Area

Getting the area right is the most important step. Here’s a reliable method:

1. Measure the length of each wall in feet (or meters).
2. Measure the height from floor to ceiling.
3. Multiply length × height for each wall and add them together.
4. Subtract the area of doors (about 20 sq ft / 1.9 m² each) and windows (about 15 sq ft / 1.4 m² each).
5. For a room, you can also use the perimeter: perimeter × height, then subtract openings.

If you’re painting a ceiling, measure the floor length × width. For trim and baseboards, measure the linear footage and multiply by the width of the trim.

Choosing the Right Coverage Rate

Paint coverage varies by surface and paint quality. The number on the can is a best-case estimate for smooth, primed drywall. In practice, you’ll get less coverage on:

• Rough or textured surfaces (stucco, brick, popcorn ceilings) – coverage can drop by 20–30%.
• Unprimed drywall – it soaks up more paint.
• Previously unpainted or highly porous wood.
• Dark colors over light – you may need an extra coat or a tinted primer.

For most interior walls, 350 sq ft per gallon is a safe average. If you’re using a cheaper paint, assume 250–300 sq ft per gallon. For premium paints, 400 sq ft per gallon is realistic on smooth surfaces.

Common Mistakes and How to Avoid Them

Even with a calculator, people often end up with too little or too much paint. Watch out for these pitfalls:

• Forgetting to subtract doors and windows – this can overestimate by 10–15%.
• Not accounting for coats – a single coat rarely covers well, especially over a darker color.
• Using the wrong units – mixing feet and meters, or gallons and liters, throws off the math completely.
• Rounding down – always round up to the nearest whole can. You’d rather have a quart extra than run out mid-project.
• Ignoring the surface condition – bare drywall and old, glossy paint absorb differently. A primer coat is often cheaper than an extra topcoat.

A good rule of thumb: buy 10% more than the calculator says. That covers touch-ups, spills, and the inevitable second coat you didn’t plan for.

Paint Calculator vs. Manual Estimation

Why use a calculator instead of just guessing? A calculator removes the arithmetic errors and gives you a consistent method. But it’s only as good as your inputs. If you measure the room wrong, the calculator will be wrong too.

Manual estimation is fine for a single wall, but for a whole room or house, a calculator saves time and reduces the chance of a costly mistake. Many paint brands have their own calculators, but they often assume a specific coverage rate. Our calculator lets you adjust for your paint’s actual spread rate.

Cost Considerations

Once you know how many gallons you need, you can estimate cost by multiplying by the price per gallon. Paint prices range widely, from budget options around $15–$25 per gallon to premium lines at $50–$70 or more. The calculator doesn’t include cost because prices vary by region and brand, but the math is simple: total cost = gallons × price per gallon.

Remember that primer is separate. If you’re painting bare drywall or a dramatic color change, add a coat of primer and factor that into your total. Primer usually costs less than topcoat and can reduce the number of topcoats you need.

Safety and Preparation

Before you open a can of paint, take a few safety steps:

• Work in a well-ventilated area – open windows and use fans.
• Wear a respirator or mask if you’re sensitive to fumes, especially for oil-based paints.
• Use a ladder safely – keep three points of contact and don’t overreach.
• Keep paint away from heat sources and open flames – most paints are flammable.
• Clean up spills immediately with a damp cloth; dried paint is much harder to remove.

For storage, keep paint cans tightly sealed in a cool, dry place. Latex paint can last several years if not frozen. Dispose of leftover paint at a local hazardous waste facility – never pour it down the drain.

Frequently Asked Questions

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

A 12×12 room has four walls, each 12 feet long. If the ceiling is 8 feet high, the wall area is 4 × 12 × 8 = 384 sq ft. Subtract about 20 sq ft for a door and 15 sq ft for a window, leaving roughly 349 sq ft. For two coats at 350 sq ft per gallon, you need about 2 gallons (349 × 2 ÷ 350 = 1.99). So buy 2 gallons.

Does one gallon of paint cover a 10×10 room?

One gallon covers about 350 sq ft. A 10×10 room with 8-foot ceilings has about 320 sq ft of wall area (4 × 10 × 8 = 320), minus a door and window (about 35 sq ft) gives 285 sq ft. One gallon will cover that with a single coat, but you’ll likely need two coats, so plan on 2 gallons.

How do I calculate paint for a wall with a large window?

Measure the wall’s full area (height × width), then subtract the window’s area (height × width of the window). For example, a 10×10 wall (100 sq ft) with a 3×4 window (12 sq ft) gives 88 sq ft of paintable surface. Multiply by the number of coats and divide by coverage.

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

Yes, buying 10–15% extra is wise. It covers touch-ups, mistakes, and the occasional thick coat. You can always return unopened cans to most stores, but running out mid-project is a hassle and may cause visible lap marks if you buy a different batch.

What if I’m painting a dark color over white?

Dark colors often need more coats – sometimes 3 or 4 – to achieve full opacity. You can reduce this by using a tinted primer. In the calculator, increase the number of coats to 3 or 4, or add a primer coat to the total.

Does the calculator work for exterior paint?

Yes, the same formula applies. Exterior surfaces like wood siding or brick absorb more paint, so reduce the coverage rate to 250–300 sq ft per gallon. Also account for trim, eaves, and other details separately.

Related

• Benjamin Moore Zero VOC Paint Calculator
• B&Q Paint Calculator: How Much Paint Do You Need?
• Asphalt Mix Calculator: How Much Do You Need?

in area | Wall area | sq ft | 400 | 0
in coats | Number of coats | - | 2 | 1
in cov | Coverage per gallon | sq ft/gal | 350 | 1
out paint | Paint needed | gallons | (area * coats) / cov | 2
out cans | Whole gallons to buy | gallons | ceil((area * coats) / cov) | 0