Paint Calculator

What this paint calculator does

Buying paint is a small decision with an annoying cost of being wrong. Too little and you are back at the store, hoping the new can matches the old one. Too much and you are storing half-full cans in the garage. Most of the confusion comes from two numbers people rarely check: how much surface there really is, and how far a gallon actually goes.

This paint calculator works out both. Enter a room size or an area you already know, choose walls, ceiling, or both, and set the number of coats and the coverage of your paint. It returns the gallons to buy, a way to split that into gallons and quarts, an optional primer allowance, and a cost if you enter a price. Four visuals show the result at a glance: a rotatable 3D model of your room in the paint color you pick, a bar graph from measured area to the area you actually paint, a chart of gallons by number of coats, and a chart of gallons by coverage rate, since coverage is the figure most likely to be off.

It is part of a set of building calculators. If you are also hanging or repairing drywall, start with the sheetrock calculator and the drywall installation cost calculator. For roofing, see the roof calculator.

How to calculate how much paint you need

If you want to calculate paint needed by hand, the method has four steps:

1. Find the area per coat. Walls: 2 × (length + width) × ceiling height. Ceiling: length × width. Subtract doors and windows.
2. Multiply by the number of coats. Two coats is standard, so double the area.
3. Divide by the coverage of one gallon. Around 350 to 400 square feet per coat is a common rule of thumb.
4. Add a little extra and round up. Paint is sold in whole quarts, gallons, and pails, and a touch-up reserve is handy.

As a formula: gallons = area × coats ÷ coverage × (1 + extra). A room with 380 square feet of wall, two coats, and a coverage of 350 needs 380 × 2 ÷ 350 = 2.17 gallons before extra. The calculator does the same sum and then shows the number you actually buy.

How much does a gallon of paint cover?

People ask this in many ways: how many square feet does a gallon of paint cover, how much will one gallon of paint cover, and how much does a gallon of paint cover per coat. The usual answer is about 350 to 400 square feet per coat on smooth, previously painted surfaces. Paint retailers give figures in that range, along with about 100 square feet per quart and 1,750 to 2,000 square feet for a five-gallon pail.

It is an estimate, not a guarantee. Rough, textured, or porous surfaces soak up more paint, so a gallon goes less far. Bare drywall, stucco, and rough siding often cover noticeably less, sometimes around 300 square feet per gallon or lower, and thin, low-quality paint can cover less than a premium product. Different brands also state different coverage on the can. For this reason the calculator gives you three starting points, 400 for smooth or primed surfaces, 350 as a typical value, and 300 for rough or porous surfaces, and lets you type your own. Whenever the can states its coverage, use that number.

How many coats and do you need primer?

Plan on two coats of finish paint for most walls. One coat can be enough when you repaint a similar color over a clean, sound surface with a good paint, but two coats gives an even color and sheen. A bold or dark color, or a big change from a dark wall to a light one, may need a third coat.

Primer is a separate layer. It helps on bare drywall, patched areas, stains, and big color changes, and it can reduce the number of finish coats you need. The calculator has a field for primer coats. It uses the same coverage you entered for the finish paint, so check the primer can for its own figure, which is often lower.

Measuring the area to paint

Measure each wall’s length and the ceiling height to the nearest inch, then convert to feet. For a simple rectangular room, the perimeter times the height gives the wall area. If the room is not a rectangle, split it into rectangles and add them up, or use the total area option.

Doors and windows reduce the area. This calculator deducts 21 square feet for a door and 15 for a window, which are typical sizes. Many painters ignore small openings, because the trim around them still takes paint, so you can enter zero if you prefer a more generous result. Ceilings are easy to forget: a 14 by 12 foot ceiling adds 168 square feet. Trim, doors, and baseboards are usually painted in a different sheen and need their own small quantity, so they are not included here.

Exterior paint calculator notes

Many people search for an exterior paint calculator when they plan to repaint a house. You can use this tool for that by choosing walls only. Enter the length and width of the house, the wall height, and the number of doors and windows. A few extras to keep in mind:

• Siding texture matters. Rough siding, stucco, and brick absorb more paint, so choose the rough or porous coverage setting.
• Gables and dormers add area that a simple perimeter times height calculation misses. Measure them separately and add their area using the total area option.
• Soffits, fascia, trim, and the garage door need their own paint and often a different product.
• Exterior paints state their own coverage, so check the can for the rate on your surface.

Gallons, quarts, and pails

Paint is sold by the quart, the gallon, and the five-gallon pail, and the price per gallon often drops as the container gets bigger. The calculator rounds your exact need up to the next whole gallon and also gives a version by the quart, so you can see whether buying a few quarts saves you from opening another gallon. For larger jobs it groups quantities into five-gallon pails.

When a job needs several containers, buy them all at once so the color stays consistent. If you do need more than one container of the same color, mixing them together in a larger bucket before you start, often called boxing, evens out slight differences between batches. Keep a small amount of leftover paint, labeled with the color and room, for touch-ups later.

Paint cost

If you enter a price per gallon, the calculator multiplies it by the whole gallons to buy to give the paint cost. That covers the paint only. A painting job also has primer, tape, rollers, brushes, drop cloths, and, if you hire someone, labor, which is often the largest part. Quotes for interior painting are usually given per square foot or per room and include labor, so they are not comparable with a paint-only figure. If you are planning drywall repairs first, the drywall installation cost calculator can help budget that stage.

Worked examples

Example 1: bedroom walls only

The room is 14 by 12 feet with 8 foot walls, one door, and one window. Walls: 2 × (14 + 12) × 8 = 416 square feet, less 36 for the door and window, leaves 380. Two coats is 760 square feet. At a coverage of 350, that is 2.17 gallons, or 2.28 gallons with 5 percent extra, so buy 3 gallons, or 2 gallons and 2 quarts. If your paint covers 400 instead, the same room needs 1.9 gallons, or 2.00 with the extra, so 2 gallons is enough. This is why checking the can matters.

Example 2: walls and ceiling

The same room with the ceiling included adds 168 square feet, for 548 per coat and 1,096 for two coats. At 350, that is 3.13 gallons, or 3.29 gallons with 5 percent extra. You buy 4 gallons, or 3 gallons and 2 quarts.

Example 3: a house exterior

A house 40 by 30 feet with 9 foot walls has 1,260 square feet of wall. Two doors and eight windows take away 2 × 21 + 8 × 15 = 162, leaving 1,098. Two coats is 2,196 square feet. On textured siding at a coverage of 300, that is 7.32 gallons, or 7.69 gallons with 5 percent extra, so buy 8 gallons. Enter these numbers in the calculator to confirm each step.

Common mistakes to avoid

• Forgetting the second coat. Dividing the area by coverage once gives enough for a single coat only.
• Using the wrong coverage. A rough surface can use a fifth to a third more paint than a smooth one.
• Skipping primer on bare or patched surfaces. The paint soaks in unevenly and you end up buying more.
• Forgetting the ceiling. It is easy to measure the walls and stop.
• Buying only the exact amount. A touch-up reserve is cheap insurance, and color matching a second batch later is hard.
• Mixing units. Entering metres while the unit is set to feet changes the answer by a factor of about ten. Select the unit first.

How this calculator works and what it assumes

The calculator runs in plain JavaScript in your browser, so your measurements are not sent anywhere. It finds the paintable area, multiplies by the coats and primer coats, divides by your coverage, adds your extra allowance, and rounds up to whole gallons. Metric inputs are converted at 3.28084 feet per metre, and coverage is always in square feet per gallon.

What it assumes, and what it leaves out:

• Coverage values are common rules of thumb, not data for your product. Use the figure on your can when you have it.
• The room is a simple rectangle with a flat ceiling and the same height on every wall. Doors are 21 square feet and windows are 15.
• Primer uses the same coverage as the finish paint.
• Trim, doors, baseboards, and gables are not included unless you add their area through the total area option.
• The 3D preview is a simple box model drawn from your length, width, and height, with doors and windows shown on one wall. It shows what is being painted and is not to scale for furniture, roof shapes, or angled walls.
• The cost is paint only, based on whole gallons at the price you enter.

I am a software engineer, not a professional painter, so treat the result as a planning figure and check the label and your supplier before you buy. Change log: October 8, 2026, first published with coats, primer, coverage presets, gallons and quarts, cost, three graphs, and a rotatable 3D preview with a color picker.

Frequently asked questions

Related calculators: browse all drywall calculators and roofing calculators, or plan the wall surface with the sheetrock calculator, budget the drywall with the drywall installation cost calculator, or size the roof with the roof calculator, roof pitch calculator, rafter calculator, and roof replacement cost calculator.

Sources and further reading

The area math is basic geometry. For typical coverage figures and the two-coat rule of thumb, see Boyd’s guide to how much paint you need and House & Home’s guide, which also points readers to the paint brands’ own coverage pages. For leftover paint, PaintCare is a nonprofit that runs paint take-back programs in several US states. Coverage varies by product and surface, so always follow the label on your can.