Paint Calculator

What Is the Paint Calculator?

The Paint Calculator estimates how much paint you need for the walls of a room. It uses the room’s length, width and wall height to find the wall area, subtracts doors and windows, multiplies by the number of coats, and divides by the coverage rate of the paint.

The result is given in gallons and liters, with the number of gallon cans to buy. It also shows the ceiling requirement separately, since ceilings are usually painted in a different product.

Buying the right amount matters for two reasons. Running out mid-wall means a trip to the store and a risk of a slightly different tint. Buying too much wastes money on a product that does not store well once opened.

How the Calculator Works

A room’s walls form a band around its perimeter. The perimeter is twice the length plus twice the width. Multiplying the perimeter by the wall height gives the gross wall area.

Doors and windows are not painted with wall paint, so the calculator subtracts 20 square feet for each door and 15 square feet for each window. These are standard allowances for an interior door and an average window. The remaining figure is the paintable area.

That area is multiplied by the number of coats and divided by the coverage rate. The default is 350 square feet per gallon, a conservative figure for interior latex on a smooth, primed wall.

The Paint Formula

Wall area = 2 × (Length + Width) × Height

Paintable area = Wall area − (Doors × 20) − (Windows × 15)

Gallons = Paintable area × Coats ÷ Coverage per gallon

About coverage rates

The figure printed on the can is the manufacturer’s estimate under good conditions. Rough plaster, textured finishes and bare drywall absorb more paint and reduce coverage, sometimes to 250 square feet per gallon. Dark colours over light, or light over dark, may need an extra coat regardless of coverage.

Step-by-Step Explanation

1. Add the room length and width, then double the sum to get the perimeter.
2. Multiply the perimeter by the wall height for the gross wall area.
3. Count the doors and windows. Subtract 20 square feet per door and 15 per window.
4. Multiply the paintable area by the number of coats.
5. Divide by the coverage rate on the can.
6. Round up to whole gallons, or to gallons plus quarts.

Worked Example

A bedroom is 14 feet long and 12 feet wide with 8 foot walls. It has one door and two windows, and you plan two coats.

• Perimeter: 2 × (14 + 12) = 52 feet.
• Wall area: 52 × 8 = 416 square feet.
• Openings: 20 + (2 × 15) = 50 square feet.
• Paintable area: 416 − 50 = 366 square feet.
• Two coats: 366 × 2 = 732 square feet.
• Gallons: 732 ÷ 350 = 2.09.

You would buy three gallons, or two gallons and a quart if the store sells quarts in your colour. The ceiling is 14 × 12 = 168 square feet, which needs about half a gallon per coat.

How to Use the Calculator

1. Enter the room length and width. Units can be feet, inches, yards, meters or centimeters.
2. Enter the wall height from floor to ceiling.
3. Set the number of coats. Two is standard.
4. Enter how many doors and windows the room has.
5. Check the coverage rate against your paint can and adjust it if needed.
6. Press Calculate. The result shows gallons, cans to buy and liters.

If your room is not a simple rectangle, find the total wall area with the Square Footage Calculator by measuring each wall, then divide by coverage yourself. If you are hanging new board first, the Drywall Calculator uses the same room dimensions. For a feature wall in paper instead of paint, see the Wallpaper Calculator. If the floor is next, the Flooring Calculator or Carpet Calculator will size that order.

Practical Tips

• Buy all the paint for one room at once and ask for it to be mixed in a single batch. This avoids colour variation between cans.
• If you have several cans of the same colour, pour them into one large bucket and stir. Painters call this boxing.
• Prime new drywall, patched areas and stained surfaces. Primer is cheaper than paint and makes finish coats cover properly.
• Use a higher coverage figure only when the can says so and the wall is smooth and previously painted.
• Paint the ceiling first, then walls, then trim.
• Keep a small labelled jar of leftover paint for touch-ups. Note the brand, colour code and sheen.
• Cut in around edges with a brush before rolling, and keep a wet edge to avoid lap marks.

Common Mistakes

Forgetting the second coat. One coat rarely gives even colour. Estimating for a single coat is the most common reason people run short.

Using floor area instead of wall area. A 12 by 14 foot room has 168 square feet of floor and more than 400 square feet of wall.

Ignoring surface texture. Textured walls can need 20 percent more paint than smooth ones.

Not subtracting large openings. Patio doors, archways and built-in cabinets remove a lot of wall. If an opening is much bigger than a standard door, reduce the wall length instead of counting it as a door.

Skipping primer on new surfaces. Bare drywall absorbs the first coat unevenly, and you end up applying a third coat of expensive paint.

Assuming trim uses the same paint. Trim and doors are usually painted in a harder-wearing enamel. Estimate about one quart per average room.

Choosing a Sheen

Flat and matte finishes hide surface flaws and suit ceilings and low-traffic rooms. Eggshell and satin wipe clean and suit living rooms, halls and bedrooms. Semi-gloss resists moisture and is used in kitchens, bathrooms and on trim. Higher sheens show every imperfection, so wall preparation matters more as gloss increases.

Primer and Ceilings

Primer coverage is typically 200 to 300 square feet per gallon, lower than finish paint. One coat is normal. Ceiling paint is usually a flat white formulated to reduce spatter. Its coverage is similar to wall paint, and the calculator shows the amount for one coat.

Estimating Paint for Other Surfaces

Exterior walls

Measure the perimeter of the house and multiply by the wall height. Add gable ends as triangles. Exterior coverage is lower than interior, often 250 to 300 square feet per gallon on siding and less on rough masonry or stucco.

Doors and trim

One side of a standard door is about 20 square feet. A quart covers roughly 90 square feet, so one quart does two coats on both sides of a door. Baseboards and casings in an average room need about a quart.

Cabinets

Add up the face area of doors and drawer fronts and double it for both sides of the doors. A typical kitchen needs about a gallon of primer and a gallon of enamel for two coats.

Stairwells and high ceilings

Enter the actual wall height. A room with a 10 foot ceiling has 25 percent more wall than the same room at 8 feet, and a two-storey stairwell can double the wall area.

Preparing the Room

Preparation takes longer than painting and decides how the finish looks. Wash greasy or dusty walls, fill holes and cracks, sand patches smooth and spot-prime them. Remove switch plates, mask trim, and cover the floor. Paint applied to a clean, dull, dry surface lasts.

How Colour Changes Affect Quantity

Covering a deep colour with a pale one usually needs a tinted primer and two finish coats. Bright reds, yellows and oranges have weaker hiding power and can need three coats even over primer. If you are making a strong colour change, enter three coats to see the upper end of what you might need.

Storing Leftover Paint

Seal the lid firmly, store the can somewhere that does not freeze, and write the room name and date on it. Latex paint keeps for several years if it has not frozen or dried out. If there is only a small amount left, move it into a smaller airtight container so less air sits above it.