Paint Calculator

Estimate paint for a room, walls, and ceiling

This paint calculator estimates how much paint and primer you may need for one or more rooms. Enter room length, width, and wall height, then choose whether to paint the ceiling. Add doors and windows to subtract their area from the walls. Finally, enter the product's coverage rate, number of coats, and a waste allowance. The result shows the surface area, estimated gallons or litres, and how many containers to buy.

The estimate is based on measured area rather than a general rule such as one gallon per room. Room size, openings, ceiling height, paint coverage, number of coats, and surface condition all matter. A small bedroom with low ceilings can need less paint than a large room with tall walls or a textured finish. If several rooms have the same dimensions and openings, use the matching-room count to estimate them together.

Use the coverage rate printed on the exact paint or primer label or product data sheet. Coverage varies by product and by the surface being painted. Sherwin-Williams notes that a theoretical coverage figure does not include effects such as surface texture and paint left on application tools. Treat this result as a buying estimate, keep the product directions handy, and adjust the inputs when you know more about the project.

How to use the paint calculator

Select feet or metres, then measure the room's inside length, width, and average wall height. Enter the count of matching rooms if the same layout repeats. The calculator estimates four walls using room perimeter multiplied by wall height. If one or more walls will not be painted, calculate only the surfaces that will receive paint or use separate wall-area calculations for an unusual layout.

Enter the number and size of doors and windows in each matching room. The calculator subtracts those openings from gross wall area. If the room has no openings, use zero for the count. Turn on the ceiling option if you are painting the ceiling. The ceiling area is room length multiplied by room width. The calculator does not include trim, baseboards, closets, built-in cabinets, doors themselves, or separate accent walls unless you add their area to the inputs.

Enter paint coverage from the product label, number of paint coats, and waste allowance. If you also need primer, enter primer coats and primer coverage separately. Set package size to the can, pail, or tub size sold by your supplier. The calculator rounds container counts up to whole packages. You can add a per-container price to get a simple material subtotal; the estimate does not include tax, labor, tools, or other supplies.

Paint calculator formula

The calculator first works out gross wall area. For a rectangular room, wall area equals two times the sum of room length and width, multiplied by wall height. It then subtracts the area of doors and windows. If the ceiling option is selected, ceiling area is added. For matching rooms, the wall and ceiling quantities are multiplied by the room count.

Paint volume is calculated by multiplying the area to coat by the number of coats and the waste factor, then dividing by product coverage. Primer is calculated separately with its own coverage and coat count. The container count is the paint volume divided by package size, rounded up to the next whole container. This method makes it easy to compare a one-coat plan with a two-coat plan or to use the spread rate listed for a particular paint.

For example, if the paintable area is 500 square feet, the coverage is 350 square feet per gallon, and you plan two coats with 10 percent extra, the estimate is 500 × 2 × 1.10 ÷ 350 = about 3.14 gallons. If paint is sold in one-gallon cans, you would round up to four cans. The actual amount can be higher or lower depending on the surface and application.

• Room wall area = 2 × (length + width) × wall height
• Opening area = (door width × door height × door count) + (window width × window height × window count)
• Paintable area = (wall area − opening area + optional ceiling area) × matching room count
• Paint volume = paintable area × coats × (1 + waste percent ÷ 100) ÷ product coverage
• Primer volume = paintable area × primer coats × (1 + waste percent ÷ 100) ÷ primer coverage
• Containers to buy = round paint or primer volume ÷ package size up to a whole container

Worked example for a 12 by 15 foot room

Imagine a room that is 12 feet long, 15 feet wide, and 8 feet high. Its four wall surfaces total 2 × (12 + 15) × 8 = 432 square feet. Suppose it has one 3 by 7 foot door and two 3 by 4 foot windows. The door covers 21 square feet and the windows cover 24 square feet, so the net wall area is 432 − 21 − 24 = 387 square feet.

If the ceiling is included, add 12 × 15 = 180 square feet. The total area to coat becomes 567 square feet. For two coats, a 10 percent waste allowance, and paint rated at 350 square feet per gallon, the estimated paint quantity is 567 × 2 × 1.10 ÷ 350 = about 3.56 gallons. With one-gallon cans, the practical purchase count is four cans.

If the project also needs one primer coat rated at 300 square feet per gallon, the primer estimate is 567 × 1 × 1.10 ÷ 300 = about 2.08 gallons, or three one-gallon containers. This is only an example. Product coverage, wall condition, color change, repairs, and application technique can change the actual amount. Follow the paint and primer labels.

How many coats of paint should you plan for?

The right coat count depends on the product, the existing finish, the new color, the surface, and the result you want. Some projects need one coat for a similar color refresh, while many jobs use two finish coats for even color and appearance. A major color change, a porous surface, repairs, or a first coat over new material may need primer or additional attention. Follow the coating manufacturer's instructions rather than relying on a fixed rule.

Primer and paint are not interchangeable quantities. Primer can seal a surface, improve adhesion, or help create a more even base, depending on the product and project. It often has a different spread rate from the finish paint. This calculator lets you enter primer coats and coverage separately so you can estimate both products. If primer is not needed, leave the primer coat count at zero.

A second coat should be applied only after the previous coat has dried for the time specified on the label. Do not use the calculator's coat count as a shortcut around drying, temperature, humidity, or recoat instructions. The amount of paint consumed by brush, roller, sprayer, and tray can differ, and spraying may require a different amount due to overspray and application method.

Paint coverage, surface texture, and product choice

Coverage is the area a stated quantity of paint is expected to coat under specified conditions. A product label or technical data sheet is the best source for the chosen paint. Smooth sealed drywall may use paint differently from rough plaster, masonry, textured walls, or bare wood. Porous surfaces can absorb more product, while an existing finish in poor condition may require preparation or primer before painting.

Use a realistic coverage figure for the surface and the way you will apply the paint. The number printed on a can is a guide, not a promise that every wall will use exactly that amount. Surface profile, porosity, patching, color contrast, applicator, overspray, and technique affect practical coverage. For a textured or unsealed surface, use product-specific advice or allow extra material rather than assuming maximum spread.

Different paint products have different intended uses. Interior wall paint, ceiling paint, trim enamel, masonry coating, exterior paint, and specialty coatings can have different coverage and application requirements. Do not transfer the rate from one product to another. Check the label for suitable surfaces, preparation, coats, temperature range, ventilation, cleanup, and drying time.

Doors, windows, ceilings, and other surfaces

Subtracting windows and doors helps avoid buying paint for areas that will not receive wall paint. Measure the width and height of each opening and enter the number of matching openings. If the doors, window frames, or trim will also be painted, estimate those surfaces separately because they may use a different product and coverage rate. Large openings make a bigger difference than small openings, but cut lines and touch-ups can still consume paint.

Ceiling paint is calculated as a flat rectangle using the room's floor dimensions. A sloped ceiling, vaulted ceiling, tray ceiling, soffit, or exposed beam layout has a different surface area. Measure the actual sloped panels or add them as separate areas. Similarly, an open-plan space may not have four full walls, while a hallway or alcove may include extra wall sections. Use the room formula as a starting point and adjust for the real layout.

Closets, pantries, stairwells, accent walls, and built-in alcoves can be estimated separately. Add their paintable area to the total or calculate those items as their own job. Separate estimates are often easier when surfaces use different colors, products, or coat counts. Remember that the calculator estimates flat area; it does not include decorative details, textured patterns, or surface damage.

Waste allowance and container sizes

A waste allowance gives a small buffer for paint left in the tray or roller, spills, cut-ins, small repairs, and touch-ups. The right allowance depends on room complexity and how accurately the surfaces are measured. Many corners, windows, textured surfaces, or color changes can increase uncertainty. Enter a modest allowance when you want a buffer, but avoid adding a large arbitrary amount that makes the estimate less useful.

Paint is sold in different container sizes. A one-gallon can, a five-gallon pail, and a litre container each cover a different area at the same spread rate. Set the package size to match what you plan to buy. The calculator rounds up because you usually cannot purchase a fraction of a sealed container. The exact sizes and labels available vary by market, brand, and product.

If you buy extra paint, store it securely according to the label and keep the product name, color, sheen, batch details, and purchase information. A small amount left over can help with touch-ups, but stored paint has a shelf life and may not match perfectly if it is not mixed or stored correctly. Ask the retailer about return policies before buying unopened containers.

Estimate paint cost and plan a shopping list

If you enter a price per container, the calculator multiplies it by the rounded paint container count to show an approximate paint subtotal. It does not include primer cost, brushes, rollers, trays, masking materials, ladders, repair compound, primer tools, delivery, labor, or taxes. Add those items separately when making a project budget. Prices vary by product, finish, brand, region, and date.

The quantity estimate can help compare coverage rates and package sizes. A higher-coverage product may require fewer containers, but purchase decisions should also consider the finish, durability, washability, warranty, indoor or outdoor use, and surface compatibility. Read product specifications and ask a paint retailer or qualified contractor if the surface has special needs.

For a useful shopping list, record each surface, product, color, sheen, number of coats, coverage rate, and package size. Keep paint and primer estimates separate. Include any trim, doors, ceilings, or accent walls not covered by the room inputs. Recheck measurements and opening counts before ordering, especially for several rooms or a whole-house project.

Common paint estimating mistakes

A frequent error is calculating only the floor area and treating it as wall area. Walls include the room perimeter multiplied by wall height, so a square room's wall area can be much larger than its floor area. Another mistake is forgetting ceilings or counting an opening twice. Enter gross walls, subtract the openings in the calculator, and add ceiling only when it will be painted.

People may also use a generic online coverage number instead of checking the product label. Coverage varies between paint lines and surfaces. If you choose a different product after estimating, update the coverage. Make sure metric coverage is entered as square metres per litre and imperial coverage as square feet per gallon; mixing those units can make the result dramatically wrong.

Finally, an area estimate cannot tell whether the surface is ready to paint. Peeling paint, moisture, stains, glossy finishes, damaged drywall, or bare material may need special preparation or primer. Follow label directions for preparation and safety. If old paint may contain lead, do not sand or scrape it casually; check local guidance and use qualified lead-safe professionals where required.

Metric and imperial paint calculations

In imperial mode, dimensions use feet, area uses square feet, coverage uses square feet per gallon, and package size uses gallons. In metric mode, dimensions use metres, area uses square metres, coverage uses square metres per litre, and package size uses litres. The calculator converts room and opening dimensions, coverage, and package size when you change units so the same physical project remains represented.

A gallon is about 3.785 litres, and one square foot is about 0.0929 square metres. These conversions let the calculator translate an entered coverage rate between systems. Product labels may use rounded values, so a converted number can show extra decimal places without adding measurement accuracy. Use the coverage figure stated on the package in the matching unit system whenever possible.

If your dimensions are in centimetres, convert them to metres before entering them, or convert inches into decimal feet for imperial mode. Keep all dimensions consistent. For example, do not enter a wall height in centimetres with room length in metres. Recheck the displayed units and package size after switching modes.

Limitations and practical guidance

This calculator assumes rectangular rooms, uniform wall height, rectangular doors and windows, and one selected coverage rate per product. It does not model sloped walls, complex ceiling shapes, trim, built-in shelving, surface porosity, color hiding, spray loss, or labor. It uses your inputs rather than inspecting the room or product. Measurements and product data determine the quality of the estimate.

Use the result to plan a purchase and compare scenarios, then check the product label and data sheet. Paint use can change with surface preparation, substrate, applicator, environment, and the required finish. Follow safety, ventilation, and disposal instructions. If the project involves a damaged surface, exterior coating, moisture problem, historic finish, or uncertain lead paint, get appropriate local advice before starting.

For exact product guidance, consult the paint manufacturer or retailer. The calculator is an educational estimate and not a guarantee of coverage, color match, or project cost. Measure carefully, prepare the surface correctly, and buy products that are approved for the surface and conditions.