How Much Paint Do You Need? Gallon Coverage, Coats and Primer

How much does a gallon of paint cover?

Whatever its label says, for one coat, on the surface the label describes. A label figure describes good conditions, and many say so outright with wording like "up to" and "on smooth surfaces." Your walls may not be that surface.

The calculator's default of 350 square feet per gallon is a planning figure, not a property of paint. Replace it with the rate on the can you are buying, or on the product's technical data sheet. If the label gives a range, use the low end for anything but a smooth, already painted wall.

Why real coverage falls short of the label

• Porosity. Bare drywall does not absorb evenly: the paper face and the joint compound over the seams take up paint differently. USG says its Sheetrock First Coat primer is made to even out how the joints and the board face take up paint. Unprimed, porous surfaces soak up the first coat, so each gallon covers less.
• Texture. Orange peel, knockdown and other textures have more surface than their flat outline, and your tape measure records only the outline. The true area is larger than the one you enter.
• Color change. Covering a dark color with a light one, or the other way round, can take more coats than you planned, and coats multiply the whole order. Follow the label's coat count for the change you are making.
• Application method. The calculator cannot know whether you will roll, brush or spray. If the data sheet gives a rate for your method, use it.

What the calculator actually computes

The paint calculator works out wall area from the room's perimeter, subtracts the doors and windows you enter as one area, optionally adds the ceiling, multiplies by the number of coats and divides by the coverage rate. It then applies the waste factor (10% by default) and rounds up to a whole gallon:

Coats multiply the whole area, so a second coat takes as much paint as the first. And because waste is applied before rounding, it can push an order over a whole-gallon line on its own, as the example below shows.

Worked example: how much paint for a 12 × 12 room

The walls-only case is closer than it looks. Set waste to 0 and the same inputs give 1.94 gallons, so the calculator says 2: 700 square feet of coverage against 680 needed, if the can's rate holds exactly. The default 10% waste is what makes it 3. If your store sells quarts, work from the exact figure instead: 2.14 gallons is covered by 2 gallons plus 1 quart (2.25 gallons). At a coverage of 300, for a textured or thirsty wall, the exact figure rises to 2.49 gallons: still 3 to buy, with less to spare.

Gallons of paint by room size

Every row uses the calculator's defaults in Imperial mode: 8 ft walls, 40 sq ft of doors and windows, 2 coats, 350 sq ft per gallon and 10% waste. Only length and width change. "With ceiling" ticks Include ceiling, same paint and coats. "Exact" is the waste-included figure; "Buy" is the rounded-up headline.

The 12 × 12 row differs slightly from the worked example because it deducts the default 40 sq ft of openings, not 44. With 9 ft walls, the same room comes to 2.46 gallons for the walls (buy 3) and 3.37 with the ceiling (buy 4), so measure the height rather than assuming 8 ft.

Ceilings and trim

Include ceiling adds length × width to the area at the walls' coverage rate and coat count, which is right only if both get the same paint. For a different ceiling paint, run the calculator separately: wall height 0, doors and windows 0, Include ceiling ticked, and that paint's coverage and coats. For the 12 × 12 room at 350, 2 coats and 10% waste, that gives 0.91 gallons, so 1 to buy.

Trim, doors and window frames are not part of the calculation, and nothing is added back for the frames of the openings you deduct. Trim is often painted in a different sheen or product. Measure its painted face, multiply length by width, and divide by that paint's own rate per coat.

When you need primer, and how much

Primer is a separate product with a separate coverage rate, and it is not one of your coats. New drywall is the clearest case for it. USG says its drywall primer is meant to even out texture differences and keep the joints from showing through the finish. It also lists that primer at approximately 180–200 sq ft per gallon, well below the 350 the calculator assumes for finish paint. If you are priming for another reason, such as stains, patches or a glossy old finish, check that the primer's label lists that use.

To estimate primer, run the calculator again with coats set to 1 and the primer's own rate. For the 12 × 12 example walls (44 sq ft of openings, 10% waste), 200 sq ft per gallon gives 1.87 gallons, so buy 2; at 180, the low end of USG's range, it gives 2.08, so buy 3. A paint-and-primer product still prints its own coverage and surface requirements, so check them before skipping primer on bare drywall.

Homes built before 1978: lead paint comes first

The federal government banned the sale of lead-based paint for use in homes in 1978, and the EPA warns that a renovation, repair or painting project in an older home with lead paint "can easily create dangerous lead dust." Scraping and sanding are the usual culprits. Under the EPA's Renovation, Repair and Painting Rule, anyone paid to disturb paint in pre-1978 housing, childcare facilities or preschools must be lead-safe certified. The rule generally does not cover homeowners working on their own home, but it does apply if you rent out all or part of it, run a childcare center in it, or renovate homes to sell. For older homes, the EPA recommends having surfaces tested by a certified lead inspector or risk assessor before work starts. If you do the work yourself, follow its lead-safe practices for DIYers.

Where paint estimates go wrong

• Using floor area instead of wall area. A 12 × 12 room has 144 sq ft of floor but 384 sq ft of wall at 8 ft high, before openings. The walls are what you paint.
• Leaving coverage at the default. 350 is a starting point. A lower rate on your can, or a textured or porous surface, means more paint than the default answer.
• Counting primer as a coat. Two finish coats over primer is two coats in the calculator, plus a separate primer estimate at the primer's own rate.
• Ticking the ceiling when it gets a different paint. The calculator then applies the wall paint's rate and coat count to the ceiling. Run it separately instead.
• Deducting every small opening. Cutting in around a window or outlet takes paint even though the opening takes none. Deduct the large openings and let the waste factor absorb the rest.
• Setting waste to zero to save a gallon. The example above shows how thin the margin gets without it.
• Reading a metric result as gallons. In Metric mode the calculator converts to liters and rounds up in liters. Every figure on this page uses Imperial mode.

Run your own room

The JoistCalc paint calculator does this arithmetic for any room. Enter your dimensions, the area of doors and windows, the number of coats and the coverage rate from your can, and it returns the paintable area, the exact amount with waste included and the whole gallons to buy.