Interior Paint: How Much to Buy, Prep, Primer and How to Paint a Room

How much paint you need for a room

Start with the coverage figure on the can you are buying, because the label is the authority for that product. The manufacturer sources we read agree on the typical range: a gallon of interior paint covers about 350 to 400 square feet, a quart about 100 square feet, and a gallon of primer about 200 to 300 square feet. A manufacturer training deck adds that its worksheet figures are for one coat on smooth, flat, nonporous surfaces. Textured, porous or previously unpainted walls will take more, so treat 350 as the planning number and 400 as the best case.

To measure walls, add the lengths of all four walls to get the room perimeter, then multiply by wall height. The training example uses 8 feet, but measure your own ceiling. Then subtract openings. The same training deck subtracts 21 square feet per door and 15 square feet per window. Those are example allowances, not universal sizes, so measure unusually large windows or glass doors. Ceiling area is simply length times width.

Here is the arithmetic for a 12 by 14 foot room with 8 foot ceilings, one door and two windows:

• Perimeter: 12 + 14 + 12 + 14 = 52 feet. Wall area: 52 x 8 = 416 square feet.
• Openings: 1 door x 21 = 21, plus 2 windows x 15 = 30, total 51. Paintable wall area: 416 - 51 = 365 square feet.
• Two coats: 365 x 2 = 730 square feet. At 350 square feet per gallon that is 2.09 gallons; at 400 it is 1.83 gallons. Buy 2 gallons plus a quart, or 3 gallons if you want leftovers for touch-ups.
• Ceiling: 12 x 14 = 168 square feet. Two coats: 336 square feet, or 0.84 to 0.96 gallon. Buy 1 gallon of ceiling paint.
• Primer on bare new drywall, one coat over the 365 square feet of wall: 365 / 300 = 1.2 gallons to 365 / 200 = 1.8 gallons.

Choosing the right product: primer and sheen

Primer is not always required, but the manufacturer data sheet specifies it for bare surfaces: a coat of primer on drywall, plaster, masonry and wood before two coats of finish paint. It also says that when a repaint involves a drastic color change, a coat of primer improves hiding. Another manufacturer page recommends a tinted primer rather than white when you are going to a darker color. Primer also seals porous surfaces, blocks stains and improves adhesion, which finish paint usually does not do on its own. Use the primer the manufacturer names for your surface, since different surfaces call for different primers.

Sheen is a trade-off between hiding flaws and standing up to scrubbing. The manufacturer sheen guide recommends flat only for ceilings, and says flat also suits uneven or textured walls but is prone to scuffs and stains. Eggshell and satin are described as easily washable and suit hallways, family rooms and children's rooms. For trim, doors and cabinets it lists eggshell or satin, semi-gloss and gloss, and calls semi-gloss easy to clean. For bathrooms it says satin or semi-gloss is ideal because it can withstand moisture. The page is internally inconsistent about kitchens, so check with the paint counter for your product.

What to buy

Buy by category and match each item to your surface. The data sheet we read specifies a nylon-polyester brush and a synthetic roller cover for latex paint, and cleanup is with soap and warm water. Another manufacturer page calls for a 3/8 to 1/2 inch nap on smooth walls and 1/2 to 1 inch on rough or stippled ones.

• Interior latex or acrylic paint in the right sheen for walls, a flat ceiling paint, and a trim paint in a higher sheen.
• Primer matched to the surface: drywall, plaster, wood or masonry, and a stain-blocking primer if you have water or smoke stains.
• A 2 to 2.5 inch angled synthetic brush for cutting in and trim, and a roller frame with synthetic covers of the right nap.
• An extension pole, a roller tray with liners, and painter's tape long enough to apply in long strips.
• Drop cloths, lightweight spackle, a putty knife, 120 grit sandpaper, a tack cloth, and paintable acrylic caulk.
• Mild soap, a sponge, and a mildew remover if you see mildew.
• A stepladder with a load rating above your weight plus tools. In the OSHA table a household Type III ladder is rated 200 pounds, so check the label.
• For pre-1978 homes: a HEPA vacuum, heavy plastic sheeting, and a NIOSH-approved respirator.

Preparing the surface

The manufacturer's training material boils preparation down to four requirements: the surface must be clean, dull, dry and sound. Wash walls and trim with lukewarm water and mild soap, rinse with a slightly damp sponge, and let everything dry fully before taping. Scrape and sand peeling paint back to a sound edge, and sand glossy surfaces dull. For glossy interior doors and trim, the manufacturer says an adhesion primer can replace sanding, though you still must wash first.

Fill holes and cracks with spackle, let it dry, sand it smooth and remove all dust. Joint compound must be cured and sanded smooth. Mildew is a fungus that will grow through new paint, so scrub it off with a commercial mildew remover before painting. For water stains, fix the leak first, then seal the stain with a stain-blocking primer. Remove switch plates, outlet covers and register grills, and tape or cover moldings, floors and windows. If the home was built before 1978, read the lead section below before you scrape or sand anything.

Step by step: painting a room

• 1. Check the age of the house. If it was built before 1978, follow the lead-safe section below before disturbing paint.
• 2. Clear the room, remove hardware and cover floors. Open windows for fresh air during painting and drying.
• 3. Wash walls and trim with mild soap and water, rinse, and let dry.
• 4. Patch holes, sand the patches and any glossy areas, and wipe off the dust.
• 5. Prime bare drywall, patches, stains and wood. Apply primer with the angled brush at trim and corners, then roll the rest.
• 6. Caulk gaps between trim and wall with paintable acrylic caulk after priming, and check the caulk label for its dry time.
• 7. Tape trim with long strips, pressing the edge down with a putty knife. Apply paint only at temperatures above 50 degrees F, the minimum on the data sheet we read.
• 8. Stir the paint thoroughly, from the bottom upward, just before you start.
• 9. Paint the ceiling first, using an extension pole and overlapping each pass. Cut in a band along the edge with the brush.
• 10. Cut in walls 2 to 3 inches wide along ceiling, corners and trim, only as far as you can roll before the paint dries. Then roll in an N or W pattern, working from dry areas back into the wet edge, in sections of about 2 by 3 feet.
• 11. Wait for the recoat time on the label, then apply the second coat the same way.
• 12. Paint trim and doors with a small brush. Remove tape once the paint is dry to the touch, pulling it back over itself, and refit hardware only when the paint is dry.

Drying and recoat times

The data sheet we read gives 1 hour to touch and 4 hours to recoat at 77 degrees F and 50 percent relative humidity, and states that times depend on temperature, humidity and film thickness. A separate manufacturer guide says to wait typically 2 to 4 hours between coats and to check the label. Treat the label as the rule and add time in cool or humid rooms. We did not find a sourced full cure time for interior latex, so we do not give one; ask the paint counter.

Bare trim primed with a trim primer needs 24 hours to dry before sanding and painting, according to the manufacturer's trim guide. Caulk has its own dry time on the tube. On tape, the sources differ: one says remove it once the paint has dried, typically within 24 hours, and another says remove it from the walls before the paint dries.

Common mistakes

• Skipping the wash. Grime and grease prevent adhesion and show through the finish.
• Painting over glossy surfaces without sanding or an adhesion primer.
• Painting over mildew or an active leak stain instead of fixing the source first.
• Cutting in too far ahead of the roller, which leaves a visible stripe where the brushed edge dried.
• Stopping in the middle of a wall, which causes lap marks when you restart on dry paint.
• Applying only one coat over a dramatic color change without primer.
• Using the top step of a stepladder. OSHA lists using the top step or cap as a step among the things not to do.
• Dry scraping or power sanding old paint in a pre-1978 home without lead precautions.

When to hire a professional

DIY painting is a reasonable project for most rooms, but some conditions change the risk. The biggest is lead. EPA recommends that homeowners planning renovation, repair or painting in a pre-1978 home hire a lead-safe certified contractor, and the federal rule applies to you if you rent the home, run a child care facility in it, or renovate to sell. Hire out stairwells and tall entries where a stepladder cannot safely reach. OSHA says to keep three points of contact, keep the spreaders locked and not use the top step.

Also call a professional for stains that return after you fix the apparent cause, widespread mildew, water-damaged ceilings where the moisture source is unknown, and large areas of failing plaster or drywall.

Cost and ordering tips

We do not quote prices because they vary by store and product line, but a few habits save money and trouble. Do the measurements before you go. Ask the paint counter what coverage the label states for your exact product, and whether your color needs a tinted primer. Order all the paint for one room in a single batch, because the data sheet notes that touching up from the same batch and by the same application method improves the match.

A quart covers about 100 square feet, which is enough for doors and trim. Buying gallons for walls and a quart for trim avoids a second gallon you barely use. Order a little extra rather than running short mid-wall, because a later can may not match.

Painting ceilings, trim and doors

Ceilings take flat paint according to the sheen guide, and a thicker roller covers textured ceilings better. The training deck counts ceiling area as length times width. Another manufacturer's calculator states that its room estimates do not include the ceiling, so compute it separately.

For trim, sand new or smooth wood with 120 grit until the shine is gone, prime bare wood and spackle spots, and let the primer dry 24 hours. Dip no more than half to one inch of an angled brush into the paint, brush a few feet in short horizontal strokes, then smooth with one long stroke. Doors count as 21 square feet each in the training example. The sources we read give no separate coverage rate for trim, so use the label figure and buy a quart.

The sources disagree on order. One manufacturer page says trim last to avoid drips onto the walls; the training deck heading says trim first yet lists ceilings, then woodwork, then walls. Either order works if you let each surface dry and tape carefully.

Painting in a home built before 1978

The EPA Renovate Right guide says lead-based paint was used in more than 38 million homes until it was banned for residential use in 1978, and that you may simply choose to assume lead paint is present in an older home. The federal rule does not impose requirements on homeowners working in their own homes, but EPA still recommends lead-safe practices.

The data sheet we read carries a warning: if you scrape, sand or remove old paint you may release lead dust, so wear a NIOSH-approved respirator and clean up with a HEPA vacuum and a wet mop. EPA's DIY guidance says to cover floors and immovable furniture with heavy-duty plastic and tape, seal doors and vents, keep children, pregnant women and pets out of the work area, use wet scraping or sanding, or sanders with HEPA attachments, then HEPA vacuum and wet mop before taking down the plastic. EPA says certain methods are prohibited for contractors because of the dust they create: open flame burning, power sanding without a shroud and HEPA vacuum attachment, and heat guns above 1100 degrees F. Do not do these yourself either. EPA-recognized test kits are sold at hardware stores. Call 1-800-424-LEAD (5323) for help.

Care and maintenance

Higher sheens wash more easily; the sheen guide describes flat as harder to maintain and eggshell and satin as easily washable. Wipe marks early, with mild soap and a soft cloth. Keep leftover paint for touch-ups in its original container, tightly closed and protected from freezing; the data sheet says not to transfer the contents to other containers for storage and lists a shelf life of 36 months unopened. Label the can with room, color and date. Clean brushes and rollers with soap and warm water right after use for latex paint.

How much paint for common room sizes

Each row assumes 8 ft walls, one door, two windows, 2 coats and 350 sq ft per gallon. Change any number in the paint calculator.

Frequently asked questions

Sources: Sherwin-Williams ProMar 200 Interior Latex Eg-Shel product data sheet (03/2025), Sherwin-Williams paint calculator and coverage FAQ, Sherwin-Williams Painter Skills Program, Part 1 (training presentation), Sherwin-Williams: The Best Order to Paint a Room, Sherwin-Williams: How to Choose Paint Finishes, EPA: Information for Do-It-Yourselfers and the RRP Rule, EPA: The Lead-Safe Certified Guide to Renovate Right, OSHA: Reducing Falls in Construction, Safe Use of Stepladders (3662). Read 7 November 2026.