BuildCalc Pro

Paint Calculator

Estimate how many gallons or liters of paint you need for any room based on dimensions, doors, windows, and number of coats.

Written & reviewed by Michael FairUpdated June 2026Based on U.S. codes & pricingHow we calculate

How to Accurately Estimate Paint for Any Room

Buying the right amount of paint saves money and reduces waste. Most homeowners either buy too much and end up storing unused cans, or buy too little and face mismatched batches. Our paint calculator helps you get it right the first time.

A standard gallon of interior paint covers approximately 350-400 square feet per coat on smooth, primed surfaces. Rough or textured walls may reduce coverage to 250-300 square feet per gallon. Always account for the number of coats — most paint jobs require two coats for proper coverage.

To calculate wall area, measure the perimeter of the room and multiply by ceiling height. Subtract standard door openings (approximately 21 square feet each) and window openings (approximately 15 square feet each). This gives you the paintable surface area.

Paint quality matters significantly. Premium paints ($40-$70/gallon) have higher pigment concentration, which means better coverage in fewer coats and more durable results compared to economy brands ($20-$30/gallon). Over the life of the paint job, premium paint often costs less because it lasts longer.

For ceilings, use flat paint which hides imperfections. For walls, eggshell or satin finishes work well in most rooms. Kitchens and bathrooms benefit from semi-gloss or gloss finishes that resist moisture and are easier to clean.

Primer is the step DIYers most often skip, and it costs them more paint in the end. Bare drywall, patched areas, and drastic color changes all absorb finish coat unevenly without primer. A single coat of primer, or a paint-and-primer product on the first coat, typically saves a full finish coat on new or repaired surfaces — cheaper per gallon than premium paint and far more effective at hiding the surface underneath.

Don't forget the ceiling and trim in your estimate — they use different products and finishes than the walls. A 12x12 room needs roughly 1.5 gallons for the ceiling (one to two coats of flat ceiling paint) on top of the wall paint, and trim, doors, and baseboards usually take about a quart per average room in a semi-gloss or gloss finish. Estimating walls alone is the most common reason people run short.

Sheen affects both durability and how much surface prep shows. Higher-gloss finishes are more washable but ruthlessly highlight dents, roller marks, and patched areas, so they demand better prep. Flat and matte finishes hide imperfections but scuff easily. For high-traffic family homes, satin on walls and semi-gloss on trim is the most forgiving balance of looks, cleanability, and prep effort.

Coverage numbers printed on the can are best-case figures for a smooth, primed, mid-tone surface. Real-world coverage drops with texture (knockdown or orange-peel walls can lose 20–30%), with very light or very dark colors that need extra coats, and with spray application, which wastes 20–40% to overspray compared to rolling. Build a modest buffer into your estimate rather than trusting the label exactly.

How to Use This Calculator

1. Select Imperial (feet) or Metric (meters). 2. Enter your room length, width, and ceiling height. 3. Enter the number of standard doors and windows (these are subtracted from the paintable area). 4. Choose how many coats of paint you plan to apply. 5. Optionally enter cost per gallon for a price estimate. 6. Click "Calculate" to see your results. Note: One gallon of paint typically covers about 350 sq ft. Standard door size used: 21 sq ft, window: 15 sq ft.

Understanding Your Result

Your result shows the gallons of paint needed per coat and the total gallons for your chosen number of coats, based on the paintable wall area after subtracting doors and windows. The estimate uses standard coverage of 350 sq ft per gallon on smooth, primed surfaces — textured walls reduce this to about 250–300 sq ft. The figure covers walls only; ceiling paint and trim paint are different products estimated separately.

Key Formulas

Wall Paint Area

(Room Perimeter × Ceiling Height) − Door/Window Area = Paintable Sq Ft

Measure the total perimeter of the room, multiply by ceiling height, then subtract door (~21 sq ft each) and window (~15 sq ft each) openings.

Gallons Per Coat

Paintable Sq Ft ÷ 350 = Gallons Per Coat

Standard interior paint covers approximately 350 sq ft per gallon on smooth, primed surfaces. Use 300 for textured walls.

Total Paint Needed

Gallons Per Coat × Number of Coats = Total Gallons

Most paint jobs require 2 coats for proper coverage. Dark color changes may need 3-4 coats.

Worked Example

A 12 × 14 ft room with 8-foot ceilings, one standard door, two windows, two coats of mid-range latex paint.

  1. 1Room perimeter: 2 × (12 + 14) = 52 feet.
  2. 2Gross wall area: 52 × 8 = 416 sq ft.
  3. 3Subtract openings: 1 door (21 sq ft) + 2 windows (30 sq ft) = 51 sq ft. Paintable area = 365 sq ft.
  4. 4Per coat: 365 ÷ 350 = 1.04 gallons. Two coats: 1.04 × 2 = 2.09 gallons.

Buy 3 gallons (round up — partial gallons are not sold). At $35–$50 per gallon for mid-range latex, that is about $105–$150 in wall paint. You will also need roughly 1.5 gallons of flat ceiling paint and a quart of semi-gloss for trim and doors, adding $40–$70. Total paint budget for this room: approximately $145–$220.

Assumptions Behind This Estimate

  • Standard coverage of 350 sq ft per gallon per coat on smooth, primed walls. Textured surfaces (orange-peel, knockdown) reduce coverage to 250–300 sq ft per gallon.
  • Door openings are approximately 21 sq ft each (3 ft × 7 ft). Window openings are approximately 15 sq ft each (3 ft × 5 ft). Adjust these if your openings differ significantly.
  • Two coats are assumed for most repaints with a similar color. Dramatic color changes (dark to light or light to dark) may need 3–4 coats or a tinted primer coat.
  • The estimate does not include ceiling paint, trim paint, or primer — these are different products with different sheens and coverage rates.
  • Spray application wastes 20–40% more paint than rolling due to overspray. If spraying, increase the total by at least 25%.

Paint Coverage by Type

Expected coverage per gallon for different paint types and surfaces
Paint TypeSmooth SurfaceTextured SurfaceCoats NeededAvg Cost/Gal
Economy Latex350–400 sq ft250–300 sq ft2–3$20–$30
Mid-Range Latex350–400 sq ft300–350 sq ft2$30–$45
Premium Latex400–450 sq ft350–400 sq ft1–2$45–$70
Primer/Sealer300–350 sq ft200–250 sq ft1$25–$40

Paint Finish Guide

Recommended paint finishes by room and surface
FinishSheen LevelBest RoomsDurabilityWashable
Flat / MatteNo sheenCeilings, low-traffic roomsLowNo
EggshellSlight sheenLiving rooms, bedroomsMediumSomewhat
SatinSoft sheenFamily rooms, hallwaysMedium-HighYes
Semi-GlossNoticeable sheenKitchens, bathrooms, trimHighYes
GlossHigh sheenDoors, cabinets, trimVery HighYes

Before You Buy: Practical Advice

  • Buy all your paint at once from the same batch — subtle color variation between batches (metamerism) is common and visible on adjacent walls, especially with deep or saturated colors.
  • Always use primer on bare drywall, patches, and drastic color changes — it is cheaper than finish paint and prevents blotchy, uneven coverage that forces an extra topcoat.
  • Invest in quality rollers and brushes: a good 3/8-inch nap roller on smooth walls applies more evenly and holds more paint per load, saving time and reducing roller marks.
  • Test your chosen color with a sample pot on the actual wall first — paint looks dramatically different under your room's lighting than on a store swatch, and flat versus satin sheen affects perceived color.
  • Keep leftover paint for touch-ups — label the can with the room, date, and sheen. Stored properly (sealed tightly, room temperature, away from freezing), latex paint lasts 2–5 years.

Frequently Asked Questions

How many gallons of paint do I need for a 12x12 room?

A 12x12 room with 8-foot ceilings has about 384 square feet of wall space. After subtracting a door and two windows, you need roughly 330 square feet of coverage. With two coats, that requires approximately 2 gallons of paint.

How much paint do I need for two coats?

Simply double the single-coat coverage requirement. If one coat needs 1.5 gallons, two coats need 3 gallons. Always round up since partial gallons are not typically sold.

Does dark paint require more coats?

Yes, dark colors and drastic color changes often require 3-4 coats for full coverage. Using a tinted primer that matches the topcoat color can reduce the number of finish coats needed.

How long does a gallon of paint last?

An unopened can of latex paint lasts up to 10 years when stored properly. Once opened, it lasts 2-5 years if sealed tightly and stored at room temperature away from freezing conditions.

How much paint do I need to paint a whole house interior?

A typical 2,000 sq ft home has roughly 5,000–6,500 sq ft of paintable wall and ceiling surface. Two coats generally require 12–18 gallons for walls, 4–6 gallons for ceilings, and 2–4 gallons for trim and doors — about 18–28 gallons total. Get an exact figure by calculating room by room, since layouts and ceiling heights vary widely.

Is paint-and-primer-in-one worth it?

On previously painted walls in good condition and a similar color, paint-and-primer products work well and save a step. But over bare drywall, stains, patches, or a dramatic color change, a dedicated primer still performs better and is cheaper. Treat paint-and-primer as a convenience for repaints, not a replacement for real primer on raw or problem surfaces.

How much does it cost to paint a room?

DIY, a standard 12x12 room costs about $80–$200 in paint and supplies. Hiring a professional runs $300–$800 per room depending on ceiling height, prep needed, and trim work — roughly $2–$6 per square foot of floor area. High ceilings, heavy repairs, and dark-to-light color changes push costs toward the upper end.

How many square feet does a 5-gallon bucket of paint cover?

A 5-gallon bucket covers roughly 1,750–2,000 sq ft per coat on smooth, primed surfaces — about the wall area of an average single-story home. Buying in 5-gallon buckets is usually cheaper per gallon than individual cans and ensures a consistent color batch across large jobs, avoiding subtle dye-lot mismatches.