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Blueprint Calc

Paint Calculator

Enter your room dimensions and get exactly how many gallons of paint and primer to buy. Accounts for doors, windows, coats, and a standard 10% waste factor. No signup, no email — just the number.

How much paint do I need?

Measurements in feet. Defaults reflect a typical 12x12 bedroom.

You need
  • Paint needed
    3 gallons
    333.00 sq ft × 2 coats, 350 sq ft/gal, 10% waste
  • Paintable area
    333.00 sq ft
    walls only, openings subtracted

What you'll need to buy

Prices approximate · Links go to retailer search results
Estimated total: ~$176 (materials, before tax)

Product links go to Home Depot search results — pick the best match for your project. Prices shown are rough averages.

How we calculated it

The formula is simple and contractor-standard:

  1. Wall area = 2 × (length + width) × ceiling height
  2. Subtract openings — 21 sq ft per door, 15 sq ft per window
  3. Multiply by the number of coats (2 is the default for a reason)
  4. Divide by 350 sq ft/gallon — the industry-standard coverage for latex on smooth drywall
  5. Add 10% waste factor, round up to the next whole gallon

Primer adds a separate single-coat calculation at 300 sq ft/gallon (primer covers less than finish paint). If you are painting new drywall, covering a stain, or going from dark to light, you need it. Otherwise modern paint-and-primer finishes can usually skip it.

Assumptions: 10% waste, 350 sq ft/gal paint coverage, 300 sq ft/gal primer coverage. Textured walls, very dark tints, or spray application can drop coverage to 250-300 sq ft/gal — buy one extra gallon if you are in that situation.

How much paint for common room sizes

Interior smooth-wall, 8 ft ceilings, 2 coats, walls only (not ceiling). Assumes 1 door and 1-2 windows per room.

Room size Wall area Paintable (with openings) Gallons for 2 coats
8 × 8 ft 256 sq ft 220 sq ft 2 gallons
10 × 10 ft 320 sq ft 284 sq ft 2 gallons
10 × 12 ft 352 sq ft 316 sq ft 3 gallons
12 × 12 ft 384 sq ft 333 sq ft 3 gallons
12 × 16 ft 448 sq ft 397 sq ft 3 gallons
14 × 16 ft 480 sq ft 429 sq ft 3 gallons
16 × 20 ft 576 sq ft 525 sq ft 4 gallons
20 × 20 ft 640 sq ft 589 sq ft 4 gallons

Door = 21 sq ft, window = 15 sq ft. Add a gallon if painting the ceiling too.

Paint coverage by surface

Sq ft per gallon (single coat), with waste factor.

Surface Coverage (sq ft/gal) Waste factor Effective coverage
Interior smooth drywall 350 10% 315 sq ft/gal
Interior textured (popcorn) 280 12% 246 sq ft/gal
Exterior wood/vinyl siding 250 15% 213 sq ft/gal
Exterior brick / CMU / stucco 200 20% 160 sq ft/gal
Primer (any surface) 300 10% 270 sq ft/gal

Frequently asked questions

How many gallons of paint do I need for a 12x12 bedroom?
For a 12x12 room with 8 ft ceilings, 1 door, and 2 windows, you will need roughly 2 gallons of paint for 2 coats. The math: 384 sq ft of walls minus 51 sq ft of openings leaves 333 sq ft × 2 coats = 666 sq ft, which rounds up to 2 gallons at 350 sq ft coverage per gallon.
Do I need primer when painting?
You need primer on new drywall, raw wood, over stains or water damage, and when going from a dark color to a light one. If you are repainting an already-painted wall with a similar color and a quality paint-and-primer product, you can usually skip dedicated primer.
Is one coat of paint ever enough?
Rarely. One coat works only with premium paint-and-primer and a same-tone repaint on a clean wall. For color changes, new drywall, builder-grade paint, or any wall with patches, plan on two coats. Touch-ups over dark colors almost always need two.
How much paint do I need for a 10x10 room?
A 10x10 room with 8 ft ceilings has about 320 sq ft of walls. After subtracting one door (21 sq ft) and one window (15 sq ft), you get ~284 sq ft of paintable surface. At 2 coats that is 568 sq ft, which fits in 2 gallons with room for touch-ups.
How do I calculate paint for walls only vs walls and ceiling?
Walls-only: perimeter × ceiling height, minus doors and windows. Walls + ceiling: add length × width of the floor. Flat ceiling paint covers a bit less per gallon than wall paint, so keep a half-gallon buffer if you are doing both in one trip.
What sheen should I pick for which room?
Flat/matte for ceilings and low-traffic walls (hides imperfections). Eggshell for living rooms and bedrooms (soft finish, wipeable). Satin for kitchens, hallways, and kids rooms (durable, scrubbable). Semi-gloss for trim, doors, and bathrooms (moisture-resistant, easy to clean).
How much does a gallon of paint cover?
Industry-standard coverage is 350 sq ft per gallon for smooth walls and one coat. Textured surfaces, porous drywall, or heavily tinted dark colors drop it to 250-300 sq ft per gallon. The 350 figure is what this calculator uses.
Should I buy extra paint for touch-ups?
Yes. Keep at least a half-gallon from the same batch for future touch-ups. Paint stores mix color by computer but there is always slight batch variation, so the safest touch-up paint is leftover from the original can.
How much paint for an exterior home?
Exterior estimates use the same perimeter × height math but need a 15-20% waste factor instead of 10% (rougher surfaces, siding profiles, spray overspray). Most 2,000 sq ft single-story homes take 10-15 gallons for 2 coats plus primer where needed.
Do ceilings need a different type of paint?
Flat ceiling paint hides imperfections and stops glare. Standard wall paint works but shows every taping flaw. If it is a kitchen or bathroom, go with a low-sheen mildew-resistant ceiling paint.

Specific room-size guides

Want a pre-filled calculator with the math already worked out for a common room size? We have dedicated guides for the most-asked sizes:

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See also

Related terms

Plain-English definitions for the terms used in this calculator.

paint coverage
Square feet of surface one gallon of paint covers in a single coat. Industry standard is 350 sq ft/gal on smooth interior drywall; drops to 250-280 on textured, 200-250 on exterior siding, and 150-200 on masonry.
primer
A preparatory coating applied before finish paint. Required on new drywall, bare wood, over stains, and when changing from dark to light colors. Covers ~300 sq ft/gal. Modern paint-and-primer products can skip separate primer on repaints of similar colors.
sheen
The light-reflective quality of a paint finish. Flat/matte hides imperfections (ceilings, low-traffic walls); eggshell and satin add scrub-resistance (living areas); semi-gloss and gloss are most durable and cleanable (trim, doors, baths).
waste factor
An over-order multiplier to account for cuts, damage, errors, and touch-ups. 10% is standard for simple jobs; 15-20% for complex layouts (diagonal tile, cathedral ceilings, rough subgrades).