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 by the BuildCalc Pro Editorial TeamUpdated June 2026✓ Fact-checked

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.

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.

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.

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

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.