Paint Calculator
How much paint do I need? This free paint calculator works it out for you: enter your room's length, width, and ceiling height, how many doors and windows to subtract, and how many coats you're applying. It shows your paintable square footage, the exact gallons of paint required, and how much to buy rounded up to the nearest quart — for the walls, and optionally the ceiling.
Enter your numbers
Your results
Adjust the numbers to see how much paint you need.
How to use this calculator
Measure your room's length, width, and ceiling height in feet, then count the doors and windows — the calculator subtracts an average door (about 21 sq ft) and window (about 15 sq ft) so you don't buy paint for openings you won't paint. Pick your number of coats (two is the usual choice) and check your paint can's coverage per gallon, then adjust the default of 350 sq ft/gal to match. Results update instantly as you change anything.
If you're also painting the ceiling, tick the checkbox to add the ceiling's square footage (length × width). The calculator shows your total paintable area, the exact gallons needed, and how much to buy rounded up to the nearest quart — paint is sold in quarts and gallons, so rounding up means you won't run out mid-coat.
What the calculator measures
Plan view of the room. Doors and windows are subtracted from the wall area since you won't paint them.
How it works
The calculator first finds the total wall area: 2 × (length + width) × ceiling height. That's the perimeter of the room times its height — the four walls. It then subtracts the openings you won't paint: about 21 sq ft per door (roughly a 3×7-foot door) and about 15 sq ft per window (roughly a 3×5-foot window). If you check "include the ceiling," it adds the ceiling's area (length × width).
The paintable area is multiplied by your number of coats and divided by the coverage per gallon to get the exact gallons needed: paintable sq ft × coats ÷ coverage. Finally, because paint is sold by the gallon and quart, the calculator rounds up to the nearest quarter gallon — that rounding is your built-in safety margin so you don't run out halfway through the second coat.
Assumptions (estimates): door and window areas are averages, not measurements of your actual openings; coverage rates vary by paint brand, surface texture, and application (roller vs. sprayer); and the quarter-gallon rounding is an estimate, not an exact waste allowance. Very porous or unprimed surfaces can need noticeably more paint than the label rate suggests.
Frequently asked questions
How much paint do I need for a 12×12 room?
For a 12×12 room with 8-foot ceilings, the four walls measure 2 × (12 + 12) × 8 = 384 square feet. Subtract roughly two doors (42 sq ft) and two windows (30 sq ft) and you get about 312 square feet of paintable wall. Two coats doubles that to 624 square feet. At 350 sq ft per gallon, that's about 1.8 gallons — so buy 2 gallons. This calculator does the same walkthrough automatically for any room size.
How many square feet does a gallon of paint cover?
A gallon of interior paint covers about 350–400 square feet per coat on smooth, primed walls. Rough or textured surfaces, unprimed drywall, and porous masonry soak up more paint, so coverage can drop to 250 square feet or less per gallon. This calculator lets you adjust the coverage rate to match your paint label and surface.
How many coats of paint do I need?
Plan on two coats for most interior walls. Two coats gives even color and full coverage, especially when you're painting over a different or darker color. One coat can work if you're refreshing the same color on a clean, primed surface. If you're painting bare drywall, add a coat of primer first — it seals the surface so your top coats cover evenly.
Do I subtract doors and windows?
Yes — you don't paint doors or windows, so subtracting them avoids buying more paint than you need. This calculator subtracts about 21 square feet per door (roughly a 3×7-foot door) and about 15 square feet per window (roughly a 3×5-foot window). These are estimates; if you have oversized doors or a picture window, just enter fewer or more openings to approximate the area.
How much primer do I need?
Use the same calculation with 1 coat: measure your wall area, subtract doors and windows, and divide by the primer's coverage rate. Primer covers about 200–300 square feet per gallon on bare drywall. For a 12×12 room with two doors and two windows (about 312 sq ft of wall), one coat of primer at 250 sq ft per gallon means buying about 1.5 gallons.