Paint Calculator · How it works
Ready to buy? Search for interior paint:
Buying paint comes down to two numbers: how much surface you are covering, and how far a gallon goes. This calculator works out the first from your room dimensions and applies a published coverage rate for the second, multiplying by the number of coats and rounding up to whole gallons.
Walls are figured by the perimeter method: it adds the four wall lengths (twice the length plus twice the width), multiplies by the ceiling height for gross wall area, then subtracts the doors and windows. The ceiling, if you include it, is simply length × width. Each surface is divided by its coverage rate and multiplied by the coats.
A 12 × 10 ft room with an 8 ft ceiling, two coats, one door and one window, walls and ceiling:
350 to 400 square feet per gallon. Major paint manufacturers publish spread rates in this range on their technical data sheets for interior wall paint applied to smooth, sealed surfaces — this tool uses 350 SF/gallon for walls and 400 for the flatter, further-spreading paint typically used on ceilings. Those figures are per coat, which is why the coat count matters so much: two coats of a 350-foot paint effectively covers 175 square feet of wall per gallon of paint bought.
Why two coats. A single coat rarely gives even color and hide; two is the standard for a durable, uniform finish and is the calculator's default. You can drop to one coat when you are repainting the same color over sound, clean paint, and you may need three or more going from a dark wall to a light one, or covering a strong red, orange or yellow.
Why texture and bare surfaces cover less. The published rate assumes a smooth, primed wall. A textured, sand-finished or porous surface has more actual area and soaks up more paint, so a gallon may cover only 250 SF or less. Bare new drywall is thirsty and uneven and should be primed first — primer seals the surface so the finish coats spread at their rated coverage. This calculator does not adjust for texture or count primer; on a rough or unprimed surface, treat its gallon count as a floor and buy a little extra.
How many square feet does a gallon of paint cover?
Manufacturers publish about 350 to 400 square feet per gallon per coat for interior wall paint on smooth, sealed surfaces. This calculator uses 350 for walls and 400 for ceilings. Rough, textured or unprimed surfaces cover less, sometimes 250 square feet or fewer, so those numbers are a best case.
Do I really need two coats?
Two coats is the standard for even color and a durable finish, and it is the tool's default. One coat can be enough when you repaint the same color over sound paint; going from dark to light, or covering a strong color, often takes three.
Does the calculator subtract doors and windows?
Yes. It removes 21 square feet for each door and 15 for each window from the wall area before working out gallons, the same standard openings the drywall calculator uses. It does not count primer, which you buy separately for bare or patched surfaces.
This is a quantity estimate for smooth, primed interior walls and ceilings. It does not size primer, trim or ceiling paint bought separately, and it does not adjust for heavy texture, deep color changes or unusually tall walls. When in doubt, buy from one batch (lot number) so a mid-job top-up matches.
The 350–400 SF-per-gallon coverage used here is the spread rate major manufacturers publish for interior wall paint on their technical data sheets; Sherwin-Williams and Behr both state rates in this range for smooth, sealed surfaces, and both note that porous or textured surfaces cover less. The door (21 SF) and window (15 SF) deductions are standard takeoff allowances. More on coverage, coats and priming is in our paint guide.