Paint
Give it your room and it hands you a store-ready can list — gallons and quarts, coats already multiplied through.
Your buy list
0
gallons + 0 quart- Wall area
- —
- Total to cover
- —
- Exact need
- —
Walls only — ceilings are priced separately.
How the paint math works
Wall area comes from walking the perimeter — twice the length plus twice the width — times ceiling height. One standard 21-square-foot door and each window come back off, because nobody paints glass. That net area times your coat count is the total surface to cover; divide by the coverage rate and you have exact gallons.
Stores sell whole gallons and quarts, so the remainder rolls into quarts — four of those become another gallon on your list. Buying the quart instead of rounding everything up to full gallons typically saves $10–15 on an average bedroom.
Why two coats
One heavy coat rarely matches one thin pass, and lighter colors over old color almost always flash through. Two coats is what paint manufacturers warranty against, which is why the form ships set to 2.
On the counter
The assumptions this calculator ships with — every one is adjustable in the form.
- Coverage rate
- 350 sq ft per gallon by default — typical for interior latex
- Coats
- Two, the standard for even color and washability
- Openings
- One 21 sq ft door and your window count deducted automatically
Questions from the aisle
How much paint does a 12×12 room take?
A 12×12 room with 8-foot ceilings has about 384 square feet of wall. Two coats at 350 sq ft/gal is 768 square feet of coverage — just over 2 gallons, so buy 2 gallons plus a quart.
What coverage rate should I use?
Most interior paints cover 350–400 square feet per gallon. Use 350 for darker colors, rough or unpainted drywall, and thick premium paints; use 400 for light colors over a sealed, previously painted wall.
Do ceilings need a separate calculation?
Yes — this calculator covers walls only. For a ceiling, treat it as its own rectangle at the same coverage rate; most rooms add roughly half their floor area again.
Is primer counted here?
No. Primer goes on at roughly 200–300 sq ft per gallon, so budget about half again your paint quantity if you are priming bare drywall or making a drastic color change.
The reasoning behind this one
How much paint do you actually need
Where the assumptions above come from, what changes them, and the mistakes that cost a second trip. Free to read, no account needed.