How the paint estimate works
The calculator measures the four walls as 2 x (length + width) x height, adds the ceiling as length x width when selected, then removes the area taken up by doors (about 21 sq ft each) and windows (about 15 sq ft each). It multiplies the remaining paintable area by the number of coats, divides by the coverage rating on your paint can, and adds a waste allowance.
Gallons = (Paintable area x coats / coverage) x (1 + waste%)
Why this matters before you shop
Buying too little paint means a second trip and a risk of slight color variation between batches. Buying too much wastes money and leaves you storing half-empty cans. A quick estimate keeps you within one can of the real need for most rooms. For a 12 x 10 ft bedroom with 8 ft ceilings, two coats and one door plus two windows, expect roughly 1.5 gallons of wall paint - so you would buy two 1-gallon cans.
Recommended tools & supplies
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Real-world use cases
- Repainting a single bedroom - subtract the window and door, pick two coats, and you know the exact can count before driving to the store.
- Whole-house planning - run each room separately, then total the gallons to negotiate a contractor quote or buy in 5-gallon buckets for a lower per-gallon price.
- Accent walls - set coats to 2-3 because saturated colors usually need extra coverage, then add a primer line item.
- Rental turnovers - property managers use the same math per unit to standardize purchasing across dozens of apartments.
Coverage ratings you will see on cans
Flat and matte paints commonly list 350-400 sq ft per gallon. Eggshell and satin are closer to 300-350. Primer is usually around 200-300 because it soaks into raw surfaces. If you are spraying instead of rolling, plan for roughly 10-15% more material due to overspray.