The short answer
One US gallon covers about 350–400 sq ft per coat. Measure wall perimeter x ceiling height, subtract 20 sq ft per door and 15 sq ft per window, then divide by 350 and multiply by the number of coats.
Buying paint by eye is how people end up with three half-used cans in the basement. The maths takes two minutes.
Step by step
- Measure the length of every wall and add them together (the perimeter).
- Multiply the perimeter by the ceiling height to get gross wall area.
- Subtract 20 sq ft for each door and 15 sq ft for each standard window.
- Divide by 350 sq ft (the realistic coverage of one gallon on primed drywall).
- Multiply by the number of coats — two if you are changing colour or covering a dark wall.
- Round up to the next whole can and add 10% if the surface is textured or porous.
What you need
- Tape measure
- Roller and 2" angled brush
- Painter's tape
- Drop cloth
- Filler and sandpaper
What it costs
| Item | Typical cost |
|---|---|
| Budget interior latex | $20–$30 / gallon |
| Mid-range washable (Best value for living areas) | $35–$55 / gallon |
| Premium one-coat | $60–$90 / gallon |
| Primer (Only needed on bare or stained surfaces) | $18–$35 / gallon |
| Rollers, tray, tape, drop cloth | $35–$60 one-off |
Common questions
Does the ceiling count?
No — calculate it separately as length x width, same 350 sq ft per gallon rule.
Do I always need primer?
Only on bare drywall, patched areas, stains, or when going light over dark.
How much does one room cost?
A typical 12x12 ft room with two coats runs about $70–$140 in paint.