How the paint calculator works
Wall area is the total length of the walls times their height.
Each door takes off about 20 square feet and each window about 15.
Paintable area times coats, divided by coverage per gallon, gives gallons. Then a little extra is added for waste.
Gallons are rounded up to whole quarts and shown as 5-gallon pails, gallons and quarts.
The formula: Gallons = (length × height - openings) × coats ÷ coverage × (1 + waste).
Tips from the trade
- Measure the walls, not the floor. A 12 by 14 room has 52 feet of wall.
- Most wall paint covers about 250 to 400 square feet a gallon. Use the low end for rough or new drywall.
- Prime first when you go dark to light or paint new drywall. Primer has its own coverage.
When you need more than a free paint calculator
This calculator answers one quick question. Running a business means doing it on every job, with your own rates saved and a quote you can send.
Free calculatorPaint & Materials Calculator
Gallons for one set of walls✓✓
Up to 40 areas: walls, ceilings, trim and more✓
Coverage adjusted for each surface✓
Paint, primer and stain with their own coats✓
Cheapest mix of pails, gallons and quarts with prices✓
Supplies and a printable shopping list✓
Paint calculator questions
How much paint do I need for a 12 by 12 room?
A 12 by 12 room with 8 foot walls has about 384 square feet of wall. Take off a door and a window and two coats need roughly 2 gallons.
How many coats of paint do I need?
Two finish coats is the usual plan. A big color change may need primer first or a third coat.