Find how many gallons of paint your project needs. Each dimension has its own unit selector (in/ft/yd/m).
Paint is estimated by area and coverage. Measure each wall (length times height, any unit), and add them up. A gallon covers about 350 sq ft on smooth walls - less on texture or porous surfaces, so use the coverage preset. Multiply the area by the number of coats and divide by coverage. Two coats is standard.