Paint Calculator

Use our free paint calculator to determine exactly how many gallons of paint you need for a room. Built for homeowners, painters, and contractors, it accounts for wall area, door and window openings, and the number of coats required, so you avoid overbuying or running short mid-project. Enter your room dimensions and openings to get an instant gallon estimate.

Paint is one of the easier materials to misjudge, because most cans list a coverage range rather than a fixed number, and every door and window you're not painting around subtracts real square footage from the job. Running the numbers before you shop keeps you from a second trip to the store mid-project, or hauling home more gallons than the room could ever use.

How to use this calculator.

1. Enter room dimensions. Input the room length, width, and ceiling height in feet.
2. Enter doors and windows. Count the number of doors and windows in the room: these get subtracted from the paintable area automatically.
3. Set the number of coats. Two coats is standard for most jobs; use one for a simple repaint in the same color.
4. Adjust the coverage rate. The default is 350 sq ft per gallon; check your paint can for the manufacturer's actual rate.
5. Review your results. Paintable wall area and gallons needed calculate instantly when you click Calculate.

How do you calculate A paint calculator helps you determine exactly how many gallons of paint you need for a room by accounting for wall area, doors, windows, and the number of coats required.?

Paint coverage is calculated by finding the net paintable wall area, then dividing by the coverage rate per gallon and multiplying by the number of coats.

Paintable Wall Area (sq ft) = 2 × (Room Length + Room Width) × Room Height − (Doors × 20) − (Windows × 15)

Gallons Needed = (Paintable Wall Area ÷ Coverage per Gallon) × Coats

Where:

• Room Length / Room Width: The interior dimensions of the room, in feet.
• Room Height: The ceiling height, in feet.
• Doors: Each door deducts a standard 20 sq ft from the paintable area.
• Windows: Each window deducts a standard 15 sq ft from the paintable area.
• Coats: The number of finish coats to be applied.
• Coverage per Gallon: The manufacturer's stated coverage rate, typically 350–400 sq ft per gallon.

Example calculation.

Let's estimate paint for a 15 ft by 12 ft bedroom with 9 ft ceilings, one door, two windows, and two coats using paint that covers 350 sq ft per gallon.

1. Calculate perimeter: (15 ft + 12 ft) × 2 = 54 ft
2. Calculate gross wall area: 54 ft × 9 ft = 486 sq ft
3. Subtract openings: 486 − 20 (1 door) − 30 (2 windows × 15 sq ft) = 436 sq ft paintable
4. Apply coats and coverage: (436 ÷ 350) × 2 = 2.49 gallons

Result: You need approximately2.49 gallons: buy 3 gallons to cover the full job with some left over for touch-ups.

Tips from the field.

• Check your paint can label. Coverage rates vary by brand and sheen. Flat paint typically covers more area than high-gloss. Always use the manufacturer's stated rate.
• Prime bare drywall first. New drywall absorbs paint differently than previously painted surfaces. A separate primer coat keeps finish coats consistent.
• Darker colors need more coats. Painting a light color over dark, or vice versa, may require a third coat beyond the standard two for full coverage.
• Texture increases consumption. Heavily textured walls can use 20–30% more paint than smooth walls; increase your estimate accordingly.
• Buy an extra quart. Keep leftover paint from the same batch for future touch-ups: color can drift between production runs.

Disclaimer: Calculations are estimates. Actual paint consumption varies with surface texture, application method (brush, roller, sprayer), paint quality, and color changes. Always consult the paint manufacturer's specifications before ordering.