Paint Calculator

How Paint Coverage Is Calculated

The paint calculator uses this formula:

Gallons = (Total Wall Area - Doors*20 - Windows*15) x Coats / 350 $$G=\frac{(\sum _i{H}_i\times W_i-20D-15N)\times C}{350}$$

Where H is wall height, W is wall width, D is number of doors, N is number of windows, C is number of coats, and 350 sq ft is the standard coverage rate per gallon.

Variable Definitions

Wall Height: The floor-to-ceiling height of the wall in feet. Standard rooms are 8 or 9 feet. Measure at the wall, not in the center of the room where there may be a vaulted ceiling.

Wall Width: The horizontal span of the wall in feet. Measure from corner to corner. For L-shaped walls, treat each segment as a separate wall.

Number of Coats: How many layers of paint you plan to apply. Two coats give full coverage and even color. One coat is only suitable for touch-ups or same-color refreshes.

Doors: Number of standard interior doors in the room. Each door subtracts 20 sq ft from the paintable area. Closet doors and French doors count individually.

Windows: Number of standard windows in the room. Each window subtracts 15 sq ft from the paintable area. Large picture windows may subtract 25-30 sq ft; adjust by adding extra "windows" to compensate.

Worked Example

A bedroom with 4 walls, 2 coats, 1 door, 2 windows:

1. Wall 1: 9 ft x 12 ft = 108 sq ft
2. Wall 2: 9 ft x 12 ft = 108 sq ft
3. Wall 3: 9 ft x 10 ft = 90 sq ft
4. Wall 4: 9 ft x 10 ft = 90 sq ft

Total wall area: 396 sq ft

Subtract: 1 door (20 sq ft) + 2 windows (30 sq ft) = 50 sq ft

Paintable area: 396 - 50 = 346 sq ft

With 2 coats: 346 x 2 = 692 sq ft of coverage needed

Gallons: 692 / 350 = 1.98 gallons (buy 2 gallons)

Liters: 2 gallons x 3.785 = 7.57 liters

Edge Cases and Advanced Scenarios

Textured walls: Textured surfaces (knockdown, orange peel, popcorn) absorb 15-20% more paint than smooth walls. If your calculator shows 2 gallons, buy 2.5 gallons for textured walls. Heavy textures like stucco may need 25% more.

Accent walls with dark colors: Deep reds, navy blues, and charcoal grays require 3 coats for full opacity even with tinted primer. Budget for 50% more paint on accent walls. Using a gray-tinted primer under dark colors reduces the number of topcoats needed.

What to Do with Your Result

• Round up to the nearest gallon when purchasing. Leftover paint stores well for touch-ups.
• Buy all paint at once from the same batch to ensure consistent color across the room.
• Prep surfaces before painting: fill nail holes, sand rough spots, tape edges, and lay drop cloths.
• Store leftover paint upside-down (lid sealed tightly) to create an airtight seal that prevents skinning.