Paint Coverage Calculator

How to Calculate Paint Coverage

Estimating the amount of paint required for a room before visiting the store prevents underbuying or overbuying. The Paint Coverage Calculator allows you to quickly estimate the number of gallons needed by entering room dimensions or total wall area, selecting the paint type, and accounting for windows, doors, coats of paint, and optional waste factors.

The Paint Coverage Formula

The calculation determines the net paintable surface area by computing the gross wall area and subtracting standard openings for doors and windows.

1. Gross Wall Area

For a single rectangular room, the perimeter is calculated first, then multiplied by the ceiling height:

$$P = 2 \cdot (L + W)$$

$$\text{Gross Area} = P \cdot H$$

Where $L$ is the room length, $W$ is the room width, and $H$ is the ceiling height.

2. Deducting Openings

To avoid buying unnecessary paint, standard deductions are subtracted for doors and windows:

$$\text{Deductions} = (\text{Doors} \cdot 21) + (\text{Windows} \cdot 15)$$

Each standard door is estimated at 21 square feet, and each window is estimated at 15 square feet.

3. Estimating Gallons Needed

The net paintable area is multiplied by the desired number of coats and divided by the coverage rate per gallon, rounded up to the nearest whole gallon:

$$G = \lceil \frac{\text{Net Area} \cdot \text{Coats} \cdot (1 + \frac{\text{Waste\%}}{100})}{\text{Coverage Rate}} \rceil$$

The coverage rate varies by paint finish, ranging from 400 square feet per gallon for flat paints to 200 square feet per gallon for stains.

Average Coverage Rates by Paint Finish

The thickness of the paint film dictates its coverage. Below are standard coverage rates used for calculations:

• Flat / Matte: 400 sq ft / gallon. High pigment density, low film thickness.
• Eggshell / Satin: 350 sq ft / gallon. Standard interior wall finish.
• Semi-gloss / Gloss: 300 sq ft / gallon. Thicker, durable film for trim and cabinets.
• Textured Wall: 250 sq ft / gallon. Rough surfaces increase actual surface area.
• Primer: 350 sq ft / gallon. Used to seal raw drywall.
• Stain: 200 sq ft / gallon. Highly absorbent wood finishes.

Tips for a Perfect Paint Estimate

• Texture absorption: Drop coverage to 250 square feet per gallon for rough drywall or popcorn ceilings.
• Color shifts: Dramatic color changes, such as white over dark navy, will require a coat of primer plus at least two finish coats.
• Touch-ups: Always round up to the next whole gallon. Having extra paint is useful for future repairs.