Paint Calculator

What this calculator does

This calculator estimates how much paint you need to cover walls, ceilings, or other surfaces based on the area to be painted, the paint's coverage rate, and the number of coats. It outputs the result in gallons so you can purchase the right amount and avoid costly return trips or leftover waste.

Estimating paint is one of those tasks that seems simple until you realize rooms have doors, windows, trim, and varying surface textures that all affect how much product you need. This calculator accounts for those factors, giving you a practical answer that matches what you will actually use on the job.

Whether you are painting a single accent wall, refreshing an entire home interior, or estimating for a client as a professional painter, accurate paint estimation keeps your project on budget and on schedule.

How to use it

1. Enter the wall height and total wall length (perimeter of the room), or enter each wall individually.
2. Enter the number and approximate size of doors and windows to subtract from the paintable area.
3. Select the number of coats (typically 2 for a color change, 1 for a refresh).
4. Enter the paint's coverage rate (default: 350 sq ft per gallon for smooth walls).
5. Review the total area, adjusted area (minus openings), and gallons needed.

Formulas used

• Wall area = Wall height × Total wall length (perimeter) Example: 8 ft ceiling × 52 ft perimeter = 416 sq ft

• Subtract openings: Standard door ≈ 21 sq ft (3 ft × 7 ft). Standard window ≈ 12 sq ft (3 ft × 4 ft). Adjusted area = Wall area − (Doors × 21) − (Windows × 12)

• Paint needed: Gallons = (Adjusted area × Number of coats) / Coverage rate per gallon Example: (370 sq ft × 2 coats) / 350 sq ft per gallon = 2.11 gallons → buy 3 gallons

Examples

Example 1 — Standard bedroom (12 × 14 ft, 8 ft ceiling): Perimeter = 52 ft. Wall area = 416 sq ft. Subtract 1 door (21 sq ft) + 2 windows (24 sq ft) = 371 sq ft. Two coats at 350 sq ft/gal: 742 / 350 = 2.12 gallons. Buy 3 gallons.

Example 2 — Living room with vaulted ceiling: Perimeter = 68 ft. Average height = 10 ft. Area = 680 sq ft. Subtract 2 doors + 4 windows = 90 sq ft = 590 sq ft. Two coats: 1,180 / 350 = 3.37 gallons. Buy 4 gallons.

Example 3 — Bathroom (small, one coat touch-up): Perimeter = 30 ft. Height = 8 ft. Area = 240 sq ft. Subtract 1 door + 1 window = 207 sq ft. One coat: 207 / 350 = 0.59 gallons. A 1-gallon can is sufficient.

Tips and common mistakes

• Textured walls need more paint. Rough, textured, or porous surfaces can reduce coverage to 250–300 sq ft per gallon. Adjust the coverage rate accordingly.
• Dark-to-light color changes need a primer coat (or a paint-and-primer-in-one). Budget for an extra coat or a separate primer gallon.
• Buy a little extra. Round up to the next full gallon. Having leftover paint is useful for touch-ups months or years later. Store sealed cans in a climate-controlled space.
• Ceiling paint has different coverage rates (often 350–400 sq ft/gal). Calculate ceiling area separately: Length × Width.
• Trim and doors require less paint (a quart often covers several doors) but use a different type (semi-gloss or satin vs. flat/eggshell for walls).
• One gallon covers about one average-sized room (2 coats, ~400 sq ft of wall area after openings). Use this as a quick sanity check.