Paint Calculator - How Many Gallons Do I Need?

How to use this tool

Choose "Room" mode and enter length, width, height - or use "Area" mode for a custom wall area.
Enter the number of windows and doors to subtract their area.
Set coverage from the paint can, choose the number of coats, and adjust the default 10% waste allowance if needed.
Optionally enter paint cost per gallon or liter for a total cost estimate.
Read the gallons needed, rounded up to whole cans.

How much paint do I need?

Estimate wall area by adding all wall surfaces: 2 × (length + width) × height for a rectangular room. Then subtract windows (~15 sq ft each) and doors (~21 sq ft each) for a more accurate estimate.

Paint coverage

Coverage depends on paint quality and surface texture. Most interior paints cover 350–400 sq ft per gallon. Primers and flat paints may cover less (~250 sq ft/gal). Check the label before purchasing.

Coats and color changes

Two coats are standard. If you are painting a dark color over a light one (or vice versa), you may need three coats for complete coverage. High-hide "one-coat" paints can sometimes cover in one coat on similar colors.

Paint coverage reference

Paint type	Coverage per gallon
Interior latex (flat/matte)	350–400 sq ft
Interior latex (eggshell/satin)	350–400 sq ft
Interior latex (semi-gloss/gloss)	350–400 sq ft
Exterior paint	250–300 sq ft
Interior primer	200–300 sq ft
Exterior primer	200–250 sq ft

Ceiling and trim

This calculator covers walls only. For ceilings, measure length × width. Trim (baseboards, door frames, window casings) is often estimated at 0.1 gallons per window and 0.1 gallons per door from a separate quart can.

Texture effect on coverage

Smooth, newly-primed drywall gives the best coverage. Textured surfaces (orange peel, knock-down, popcorn ceilings) have significantly more surface area and will require 10–25% more paint. New drywall also absorbs more paint on the first coat; plan for a full prime coat before painting.

Waste and overage

This calculator starts with a 10% allowance for cutting in, paint left in a roller tray, and small touch-ups. The allowance is applied after subtracting openings and multiplying the net wall area by the number of coats, then the adjusted area is divided by the coverage on the paint can. For example, 301 square feet of net wall area over two coats is 602 square feet; with 10% waste, plan to cover 662.2 square feet before rounding the paint purchase up to whole gallons. Choose 0% for a precise estimate on smooth walls, or 15–20% when textured walls or new drywall need more allowance. New drywall usually needs a dedicated primer coat, which is not included in the wall-paint result.

Frequently asked questions

How many square feet does a gallon of paint cover?

Most interior paints cover 350–400 square feet per gallon on a smooth surface. Textured surfaces or primer coats may cover less (~250 sq ft/gal). Check the label - this calculator defaults to 350 sq ft/gal.

How much extra paint should I allow for waste?

The calculator adds 10% by default for cuts, roller and tray residue, and touch-ups. Use 0% for a precise smooth-wall estimate or raise it to 15–20% for textured walls or new drywall. A primer coat may need a separate estimate.

Should I subtract windows and doors?

Yes, for a more accurate estimate. This calculator deducts ~15 sq ft per standard window and ~21 sq ft per standard door.

How many coats do I need?

Two coats is standard for good coverage when painting over a different color. One coat may suffice for touch-ups or when using high-coverage paint. Dark-to-light color changes may need three coats.

What about the ceiling?

This calculator covers walls only. For the ceiling, add a separate estimate: multiply room length × width, then divide by the coverage per gallon.