Paint Calculator

Estimate how many gallons of paint you need for walls and ceilings — including doors, windows, coats and waste.

Last updated: September 2026 · Reviewed for standard interior paint coverage practices

Room Dimensions

Length

Width

Ceiling Height

Include ceiling

Openings

Number of Doors

Number of Windows

Paint Options

Number of Coats

Coverage (sq ft per gallon)

Waste Factor

Price per Gallon (optional)

Include primer (1 coat)

How to Use This Paint Calculator

Enter the room length, width and ceiling height
Optionally check Include ceiling if you will paint it
Enter number of doors and windows (they are subtracted from wall area)
Choose coats (usually 2) and coverage rate from your paint can
Add waste (10% is typical) and optional primer / price
Click Calculate Paint for gallons needed

Common Room Examples (8 ft Ceiling, 2 Coats, 350 sq ft/gal, 10% Waste)

Quick reference for typical rooms. Assumes 1 door + 2 windows unless noted. Re-calculate for your exact openings and ceiling option.

Room Size	Walls Only	Walls + Ceiling	With Primer (walls only)
10 × 12 ft	~2.0 gal	~2.75 gal	~3.0 gal paint + primer
12 × 15 ft	~2.5 gal	~3.5 gal	~3.75 gal paint + primer
15 × 20 ft	~3.5 gal	~5.0 gal	~5.25 gal paint + primer
20 × 20 ft	~4.0 gal	~5.75 gal	~6.0 gal paint + primer

Figures are approximate and rounded up. Always verify with your paint’s labeled coverage rate.

Typical Paint Coverage

Surface / Product	Approx. Coverage per Gallon
Smooth interior walls	350 – 400 sq ft
Textured or porous walls	250 – 350 sq ft
Ceiling (flat paint)	300 – 400 sq ft
Primer	300 – 400 sq ft
Exterior siding	250 – 350 sq ft

Always use the coverage number printed on your paint can for the most accurate result.

Formulas Used

Wall area = 2 × (Length + Width) × Height − (Doors × 20) − (Windows × 15)

Ceiling area = Length × Width (if included)

Paint gallons = (Paintable area × Coats × (1 + Waste%)) ÷ Coverage

Results are rounded up to the nearest quart so you do not run short.

Painting Tips

Two coats are standard for a durable, even finish
Use primer on new drywall, stains, or big color changes
Buy from the same batch when possible to avoid shade variation
Keep leftover paint for touch-ups (label the lid with room and date)
Cut in edges first, then roll walls for a professional look

Frequently Asked Questions

How much paint do I need?

Calculate wall area (perimeter × height), subtract doors and windows, multiply by number of coats, then divide by coverage rate (typically 350 sq ft per gallon). Add 10% waste for a safe estimate. This calculator does it automatically.

How many square feet does a gallon of paint cover?

Most interior paints cover about 350–400 square feet per gallon on a smooth surface with one coat. Rough or porous surfaces may need more. Always check the can label.

Do I need primer before painting?

Primer is recommended on new drywall, stained walls, dark-to-light color changes, or glossy surfaces. Many projects need one coat of primer plus two coats of paint.

How many coats of paint should I use?

Two coats are standard for most interior walls. One coat may work for a similar color touch-up; three coats can help when covering a very dark color.

Should I paint the ceiling too?

Optional. Check “Include ceiling” if you plan to paint it. Ceilings often use flat paint and may need less coverage adjustment than walls.

How do I account for doors and windows?

Enter the number of doors and windows. The calculator subtracts about 20 sq ft per door and 15 sq ft per window so you do not over-buy paint.

How much waste should I add?

10% is a good default for most rooms. Use 5% for simple rectangular rooms and 15% if the room has many corners, trim, or odd shapes.

Related measurements

Room floor area pairs well with the square footage calculator. After painting, estimate flooring with the flooring calculator or wall finish with the drywall calculator. Browse all construction calculators.