Calwarden

Room Painting Estimator

Estimate gallons of paint required for walls based on room dimensions.

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Inputs

ft
ft
ft

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Results

Gallons Needed
Paintable Area301

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Assumptions

  • The room is treated as a simple rectangle using wall perimeter and ceiling height.
  • Door and window deductions use standard opening sizes rather than custom measurements.
  • Room dimensions and coverage must be positive, while counts and coats should be zero or greater.

Sensitivity preview

Room Length -10%: Gallons Needed 1.61 gal

Room Length Current: Gallons Needed 1.72 gal

Room Length +10%: Gallons Needed 1.83 gal

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About this Paint Estimator

Painting is the easiest way to transform a room, but buying the wrong amount of paint is frustrating. Buying too little forces a mid-project store run (risking color mismatches), while buying too much wastes money. This calculator works out the exact wall area minus doors and windows.

Who this calculator is for

  • Homeowners estimating gallons before painting a bedroom, hallway, or living room.
  • Painters checking wall area, coats, and coverage before buying materials.
  • Renters or remodelers comparing how coverage changes with doors, windows, and multiple coats.

When to use it

  • Estimate gallons for repainting or changing colors in a rectangular room.
  • Adjust for standard door and window openings and multiple finish coats.
  • Compare coverage rates from different paint brands or product lines.

How to use this calculator

  1. Measure the Length, Width, and Height of the room.
  2. Count the number of Doors and Windows (standard sizes assumed).
  3. Specify how many Coats you plan to apply (2 is standard for color changes).

Key assumptions

  • The room is treated as a simple rectangle using wall perimeter and ceiling height.
  • Door and window deductions use standard opening sizes rather than custom measurements.
  • Room dimensions and coverage must be positive, while counts and coats should be zero or greater.

Accuracy and testing

Review formula sources, default test cases, assumptions, unit handling, rounding rules, and changelog for this calculator.

View accuracy page

How this calculator works

1. Gross Area: Perimeter (2L + 2W) × Height. 2. Net Area: Gross Area - (Doors × 21 sqft) - (Windows × 15 sqft). 3. Gallons: (Net Area × Coats) / 350 sqft/gal.

Inputs explained

Room Dimensions

Length and Width of the floor, and Height of the ceiling.

Doors/Windows

We subtract 21 sq ft for a standard door and 15 sq ft for a standard window.

Coverage

Most paint cans state coverage of 350-400 sq ft per gallon. Textured walls absorb more paint, reducing coverage.

Understanding your results

The result is rounded to the nearest decimal. Always round up to the nearest quart or gallon. Having a little leftover is good for future touch-ups.

Example scenarios

Bedroom

12x12 room, 8ft ceilings. 1 door, 1 window. 2 coats. Needs ~2.0 gallons.

Hallway

20x4 hallway, 8ft ceilings. 4 doors. 2 coats. Needs ~1.5 gallons.

Key Features

  • Automatic door/window deduction
  • Custom coverage rates
  • Multi-coat support

Tips & Best Practices

  • If painting a light color over a dark wall, assume you need a primer coat plus two finish coats.
  • Buy all your paint at once and mix them (box them) to ensure consistent color across the room.
  • Don't forget the ceiling! Ceiling paint is usually separate.

Glossary of Terms

Primer
A preparatory coating put on materials before painting to ensure better adhesion and durability.
Cut-in
Painting the edges/corners with a brush before rolling the main walls.

Frequently Asked Questions

How much paint does 1 gallon cover?
A standard gallon of quality paint typically covers 350 to 400 square feet (one coat). Rough or textured walls may require more paint (lower coverage).
Do I really need two coats?
Yes, two coats are highly recommended for true color fidelity and durability. If you are painting a light color over a dark wall, you might even need a primer plus two coats.
How do I account for trim and ceilings?
This calculator focuses on walls. For ceilings, calculate L x W. For trim, a quart is usually sufficient for an average room, but a gallon is safer for a whole house.
What if my room is not a rectangle?
Break the room into rectangles, calculate each section separately, and add the paintable areas together. If you prefer one run, use equivalent positive length and width values that preserve the same perimeter, then sanity-check the result against your manual wall measurements.