Paint Calculator for a Room
Enter your room dimensions to find out exactly how many gallons of paint you need — doors, windows, and ceiling included.
1Room Dimensions
2Doors and Windows
Standard door = 21 sq ft. Standard window = 15 sq ft. These are subtracted from your wall area automatically.
3Paint Settings
Check your specific paint can for exact coverage
Your Paint Estimate
Buying Guide
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Tip: Always buy a little extra. Paint is mixed in batches — if you run short and need to remix, the color may not match exactly. Keep the leftover for touch-ups.
How to Use This Calculator
Using this paint calculator for a room ensures you buy the exact right amount of paint. Follow these five steps:
- 1Measure room dimensions. Use a tape measure to find the length, width, and ceiling height of your room in feet and inches.
- 2Count doors and windows. Enter the number of doors and windows in the room. This paint calculator for a room uses standard sizes (21 sq ft per door, 15 sq ft per window) and subtracts them automatically from your wall area.
- 3Select number of coats. Applying 2 coats is standard for a durable, professional finish. You will usually only need 3 coats for drastic color changes, like painting bright white over dark blue.
- 4Select coverage rate. Check your specific paint can label for the manufacturer's expected coverage rate to get the most accurate estimate.
- 5Choose ceiling options. Select whether you are painting the ceiling the exact same color as the walls, or if you need a separate calculation for flat ceiling paint.
The Formula
This paint calculator for a room uses basic geometry to determine your exact square footage. Here is the math in plain English:
Perimeter = (2 × Length) + (2 × Width)
Gross Wall Area = Perimeter × Ceiling Height
Net Wall Area = Gross Area − (21 sq ft per door) − (15 sq ft per window)
Gallons Needed = (Net Wall Area × Number of Coats) / Coverage Rate
Ceiling Area = Length × Width
First, find the wall area by multiplying the room perimeter by the ceiling height. To ensure you don't overbuy, subtract 21 sq ft for each door and 15 sq ft for each window. The gallons needed equals that net area multiplied by your number of coats, divided by the paint's coverage rate. The ceiling area simply equals the room length times the width.
According to technical data sheets from major brands like Sherwin-Williams and Benjamin Moore, standard interior paint typically covers 350 to 400 sq ft per gallon.
Frequently Asked Questions
Should I buy a quart or a gallon for small rooms?
A standard quart of paint covers roughly 100 square feet (one coat). Even in a small bathroom, applying two coats usually pushes your requirement near or slightly over the 200 square foot mark. Because a gallon is often heavily discounted compared to buying three individual quarts, it is almost always more economical and safer to buy the full gallon.
Why are two coats almost always necessary even when painting the same color?
A single coat of paint rarely provides an even sheen. Two coats ensure full, even coverage, hide roller marks (lap lines), and provide true color depth and durability, even when you are simply refreshing the exact same color.
Does primer count as a coat of paint?
No. Primer is formulated specifically to bind to bare drywall, seal stains, and create a uniform surface. It does not contain the protective resins and pigments of topcoat paint. You still need your standard 1 to 2 coats of actual paint on top of your primer layer.
How do I calculate paint for an accent wall only?
To calculate a single accent wall, simply measure that specific wall's length and height. Multiply them together for the square footage, subtract any doors or windows on that wall, and multiply by 2 coats. Divide that number by your coverage rate. Usually, 1 quart or 1 gallon is plenty.
Should I buy extra paint, and how much?
You should always aim to have about 10% to 15% leftover paint. You will inevitably need this leftover paint for future touch-ups from wall scuffs, moving furniture, or patching small nail holes. Storing leftover paint in a sealed, airtight container is a best practice for homeowners.
What is the difference between paint coverage on the can versus real-world coverage?
The coverage rate printed on the paint can (usually 400 sq ft) assumes you are painting over a perfectly smooth, fully primed surface. In the real world, heavily textured walls, highly porous builder-grade flat paint, and thick roller naps will soak up significantly more paint, dropping your actual coverage rate closer to 300 or 350 sq ft per gallon.