Room Paint Calculator
Figure out exactly how many gallons of paint you need for your room. Enter the room dimensions, number of doors and windows, and how many coats you plan to apply.
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Calculating Paint for Your Room
The basic formula is straightforward. Measure the perimeter of the room by adding all wall lengths together, then multiply by the ceiling height. This gives you the gross wall area. Subtract the area of doors (about 21 square feet each) and windows (about 15 square feet each) to get the net paintable surface.
Multiply the paintable area by the number of coats, then divide by the coverage rate of your paint, typically 350 square feet per gallon. Round up to the nearest whole gallon since paint is sold in gallon and quart sizes. Two coats on a 12-by-10-foot room with 8-foot ceilings, one door, and two windows requires about 2 gallons.
If you are also painting the ceiling, add the floor area (length times width) to your calculation. Ceiling paint is usually the same type as wall paint but in flat white, which hides imperfections better than higher-sheen finishes.
Choosing the Right Paint and Finish
Interior paint comes in several sheen levels. Flat or matte hides wall imperfections best, ideal for ceilings and low-traffic rooms. Eggshell has a slight sheen and cleans easier, making it the most popular choice for living rooms and bedrooms.
Satin resists moisture and staining, good for kitchens and bathrooms. Semi-gloss is more durable, commonly used for trim, doors, and cabinets. High-gloss is the most washable but shows every imperfection, typically reserved for accent pieces.
Quality matters more than brand loyalty. Higher-quality paints contain more pigment and resin, covering better in fewer coats. A premium gallon at $40 to $50 that covers in two coats is better value than a budget gallon at $25 that needs three coats and fades quickly.
Preparation and Painting Tips
Preparation accounts for 80 percent of a professional result. Clean walls with a damp cloth to remove dust. Fill nail holes with spackle, let dry, and sand smooth. Apply painter's tape along trim, ceiling lines, and window frames for clean edges.
Use primer when covering dark colors, painting over stains, or on new drywall. Tinted primer matched to your paint color reduces topcoats needed. Self-priming paints work for minor color changes but rarely replace dedicated primer for major transformations.
Roll walls in a W pattern, keeping a wet edge to prevent lap marks. Paint the ceiling first, then walls, then trim. Allow each coat to dry 2 to 4 hours before applying the next. Proper ventilation speeds drying and reduces fumes.
Frequently Asked Questions
How much does one gallon of paint cover?
One gallon of interior paint covers approximately 350 to 400 square feet on a smooth surface with one coat. Textured walls, porous surfaces, and darker base colors reduce coverage to 250 to 300 square feet per gallon. This calculator uses 350 sq ft per gallon as a standard estimate, which accounts for typical wall textures and rolling technique.
Do I really need two coats of paint?
In most cases, yes. Two coats provide better coverage, more uniform color, and greater durability. A single coat often shows roller marks, lap lines, and the old color bleeding through, especially with lighter colors over darker ones. The exception is when repainting with a very similar color over a well-prepped surface, where a thick single coat may suffice.
How does the calculator account for doors and windows?
Standard doors (including frame) are estimated at 21 square feet each, and standard windows (including frame) at 15 square feet each. These areas are subtracted from the total wall area since you don't paint over them with wall paint. If your doors or windows are significantly larger or smaller than standard, adjust the count accordingly.
Should I buy an extra gallon just in case?
Buying one extra gallon is smart for several reasons. You may need touch-ups after moving furniture or hanging pictures, and paint from the same batch guarantees a perfect color match. Store the leftover paint sealed at room temperature. If you calculated exactly to the gallon, rounding up ensures you don't run short mid-wall, which can leave visible lap marks.
How do I estimate paint for an oddly shaped room?
Break the room into rectangular sections and calculate each wall separately. For a room with an alcove or L-shape, treat each wall segment as its own rectangle (width times height) and add them together. This calculator assumes a standard rectangular room. For rooms with cathedral ceilings or angled walls, calculate the triangular sections separately using half base times height.