Find out exactly how much paint you need for any room — walls, ceiling, and trim — with per-surface gallon breakdowns and a shopping list optimized to minimize waste and cost.
Pro tip: Dark colors covering light walls may need 3 coats for full opacity, while light-over-light often needs just 1. Primer counts as a coat and costs half as much — always prime over dark or patched surfaces.
Standard door = 21 sq ft (3×7). Standard window = 12 sq ft (3×4).
Add up to 8 rooms and get a combined shopping list.
Pick a base color to see complementary, analogous, and triadic palettes.
How Much Paint Do I Need? The Complete Formula
The standard formula for estimating paint is straightforward: calculate the total paintable surface area in square feet, then divide by the coverage rate on the can. For walls, multiply the perimeter of the room (2× width + 2× length) by the ceiling height, then subtract the area of every door and window. A standard interior door removes roughly 21 square feet and a typical window removes about 12 square feet. Multiply the net wall area by the number of coats and add a 10% waste factor for spills, touch-ups, and roller nap absorption.
For ceilings, multiply room width by length. Trim area is estimated by adding the perimeter’s linear footage and multiplying by the trim width (usually 3–6 inches converted to feet). This calculator automates every step and rounds up to the nearest purchasable can size so you buy exactly what you need.
Paint Coverage Rates by Finish Type
Coverage rates vary by sheen level because higher-gloss paints contain more binders and fewer solids. Flat and matte finishes cover the most area — typically 350–400 square feet per gallon. Eggshell and satin offer moderate coverage at 350–375 square feet per gallon with better moisture resistance. Semi-gloss and gloss cover 300–350 square feet per gallon but provide superior durability and scrub resistance, making them ideal for kitchens, bathrooms, and trim.
Surface texture significantly affects actual coverage. Smooth drywall absorbs less paint than textured knockdown or orange-peel finishes, which can reduce coverage by 15–25%. If your walls are heavily textured, add an extra 20% to the estimate.
When Do You Need Primer?
Primer is not always necessary, but skipping it in certain situations causes poor adhesion and uneven color. Always prime when painting over new drywall, when covering dark colors with a lighter shade, and after patching or repairing walls. Stained surfaces from water damage or smoke also require a stain-blocking primer. When painting the same color or a similar shade over properly prepped walls, you can usually skip primer and go straight to two coats.
How to Estimate Paint for Odd-Shaped Rooms
For L-shaped rooms, split the space into two rectangles and calculate each separately. Rooms with cathedral ceilings have more wall area above the standard 8-foot line — measure the tallest point and average it with the shortest. Bay windows and alcoves add extra wall segments; measure each flat panel individually. For stairwells, measure the wall height at its tallest point and use that for the full perimeter calculation.
Gallon vs. Quart: When to Size Down
If the calculator shows 1.3 gallons, purchasing one gallon and one quart is more economical than two full gallons. Most stores sell quarts at roughly 35–40% of the gallon price. However, if the total falls between 1.6 and 2.0 gallons, buying two gallons is cheaper per square foot. Keep in mind that leftover paint is valuable for touch-ups — a spare quart stored properly can last 5–10 years unopened.
Paint Calculator FAQ
- How many square feet does a gallon of paint cover? Most interior latex paints cover 350–400 square feet per gallon on smooth surfaces with a single coat.
- How many coats do I need? Two coats is the industry standard. One coat may suffice for same-color repainting; three coats are recommended for drastic color changes.
- Should I subtract closet doors? Yes. Any opening larger than 8 square feet should be deducted from paintable wall area.
- Does texture affect paint quantity? Heavily textured surfaces increase consumption by 15–25%. Raise the waste factor slider to 15–20%.
- Can I paint over wallpaper? It is possible but not recommended. Removing the wallpaper first and priming produces far better results.
Looking for related tools? Try our Renovation ROI Calculator to estimate the return on your painting project, or explore all Home & Real Estate tools.