Paint Calculator
Buy the right amount of paint the first time. Enter wall dimensions, account for doors and windows, and get gallons needed for the number of coats you plan. This free online other calculator runs entirely in your browser — no signup, no data sent anywhere.
Inputs
Results
How It Works (Formula & Method)
Paintable area = perimeter × height − (doors × 21 ft²) − (windows × 15 ft²). Gallons needed = paintable area × number of coats ÷ coverage per gallon. Round up to the next whole gallon when buying — partial gallons are wasted.
Worked Example
Below is a worked example using the calculator's default values. The same numbers are pre-filled in the form above so you can press Calculate and see the result without typing anything.
Inputs used:
- Total Wall Perimeter (ft): 60
- Wall Height (ft): 9
- Number of Doors: 2
- Number of Windows: 4
- Coats of Paint: 2
- Coverage per Gallon (ft²): 350
With these inputs, the calculator computes the metrics shown in the Results panel. Change any value and press Calculate again to see how the result responds — the live widget and the chart both update instantly.
About the Paint Calculator
Buying too little paint means an extra trip to the store and the risk of slightly different batches (which can show as visible streaks). Buying too much is wasteful — paint is expensive and disposing of leftover paint is a hassle. This calculator gives you a realistic estimate for an interior room based on a standard formula contractors use.
How to Use This Calculator
Add up the total wall perimeter (walk around the room and sum the wall lengths in feet). Enter the ceiling height. Count the number of standard doors and windows — the calculator subtracts a standard area for each. Choose how many coats you plan; primer counts as a coat. Coverage per gallon is usually 350–400 square feet for a typical latex paint on a smooth surface.
Tips & Considerations
- Two coats are standard. One coat is rarely enough for color changes; primer plus two coats is best when going from dark to light.
- Textured walls (stucco, popcorn ceilings, rough plaster) need 10–20% more paint than smooth drywall.
- Always buy paint in the same batch for a single project — slight batch-to-batch color variations are common.
- Add a quart or two for trim, baseboards, and touch-ups even if you do not plan to paint them yet.
Frequently Asked Questions
How do I measure perimeter?
Walk around the room and add up the length of each wall. A 12×10 ft room has a perimeter of 44 ft.
What about the ceiling?
Ceiling area = room length × width. Add that to wall area, then apply the coats and coverage formula.
Do I need primer?
Yes when painting new drywall, over stained surfaces, or when going from a dark color to a light color. Otherwise modern paint+primer products skip the separate step.
Why is my paint not covering well?
Common causes: under-mixed (paint settles), poorly primed surface, very glossy underlying paint, or too-thin coats. Two thin coats always beat one thick coat.