Paint Calculator
Estimate how many gallons of paint you need for one room or multiple rooms. Enter room dimensions, doors, windows, coats, paint coverage, and price per gallon to get an instant material and cost estimate.
How to Use the Paint Calculator
Measure each room
Enter the room length, width, and wall height. For most homes, standard wall height is around 8 feet, but measure if you are unsure.
Subtract doors and windows
The calculator subtracts a standard 21 square feet for each door and 15 square feet for each window.
Choose coats and coverage
Two coats are selected by default. Adjust coverage per gallon if your paint can lists a different number.
Calculate gallons and cost
The result rounds up to whole gallons because paint is normally purchased by the gallon.
Common Paint Coverage Estimates
| Surface / Paint Type | Typical Coverage | Notes |
|---|---|---|
| Interior wall paint | 350–400 sq ft per gallon | Best for smooth, previously painted walls |
| Primer | 250–350 sq ft per gallon | Raw drywall and porous surfaces use more primer |
| Textured walls | 250–350 sq ft per gallon | Texture increases paint absorption |
| Ceiling paint | 350–400 sq ft per gallon | Usually flat finish |
| Exterior paint | 250–400 sq ft per gallon | Coverage depends heavily on siding material |
Paint Estimating Tips
Smart Buying Tips
- Buy all paint for the same room at the same time to avoid slight color differences between batches.
- Use primer when covering dark colors, raw drywall, stains, or glossy surfaces.
- Keep leftover paint for future touch-ups and label the can with the room name.
- Round up to the nearest gallon if you are close. Running out mid-project can create color matching problems.
Common Mistakes
- Forgetting the second coat.
- Using the advertised coverage number without considering wall texture.
- Not subtracting large doors and windows.
- Forgetting ceilings when the project includes ceiling paint.
Paint Calculator FAQ
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Paint Calculator: Complete Buying Guide
Painting is one of the most common home improvement projects, but estimating paint can be frustrating because every room has different dimensions, doors, windows, ceiling height, surface texture, and number of coats. Buying too little paint can stop the project halfway through. Buying too much can waste money, especially when using premium paint.
This paint calculator helps estimate how many gallons you need by using the actual room dimensions instead of a rough guess. It calculates the total wall area, subtracts standard doors and windows, optionally adds ceiling area, multiplies by the number of coats, applies a waste factor, and divides the final area by your paint coverage per gallon.
Why Coverage Per Gallon Matters
Most interior wall paint lists coverage somewhere around 350 to 400 square feet per gallon. That number is usually based on one coat over a smooth, properly prepared surface. Rough plaster, orange peel texture, raw drywall, brick, concrete block, and major color changes can reduce actual coverage. If the surface is porous or uneven, use a lower coverage estimate.
Why Two Coats Are Usually Better
One coat may look acceptable when painting a similar color over a clean wall, but two coats usually produce better coverage, stronger color, and a more even finish. Two coats are especially important when changing from dark to light, light to dark, or when using deep accent colors.
Doors, Windows, and Ceilings
Doors and windows reduce the amount of wall paint needed. This calculator subtracts standard estimating amounts for each opening. Ceilings are optional because many projects only involve walls. If you plan to paint the ceiling, include it in the estimate so the final gallon count reflects the full job.
Final Buying Advice
The calculator gives a practical estimate, but the final number should be checked against the paint can label. When the estimate is close to the next gallon, rounding up is usually safer. Leftover paint is useful for touch-ups, especially in high-traffic rooms, hallways, rentals, and children’s bedrooms.