Flooring Calculator
Refreshing the floor along with the walls? Estimate how much hardwood, laminate, vinyl, carpet, or tile you need next.
Calculate Flooring NeededPaint Calculator
Work out exactly how much paint you need for your next project. Enter your room dimensions, doors, windows, and number of coats — the calculator instantly figures out the paintable area and how many gallons (or litres) to buy.
Estimated Paint Needed
— gallons
Total wall area—
Door area—
Window area—
Paintable area—
Number of coats—
Total coverage required—
How this calculator works
The paint calculator adds up the total surface area of your walls, subtracts the space taken up by doors and windows (since you don't paint those), then multiplies by how many coats you plan to apply.
1. Total wall area — for each room, we take the perimeter of the room and multiply it by the wall height, then multiply by the number of rooms:
Total Wall Area = 2 × (Length + Width) × Height × Number of Rooms
2. Door area — the space taken up by all your doors:
Door Area = Number of Doors × Door Width × Door Height
3. Window area — the space taken up by all your windows:
Window Area = Number of Windows × Window Width × Window Height
4. Paintable area — the actual surface you'll be painting:
Paintable Area = Total Wall Area − Door Area − Window Area
5. Total coverage required — multiply by the number of coats you're applying (most walls need 2 coats):
Total Coverage Required = Paintable Area × Number of Coats
6. Paint needed — divide by how much area one gallon (or litre) of paint covers:
Paint Needed = Total Coverage Required ÷ Coverage per Gallon (or Litre)
The result is rounded up to the nearest whole gallon or litre, since paint is sold in fixed container sizes and it's always better to have a little extra for touch-ups.
Example calculation — using this calculator's default values: one 12 ft × 10 ft room with 8 ft walls, one 3 ft × 6.7 ft door, one 3 ft × 4 ft window, 2 coats, and 350 sq ft of coverage per gallon:
Total Wall Area = 2 × (12 + 10) × 8 × 1 = 352 sq ft
Door Area = 1 × 3 × 6.7 = 20.1 sq ft
Window Area = 1 × 3 × 4 = 12 sq ft
Paintable Area = 352 − 20.1 − 12 = 319.9 sq ft
Total Coverage Required = 319.9 × 2 = 639.8 sq ft
Paint Needed = 639.8 ÷ 350 = 1.83 → rounded up to 2 gallons
With these inputs, the calculator recommends buying 2 gallons — enough to cover both coats with a small margin left over for touch-ups.
Factors That Affect Your Paint Estimate
Paint coverage rating
Coverage varies by brand, finish, and paint quality — most cans list a specific coverage rating (often 250–400 sq ft per gallon) on the label. Use that number in the "coverage" field instead of the default for a more accurate result.
Surface porosity and prior condition
Unpainted drywall, plaster, masonry, or previously unpainted surfaces absorb more paint than a smooth, already-painted wall. Porous or textured surfaces reduce effective coverage, so you may need more paint than this estimate suggests.
Color change
Going from a dark wall to a light color (or vice versa) often requires an extra coat or a tinted primer beyond what's entered here, since this calculator only accounts for the number of coats you specify.
Application method
Rollers, brushes, and sprayers all use paint at different rates. Sprayers in particular tend to use more paint due to overspray, so factor that into your purchase if you're spraying rather than rolling.
Door and window accuracy
This calculator subtracts a single average size for all doors and all windows. If your doors or windows vary significantly in size, your paintable area — and therefore your paint total — will be slightly under- or overestimated.
Tips for a More Accurate Estimate
Always round up, never down
Paint is sold in fixed container sizes, so there's no way to buy "1.8 gallons." Rounding up avoids a mid-project trip back to the store — and mismatched paint batches can have slightly different tints.
Buy from the same batch or lot number
If you need more than one can, try to buy them from the same production batch, or have the store "box" (mix) multiple cans together, so subtle color variations between batches don't show up on the wall.
Account for primer separately
This calculator estimates topcoat paint only. If you're priming bare drywall, a dramatic color change, or a stained surface, budget for primer as a separate line item using the same paintable area.
Measure irregular rooms wall by wall
This calculator assumes a simple rectangular room. For L-shaped rooms, alcoves, or rooms with very different wall lengths, it's more accurate to measure and calculate each wall individually and add the totals together.
Limitations of This Estimate
This calculator provides a planning estimate, not an exact purchase quantity. A few things it can't account for: the exact coverage rating of your specific paint product, the porosity and condition of your walls, texture (stucco or heavily textured walls absorb more paint), and paint lost to spills, tray residue, or spraying overspray.
It also assumes every room you enter shares the same dimensions and that all doors and windows are the same average size. For projects with significantly different rooms, run the calculator once per room for a more precise total. When in doubt, buying slightly more than the estimate is safer than running short mid-project.
Frequently Asked Questions
Most standard interior paints cover between 300–400 square feet (about 28–37 square meters) per gallon for a single coat. Check your specific paint can for its exact coverage rating and adjust the "coverage" field above.
Yes. Doors and windows aren't painted the same way as walls, so subtracting their area gives you a more accurate paintable area and prevents you from buying more paint than you need.
Two coats is standard for most walls, especially when changing colors or painting over a darker shade. A single coat may be enough for touch-ups or when using a paint-and-primer product on a similar existing color.
Yes — this calculator automatically rounds up to the nearest whole gallon or litre so you don't run out mid-project. Buying a little extra also ensures you have matching paint for future touch-ups.
Yes, significantly. Flat and matte finishes often cover slightly more area than glossier finishes like semi-gloss or high-gloss, and coverage varies by manufacturer and product line. Always check the coverage rating printed on your specific paint can and enter it in the "coverage" field for the most accurate result.
No — this calculator only estimates wall area. If you're also painting the ceiling, calculate its square footage separately (room length × room width), then divide by your coverage rate and multiply by the number of coats to get the extra paint needed.
Set "Number of rooms" to 1 and enter just that single wall's length and height as the wall length and width — since an accent wall is one surface, treat it as a small room with only one long wall for a rough estimate, or measure the exact wall area and divide by your coverage rate.
It's a good idea to keep at least a small amount left over (even a quart) after your project, properly sealed and labeled with the color name and code, so you can touch up scuffs or repairs later without needing to color-match again.
Yes. Covering a dark wall with a light color (or vice versa) often requires an extra coat beyond the two this calculator defaults to. If you're making a dramatic color change, consider setting "Number of coats" to 3, or use a tinted primer first.
This calculator assumes a standard flat-ceiling rectangular room. For vaulted or sloped ceilings, use the average wall height as a rough approximation, or measure the actual wall area (including the triangular sections under the slope) and divide by your coverage rate for a precise number.
Primer and paint are typically sold separately and have their own coverage ratings, though they're often similar (around 300–400 sq ft per gallon). If your project needs primer, calculate it as its own line item using the same paintable area from this calculator.
The math works the same way, but exterior surfaces (stucco, brick, wood siding) are typically more porous and textured than interior drywall, so real-world coverage is often lower. Use your exterior paint's specific coverage rating rather than the default, and expect to round up more generously.
Related Calculators
Tile Calculator
Tiling a bathroom or kitchen backsplash in the same project? Figure out how many tiles and boxes you'll need.
Calculate Tiles NeededFence Calculator
Painting or staining a fence too? Estimate posts, rails, and pickets or boards for your fence project.
Calculate Fence Materials