Paint Coverage & Cost Calculator
What this paint coverage & cost calculator does
This calculator estimates how much paint you need and how much it will cost to paint the interior walls of a simple rectangular room. By entering your room dimensions, number of doors and windows, desired number of coats, paint coverage per gallon, and price per gallon, you get a quick, budget-ready estimate.
- Inputs you provide: room length, width, and ceiling height; number of doors and windows; number of coats; coverage per gallon; cost per gallon.
- Outputs you see: total paintable wall area, exact gallons needed, recommended gallons to buy (rounded up), and estimated paint cost.
Use these results to compare brands, plan store trips, or discuss options with a contractor.
How the paint coverage formula works
The calculator treats your room as a simple box and focuses on wall surfaces only. It starts with the perimeter of the room, multiplies by the ceiling height to get total wall area, then subtracts standard areas for each door and window. Finally, it accounts for the number of coats and divides by your paint coverage rate.
Step 1: Wall area
The basic wall area is the perimeter times the height:
where:
- L = room length (ft)
- W = room width (ft)
- H = ceiling height (ft)
Step 2: Subtract doors and windows
The tool assumes standard sizes of approximately 20 ft² per door and 15 ft² per window. It subtracts these from the wall area:
where:
- D = number of doors
- W = number of windows (different from width above; context determines meaning)
Step 3: Account for number of coats
Each additional coat multiplies the paintable area:
where C is the number of coats and T is the total area to be painted in square feet.
Step 4: Gallons of paint required
Divide the total area by the coverage per gallon to get the exact gallons of paint required:
where Cov is the coverage in square feet per gallon, usually listed on the paint can (for example, 300–400 ft²/gal).
Step 5: Paint cost
Finally, the calculator multiplies gallons by the cost per gallon:
where P is the price per gallon, and
Worked example: bedroom paint estimate
Imagine you are painting a bedroom with the following characteristics:
- Length: 12 ft
- Width: 15 ft
- Ceiling height: 8 ft
- Doors: 1 standard door
- Windows: 2 standard windows
- Number of coats: 2
- Coverage: 350 ft² per gallon
- Cost per gallon: $40
1. Wall area
Perimeter = 2 × (L + W) = 2 × (12 + 15) = 2 × 27 = 54 ft.
Wall area A = perimeter × height = 54 × 8 = 432 ft².
2. Subtract doors and windows
Door area = 1 × 20 = 20 ft².
Window area = 2 × 15 = 30 ft².
Adjusted single-coat wall area A' = 432 − 20 − 30 = 382 ft².
3. Account for coats
Total paintable area T = A' × coats = 382 × 2 = 764 ft².
4. Exact gallons needed
G = T ÷ coverage = 764 ÷ 350 ≈ 2.18 gallons.
Exact gallons (mathematical) ≈ 2.18.
Recommended gallons (rounded up) = 3 gallons to allow for waste and touch-ups.
5. Estimated cost
Estimated cost = recommended gallons × price per gallon = 3 × $40 = $120.
In this example, the calculator would report roughly:
- Paintable area: about 764 ft²
- Exact gallons: ~2.18
- Recommended gallons to buy: 3
- Estimated cost: ~$120
How to interpret your results
When the calculator returns your results, you will typically see four key numbers. Here is how to use them:
- Paintable area: The total square footage of wall surface the tool assumes you will paint, after subtracting standard doors and windows and multiplying by the number of coats.
- Exact gallons: The precise mathematical amount of paint required if application were perfectly efficient and cans came in fractional sizes.
- Recommended gallons: A rounded-up value, giving you a practical number of whole gallons to buy so you do not run short.
- Estimated cost: Paint cost only, based on the recommended gallons and your price per gallon.
Most people should base their purchase on the recommended gallons, not the exact gallons number. The difference between these numbers is a safety margin that helps cover roller absorption, tray losses, minor mis-measurements, and future touch-ups.
Typical coverage rates and when to adjust them
The default coverage of 350 ft² per gallon is a reasonable average for many interior paints on smooth, previously painted walls. However, you should adjust this value to reflect your specific project. You can usually find the manufacturer’s recommended coverage range on the back of the paint can or in the product data sheet.
| Surface / situation | Typical coverage (ft²/gal) | Suggested adjustment |
|---|---|---|
| Smooth, previously painted drywall in similar color | 325–400 | Use 350–400 if walls are in good condition. |
| New drywall without primer | 250–325 | Use a lower coverage (e.g., 275) or plan for extra coats. |
| Dark color being covered by light color | 225–300 | Reduce coverage and/or increase the number of coats. |
| Textured or rough surfaces (e.g., masonry, heavy orange peel) | 150–250 | Use a much lower coverage and expect higher paint use. |
| Ceilings with flat paint | 275–350 | Similar to walls, but may vary with texture and color. |
If your project does not fit neatly into one of these categories, choose a middle-of-the-road coverage value, run the calculation, then re-run with slightly higher and lower coverage numbers to see a best-case and worst-case range.
Assumptions & limitations of this calculator
This tool is designed as a fast planning aid, not an engineering-grade estimator. It makes several assumptions that you should be aware of when interpreting the output:
- Simple rectangular rooms: The calculator assumes four straight walls forming a rectangle. L-shaped rooms, bay windows, alcoves, and angled ceilings are not modeled directly.
- Walls only by default: Estimates are based on wall area. Ceilings, trim, baseboards, doors, and built-ins are not explicitly included. If you want to include the ceiling, you can manually add its area (length × width) to your wall area and adjust your coverage estimate, or run a separate calculation using the ceiling dimensions as a simplified "wall".
- Standard door and window sizes: Each door is treated as 20 ft² and each window as 15 ft². Oversized patio doors, picture windows, or very small openings are not accounted for precisely. You can compensate by entering a slightly higher or lower door/window count to approximate the actual area.
- No separate primer calculation: The calculator assumes you are estimating finish paint only. If you plan to use primer, you may want to add an additional coat in the coats field or run a separate estimate for primer using a coverage value from the primer can.
- Uniform surface condition: It assumes similar coverage across all walls. In reality, patched, stained, or highly textured sections may absorb more paint than smoother areas.
- Even application: The math does not account for technique (roller vs. sprayer vs. brush), spillage, or heavy application in some spots. The recommended gallons value is intentionally rounded up to create a buffer for these effects.
- Paint only, not full project cost: The cost estimate includes paint only. It does not include primer, supplies (rollers, tape, drop cloths), or labor.
Because of these limitations, consider the calculator’s output a useful estimate rather than an exact prediction. If your room or surfaces are complex, you may want to add an extra 10–20% to the estimated paint volume as a contingency.
Practical tips and FAQs
How many coats of paint do I need?
For most interior walls that are already painted a similar color, two coats of quality paint provide good coverage and durability. New drywall often needs primer plus two coats. Covering a very dark color with a light one may require a tinted primer and two finish coats.
Should I include the ceiling in my estimate?
This calculator is focused on walls, but you can approximate ceiling paint by treating the ceiling as an additional surface. Use length × width to get ceiling area, choose an appropriate coverage rate, and either add that to your total or run a separate calculation. Remember that ceiling paint is often flat and may have slightly different coverage than wall paint.
Where do I find coverage (sq ft per gallon)?
Look on the paint can label or the product page online. Most manufacturers list a coverage range, such as 300–400 ft² per gallon. Use the lower end for rough or unprimed surfaces and the higher end for smooth, previously painted walls.
What if my doors or windows are not standard size?
If you have very large glass sliders or tall windows, treat them as multiple standard units. For example, a large patio door might be roughly equivalent to two doors. The goal is to roughly match the total unpainted area so the remaining wall area is reasonably accurate.
How can I refine the estimate for a complex room?
For rooms with multiple sections, you can break the space into simpler rectangles, calculate wall area for each section separately, and then add them together. You may also want to increase the number of coats or reduce the coverage rate to build in a margin for error.
Using your estimate to plan your project
Once you have your paintable area, gallons, and cost estimate, you can use the numbers to plan a realistic paint budget. Compare the cost of different paint lines, decide whether you need one large can plus a smaller one, and schedule your shopping so you have everything on hand before you start painting.
If your exact gallons value is very close to a whole number (for example, 2.05 gallons), you may be comfortable rounding down if you are an experienced painter and your walls are in good shape. If you are unsure, or if your surfaces are rough or patched, rounding up to the recommended gallons is usually safer and avoids mid-project trips back to the store.