Paint Coverage Calculator

How to Use This Paint Coverage Calculator

This paint coverage calculator estimates gallons and optional cost for a rectangular room, with the ceiling included only when you choose it. The workflow is quick, but each field affects the final purchase recommendation.

1. Measure room length and width in feet. Use a tape measure along the floor from wall to wall. Enter these as Room Length and Room Width.
2. Measure wall height from finished floor to finished ceiling and enter this as Wall Height.
3. Estimate door and window area. Add up the square footage of all doors and windows you will not paint and enter the total as Door/Window Area.
4. Choose whether to include the ceiling by checking or unchecking the Include Ceiling box.
5. Set the number of coats. Most repaint jobs use 2 coats; new drywall or drastic color changes may need 3 or more.
6. Enter coverage per gallon. If you are not sure, keep the default value, or refer to the coverage table below based on your paint finish.
7. Optionally add a price per gallon to estimate total paint cost.
8. Click Calculate to see the total coated area, exact gallons required, a rounded-up recommendation, and, if provided, an estimated cost.

All calculations run directly in your browser. No room dimensions or prices are sent to a server, so you can adjust the inputs as often as needed while you measure, budget, and compare paint options.

How the Paint Coverage Calculator Works

The calculator treats the job as a simple rectangular room with four vertical walls and an optional flat ceiling. It measures the paintable surface first, then multiplies that surface by the number of coats, and finally divides by the chosen coverage rate to estimate gallons.

Wall and ceiling area formulas

Let:

• = room length (ft)
• = room width (ft)
• = wall height (ft)
• = total area of doors and windows to exclude (sq ft)

The combined area of the four walls is:

If you include the ceiling, its area is simply:

The total paintable area before coats is then:

If you leave the ceiling unchecked, the calculator uses only the wall area and subtracts openings. After that, it converts the one-coat area into the total coated area used for the paint estimate:

Here, is the total square footage that will actually receive paint across every coat, and is the number of coats.

From area to gallons and cost

Manufacturers publish a typical coverage value such as 300-400 sq ft per gallon under ideal conditions. Let:

• = coverage per gallon (sq ft)
• = number of coats
• = price per gallon

The exact gallons required are:

Because paint is sold in discrete cans, you rarely buy a fractional gallon. The calculator therefore also shows a rounded practical value:

Recommended gallons = ceil(Exact gallons)

If you enter a price per gallon, the tool multiplies the recommended gallons by that price to estimate material cost:

Estimated cost = Recommended gallons × P

In practical terms, the estimate is driven by four inputs: the room size, the square footage you exclude, the coat count, and the coverage printed on the product label. A change in any one of those values can move the result enough to matter when you are deciding what to buy.

Interpreting Your Paint Coverage Results

When you click Calculate, the results area shows both a short summary sentence and a small table. The outputs are easiest to use when you read them in the same order the calculator presents them.

• Total area across coats — the total square footage that will actually receive paint after subtracting openings and then multiplying by the number of coats. This is the coverage demand used in the gallons estimate.
• Exact gallons — the mathematical result of dividing the total coated area by your coverage-per-gallon value.
• Recommended gallons — the exact gallons rounded up to the next whole gallon to reflect how paint is normally purchased.
• Estimated cost — an approximate materials budget based on the price per gallon you entered.

Most people use the recommended gallons when they are buying paint, because a paint job rarely behaves like a perfect formula. Coverage shifts with roller technique, how evenly the wall was prepared, how drastic the color change is, and whether small touch-ups are likely afterward. If your exact result is close to a whole number, the rounded recommendation may be enough on its own. If the number is close to the top of the gallon range, buying a little extra can reduce the risk of running short before the last coat is finished.

Typical Coverage by Paint Finish

Use the following table as a starting point for the Coverage per Gallon input if your paint can does not clearly state a value. These are planning ranges, not guarantees.

Always check the specific product label. If the manufacturer provides a range, choose a value near the lower end for rough, unprimed, or highly absorbent surfaces, and use a higher value only when conditions are smooth and favorable.

Worked Example: Single Room with Ceiling

Consider a 15 ft by 20 ft room with 8 ft ceilings. You plan to paint the four walls and the ceiling. There are two doors totaling 42 sq ft and two windows totaling 30 sq ft, and you will not paint those areas. You intend to apply 2 coats of eggshell paint rated at 350 sq ft/gal, priced at $32 per gallon.

1. Compute wall area: A_walls = 2 × H × (L + W) = 2 × 8 × (15 + 20) = 560 sq ft
2. Compute ceiling area: A_ceiling = L × W = 15 × 20 = 300 sq ft
3. Add wall and ceiling area, then subtract openings: the total door and window area is O = 42 + 30 = 72 sq ft, so A = 560 + 300 - 72 = 788 sq ft.
4. Apply coats: across two coats, T = 788 × 2 = 1576 sq ft. This is the total coated area the calculator uses in the first result row.
5. Convert to gallons: Exact gallons = 1576 / 350 ≈ 4.5 gallons.
6. Round for purchasing: the recommendation becomes 5 gallons.
7. Estimate cost: 5 × $32 = $160.

This example shows why the number of coats matters so much. The room itself is not 1,576 square feet, but the paint still has to cover that much surface once you count both passes.

Exact vs. Recommended Gallons

The calculator separates the raw mathematical result from the value you are likely to buy in the store. That distinction matters because paint is sold in whole cans, but the needed coverage often lands on a fractional number.

For smaller projects, it can be helpful to keep a little extra paint for future touch-ups, especially if the color was custom mixed. That reserve is often more useful than saving a few dollars by buying too little and trying to match the color later.

Assumptions and Limitations for Paint Coverage

This calculator is intentionally simple so you can get a room estimate quickly. The tradeoff is that it assumes a fairly regular room shape and a straightforward painting plan, which means some projects will need extra judgment.

• Rectangular room only: The calculation assumes a simple rectangle with four straight, vertical walls and a flat ceiling. It does not directly model L-shaped rooms, angled walls, or alcoves.
• Uniform wall height: All walls are assumed to be the same height. Vaulted ceilings, stairwells, and sloped ceilings need separate treatment.
• Single room per calculation: The tool is designed for one room at a time. For larger jobs, run one estimate per room and combine the results.
• Openings entered as area: Doors and windows are handled as one combined square-footage input rather than individual objects.
• Trim and specialty surfaces excluded: Baseboards, cabinets, crown molding, doors, trim, and built-ins are not included unless you estimate them separately.
• Coverage rate is an estimate: Real coverage varies with surface texture, porosity, application method, color change, and weather or indoor conditions.
• Primer is separate: If you plan to prime, treat that as an additional coating step or estimate it on its own with the same area logic.
• Measurement error is real: Small measuring mistakes are common. Rounding up helps, but complex rooms still benefit from double-checking.

Within those limits, the estimate is usually good enough for budgeting and shopping. For expensive paint, specialty finishes, or unusual surfaces, use this calculator as a first pass and then confirm the label details with the paint supplier or product packaging.

Practical Tips for Better Paint Coverage Estimates

Accurate paint planning usually depends more on careful measuring than on complicated math. A few small habits can improve your estimate quickly:

• Measure to the nearest inch and convert to feet when possible. A room listed as 12 ft 6 in should be entered as 12.5 ft.
• Use realistic opening sizes if you do not want to measure every door and window. A common interior door is roughly 20-21 sq ft, while many windows fall near 10-15 sq ft.
• Be conservative with coverage on rough, repaired, unprimed, or highly porous walls. Lower coverage assumptions usually produce fewer surprises.
• Increase coats when color change is dramatic. Painting light over dark, or dark over light, often needs more material than a same-color refresh.
• Keep a little surplus for touch-ups. A small reserve is especially helpful for high-traffic rooms, custom tints, and kids' spaces.

Paint Coverage Calculator Frequently Asked Questions

How many square feet does a gallon of paint cover?

Most interior wall paints cover between 300 and 400 sq ft per gallon under ideal conditions. Rough, unprimed, or very dry surfaces can reduce coverage, while smooth, previously painted walls can improve it. Always use the figure on the paint can as your primary reference.

How much extra paint should I buy?

Beyond the calculator’s recommended gallons, many painters add about 10% extra for touch-ups and future repairs, especially for custom colors. For small rooms, this might mean buying an additional quart; for larger projects, adding a full gallon can be reasonable.

Does the calculator account for primer?

No. Primer is not included in the calculation. If your project requires primer, you can either treat it as an additional coat with a lower coverage rate or estimate it separately using the same surface area.

Can I use this for exterior walls?

You can use the same basic approach for exterior walls, but exterior surfaces often have different coverage rates and weathering considerations. Check the exterior paint’s stated coverage and be conservative in your assumptions.