Paint Coverage Calculator

How to Use This Paint Coverage Calculator

This calculator estimates how many gallons of paint you need for a rectangular room and, if you like, the approximate material cost. It is designed mainly for interior walls and an optional flat ceiling, so the steps are straightforward and fast.

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 compare as many scenarios as you want while you measure, budget, and plan.

How the Paint Coverage Calculator Works

The calculator models a simple rectangular room with four vertical walls and an optional flat ceiling. It first finds the base paintable area, then multiplies that area by your selected number of coats, and finally divides by the coverage rate to estimate gallons.

Wall and ceiling area formulas

Let:

• $L$ = room length (ft)
• $W$ = room width (ft)
• $H$ = wall height (ft)
• $O$ = 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, $T$ is the total square footage that will actually receive paint across every coat, and $n$ 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:

• $C$ = coverage per gallon (sq ft)
• $n$ = number of coats
• $P$ = 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 plain language, this means the result is driven by four levers: the size of the room, the amount of area you exclude, how many coats you want, and how efficiently the paint covers. Changing any one of those inputs can noticeably change the gallons recommendation.

Interpreting Your Results

When you click Calculate, the results area shows both a quick summary sentence and a small table. The numbers are easiest to use when you read them in order.

• 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 make their shopping decision from the recommended gallons rather than the exact gallons. That extra rounding margin is practical because real walls are rarely perfect, roller technique varies, and touch-ups almost always happen. If your exact value is close to a whole number, rounding up may be plenty. If the number sits near the upper end of a gallon, a little extra paint can save hassle later, especially for custom colors.

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 tool intentionally separates the raw mathematical result from a more practical buying recommendation. That helps you understand the calculation without confusing the math with the way paint is actually sold.

For small jobs, many homeowners still like to keep a little extra paint on hand for future repairs. That is especially sensible when a color was custom mixed and may be hard to match exactly later.

Assumptions and Limitations

This calculator is intentionally simple and fast. To stay simple, it makes a few assumptions. Keeping those assumptions in mind helps you decide when the estimate is close enough and when the project deserves a more detailed takeoff.

• 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 boundaries, the estimate is usually strong enough for planning, budgeting, and buying. For high-cost paints, specialty finishes, or unusual surfaces, use the calculator as a solid first pass and then confirm the plan with the product label or your paint supplier.

Practical Tips for Better Estimates

A careful estimate is usually more about good measuring habits than complicated math. These small habits improve results 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.

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.