Paint Coverage Calculator

How to Use This Paint Coverage Calculator

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

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 your paintable area, exact gallons required, a rounded-up recommendation, and (if provided) an estimated cost.

All calculations run in your browser. No room dimensions or prices are sent to a server, so you can try as many scenarios as you like without sharing any data.

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 total paintable area and then determines how many gallons are required for your chosen number of coats.

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 is then:

If you keep the ceiling unchecked, the calculator just uses the wall area and subtracts openings:

A = A_walls - O

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

The total painted area across all coats is A × n. The exact gallons required are:

Exact gallons = (A × n) / C

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

Recommended gallons = ceil(Exact gallons)

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

Estimated cost = Recommended gallons × P

Interpreting Your Results

When you click Calculate, you will see:

• Paintable area — the total square footage of walls (and optionally ceiling) that will receive paint after subtracting door and window openings.
• Exact gallons — the mathematical result of the area, coats, and coverage values you entered. This is useful for comparing different scenarios.
• Recommended gallons — the exact gallons rounded up to the next whole gallon to reflect how paint is sold in stores.
• Estimated cost (optional) — an approximate material budget based on the price per gallon you entered.

In practice, most people base their purchase on the recommended gallons, possibly adding an extra quart or gallon if they know conditions are challenging (very rough surfaces, painting over a dark color, or expecting future touch-ups).

If your exact gallons are already very close to a whole number (for example, 4.1 gal), you may be comfortable buying exactly the rounded recommendation. If the value is close to a half gallon (for example, 4.6 gal), you may want to consider whether you prefer a bit more safety margin (buy 5 gallons) or whether you can safely reduce coats or improve priming to use less.

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.

Always check the specific product label. If the manufacturer provides a range (for example, 250–400 sq ft/gal), choose a value near the lower end for rough, unprimed, or highly absorbent surfaces, and a higher value for smooth, previously painted walls.

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)

   A_walls = 2 × 8 × 35 = 560 sq ft

2. Compute ceiling area:

   A_ceiling = L × W = 15 × 20 = 300 sq ft

3. Subtract total openings:

The total area of doors and windows is O = 42 + 30 = 72 sq ft.

Total paintable area:

A = A_walls + A_ceiling - O = 560 + 300 - 72 = 788 sq ft

Across two coats, the total painted area is A × n = 788 × 2 = 1576 sq ft.

Exact gallons required are:

Exact gallons = 1576 / 350 ≈ 4.5 gallons

The calculator will also show:

• Exact gallons: approximately 4.5
• Recommended gallons: 5 (rounded up)
• Estimated cost: 5 × $32 = $160

Exact vs. Recommended Gallons

The tool explicitly separates the raw mathematical result from a more practical buying recommendation. This helps you decide how aggressively you want to round.

For small jobs, you may want to add an extra quart or gallon beyond the recommended value if matching color in the future is important or if you expect extensive touch-ups.

Assumptions and Limitations

This calculator is intentionally simple and fast. To keep it that way, it makes several assumptions. Keep these in mind when interpreting the results:

• Rectangular room only: The model assumes a simple rectangle with four straight, vertical walls and a flat ceiling. It does not account for L-shaped rooms, angled walls, or alcoves.
• Uniform wall height: All walls are assumed to be the same height. Vaulted ceilings, sloped ceilings, and stairwells are not modeled directly.
• Single room per calculation: The calculator is designed for one room at a time. For multi-room projects, run a separate calculation for each room and add the recommended gallons or area values.
• Openings entered as area: Doors and windows are handled as a combined square footage. The calculator does not separately model door count, trim width, or casing details.
• Trim and non-wall surfaces excluded: Baseboards, crown molding, doors, windows, radiators, cabinets, and other features are not included. If you plan to paint these, estimate them separately.
• Coverage rate is an estimate: The coverage value you enter is treated as an average. Actual coverage can vary with surface texture, porosity, color, application method (brush, roller, sprayer), and temperature/humidity.
• Primers and specialty coatings: Primer, stain-blocking paints, and very high-build or textured products may cover significantly fewer square feet per gallon than standard wall paint.
• Minor measurement error: Small inaccuracies in measuring length, width, or height are common. The recommended gallons already include some rounding, but you may wish to add a bit of extra margin for complex spaces.
• Interior use focus: The calculator is oriented toward interior walls and ceilings. Exterior surfaces, siding, and masonry often have different coverage characteristics and weather-related considerations.

Within these boundaries, the estimates are typically close enough for planning and budgeting. For high-stakes projects (custom tints, specialty finishes, or tight budgets), double-check measurements and coverage with your paint supplier.

Practical Tips for Better Estimates

• Measure to the nearest inch and convert to feet (for example, 12 ft 6 in = 12.5 ft) for more accurate results.
• Use standard opening sizes if you do not want to measure exactly: a typical interior door is about 20–21 sq ft (e.g., 3 ft × 6 ft 8 in), and a common double-hung window might be 10–15 sq ft.
• Increase coats when painting over dark colors, raw drywall, or large patches; decrease only if you are doing a light touch-up on a similar color.
• Choose conservative coverage values (lower sq ft/gal) for rough, unprimed, or previously unpainted substrates, and slightly higher values for smooth, sealed, previously painted walls.
• Keep a small surplus for touch-ups, especially for accent walls or custom-mixed colors that are hard to match later.

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.