Paint Calculator

About the Paint Calculator

Buying paint is a small decision with an annoying penalty for getting it wrong. Too little and you are making a second trip mid-project, hoping the store can match a batch that has already started to cure on your walls. Too much and you are storing half-full cans for years. This calculator estimates the volume you need for the walls of one rectangular room by working out the surface area, taking away the openings that will not be painted, and dividing by how far a gallon or liter of paint spreads.

The model is straightforward. Wall area is the room's perimeter multiplied by the wall height. From that it subtracts a fixed 20 square feet for every door and 15 square feet for every window, multiplies the remainder by the number of coats, and divides by the coverage rate on the can, which defaults to 350 square feet per gallon or 10 square meters per liter. It then rounds up to the nearest quarter gallon or half liter. Because the output follows from the inputs you control, you can see exactly how changing a coat count or a coverage figure shifts the amount.

That transparency is useful for homeowners repainting a bedroom, renters planning a weekend refresh, and contractors giving a quick ballpark before a site visit. It estimates walls only. Ceilings, trim, baseboards, closets, and any primer are separate calculations. The result is best treated as a well-informed shopping number rather than an exact figure, since porous drywall, texture, and dramatic color changes can all make real consumption differ from the label coverage.

When to Use It

How to Use It, Step by Step

1. Select U.S. Customary or Metric at the top. The inputs relabel to feet or meters, and the coverage field switches between square feet per gallon and square meters per liter. Changing the setting after typing does not convert your numbers.
2. Measure the Room length and Room width along the floor, wall to wall. Only the four walls matter here, so a rough rectangle is fine. Use the longest dimensions for an L-shaped room and calculate each section separately for better accuracy.
3. Enter the Wall height from floor to ceiling. Standard rooms are often around 8 feet, but older houses, basements, and rooms with vaulted ceilings can be quite different, so measure rather than assume.
4. Enter the Number of doors and Number of windows. The tool subtracts 20 square feet for each door and 15 for each window. If your openings are much larger or smaller than typical, adjust the final volume by hand.
5. Choose the Number of coats from the drop-down: 1, 2, or 3. Two coats is the normal choice for a color change or for new drywall, while one coat can suffice when repainting a similar color with good-quality paint.
6. Check the Paint coverage field. It starts at 350 square feet per gallon or 10 square meters per liter. Replace it with the number printed on your actual can, since coverage differs between brands and finishes.
7. Click Calculate Paint Needed. Read the wall area, the total paintable area including coats, the exact paint required, and the Recommended purchase, which is rounded up to the nearest quarter gallon or half liter.
8. Decide on your purchase. Because stores sell in fixed can sizes, translate the rounded volume into the combination of cans closest to it, and add leftover for touch-ups if your painting plan allows.

Understanding Your Results

You will see four lines. Wall area (net of doors and windows) is the single-coat surface you are covering. Total paintable area multiplies that by the coats you chose. Paint needed is the exact, unrounded volume at your coverage rate. Recommended purchase is the figure to buy, rounded up to a quarter gallon in U.S. mode or half a liter in metric. The metric result also gives an approximate U.S. gallon equivalent for reference.

To sanity-check the outcome, remember a rule of thumb: a gallon of typical wall paint covers somewhere in the range of 300 to 400 square feet per coat. A medium bedroom with two coats usually lands between one and two gallons, and a living room or open plan space can require several. If your answer is wildly outside that scale, recheck that the height and length are in the correct unit and that you have not typed a dimension in inches.

Next, convert the recommended purchase into real cans. Paint is typically sold in quarts, gallons, and larger pails, so choose the smallest combination that equals or exceeds the number. Consider buying a bit extra when the color is custom-mixed, since matching it later can be difficult. If the room has heavy texture, unsealed drywall, or a strong existing color, plan on the upper end of your range or on a separate primer.

More Worked Examples

Tips for Better Results

• Use the coverage number printed on your paint can instead of the default, and use the figure for one coat. Premium paints, primers, and specialty finishes can differ noticeably from the generic 350 square feet per gallon.
• Estimate ceilings separately by multiplying length by width, then divide by the same coverage rate. Ceilings often take a different, flat paint, so the volume should be considered as its own purchase, not an add-on to the wall total.
• For rooms with unusual shapes, break them into rectangles. Run the calculator for each section and add the results, which is more accurate than forcing an L-shaped or alcove room into a single length and width.
• When a room has big windows, glass doors, or built-in shelves, measure their actual area and subtract the difference from the standard deduction. A wall of glass can free up many square feet that the default would still count as paint.
• Buy slightly more than the rounded recommendation for dark-to-light changes. The extra covers an additional coat or a patch of poor coverage, and it avoids the problem of trying to match a custom tint from a different batch later.
• Mix all cans of the same color together in one larger container before you start. Slight variations between cans are common, and blending them, sometimes called boxing, keeps the color uniform across the whole room.

Common Mistakes to Avoid

• Measuring in the wrong unit. Mixing inches with feet, or typing a ceiling height in centimeters while in U.S. mode, gives an answer that looks plausible but is far off. Confirm the unit labels before you read the result.
• Forgetting to count each wall surface. The perimeter formula assumes four full walls. If a wall is covered with tile or paneling, or you are only painting an accent wall, the room-based figure will be too high, so calculate those cases separately.
• Relying on one coat after a big color change. New drywall and dramatic shifts, such as dark red to white, rarely finish in one coat, and underestimating coats is the most common reason people end up short.
• Ignoring primer and ceilings. This estimate only covers finish paint on the walls, so budget primer, ceiling paint, and trim paint as their own quantities to avoid surprises at the checkout.
• Trusting the default coverage for rough surfaces. Textured walls, raw wood, and porous masonry absorb more paint, so the real coverage can be well under the number on the can. Increase the estimate for such surfaces.

Assumptions and Limits

• Door and window deductions are fixed averages, 20 square feet per door and 15 per window, regardless of their actual size. This is reasonable for standard interior openings but will misstate the area for French doors, picture windows, or tiny casements.
• The estimate treats every wall as a flat rectangle of the same height. Vaulted ceilings, gables, stair walls, built-in cabinetry, and recesses are not modeled, so rooms with such features need a manual adjustment or a separate calculation.
• Coverage is applied evenly to the area of each coat. In practice, the first coat on bare or porous surfaces uses more paint, and rolling technique, tools, and application thickness all affect consumption. A can's stated coverage assumes ideal conditions.
• The metric notes always describe a 10 square meter per liter standard, even if you change the coverage value, but the calculation itself uses the number you enter. For large jobs or professional estimates, measure carefully and consult the paint manufacturer's data sheet.

Key Terms

Coverage rate

How much surface a given volume of paint spreads over in one coat, usually printed on the can in square feet per gallon or square meters per liter.

Net wall area

The wall surface left to paint after subtracting doors, windows, and other openings from the total perimeter-times-height area.

Primer

A preparatory coating that seals the surface and improves adhesion and color coverage, often used on bare drywall, stains, or before a dramatic color change.

Cutting in

Painting the edges and corners by brush where a roller cannot reach, such as along ceilings, trim, and corners, before rolling the main wall.

Sheen

How glossy a paint finish is, from flat to high gloss. Higher sheens are more washable but show surface flaws more clearly.

Boxing paint

Combining multiple cans of the same color into one container before painting so that small differences between batches do not show on the wall.