Paint Calculator

A paint calculator works out how much paint you need for a room, a whole house, or an exterior project based on wall dimensions, number of coats, and surface type. It is used by homeowners planning DIY paint projects, professional painters generating client quotes, interior designers specifying materials, and contractors ordering supplies for jobs.

Paint is sold by the gallon (US) or liter (most of the world), and the coverage varies by surface, paint quality, and application method. A typical interior latex paint covers 350-400 square feet per gallon on smooth drywall, but only 250-300 on textured or porous surfaces like brick or stucco. The calculator accounts for these variables to give a realistic estimate that prevents over-buying (waste) or under-buying (extra trips to the store).

How to Use the Paint Calculator

1. Measure each wall's width and height in feet (or meters). Add the areas together.
2. Subtract areas that won't be painted: doors (~20 sq ft each), windows (~15 sq ft each).
3. For ceilings: enter length and width separately.
4. For trim (baseboards, door frames, window frames): enter linear feet.
5. Select the surface type (smooth drywall, textured, plaster, brick, etc.).
6. Select the number of coats (typically 2 for new color over white, 1 for refresh).
7. The calculator outputs gallons needed plus a 10% waste buffer.

Coverage Rates by Surface

These coverage rates are typical for latex paints applied with a roller. Spraying uses 10-20% more paint due to overspray; brushing uses 5-10% more due to less efficient transfer.

The Formula

Total Wall Area = Sum of (Wall Width x Wall Height) for all walls Paintable Area = Total Wall Area - Door Areas - Window Areas Gallons Needed = Paintable Area / Coverage Rate per Gallon x Number of Coats With Waste Buffer = Gallons Needed x 1.10

Example: A 12x14 room with 8-foot ceilings has 4 walls totaling (12+14+12+14) x 8 = 416 sq ft. Subtracting one door (20 sq ft) and two windows (30 sq ft): 416 - 50 = 366 sq ft of paintable wall area. With 2 coats at 400 sq ft/gallon: 366 / 400 x 2 = 1.83 gallons. With 10% buffer: 1.83 x 1.10 = 2.01 gallons. Buy 2 gallons (or 2.5 if you want extra).

Ceiling and Trim Calculations

Ceilings: Length x Width = total sq ft. With popcorn texture, use the lower coverage rate (150-200 sq ft/gallon). Most ceilings need only 1 coat unless there's significant staining.

Trim: Measure the linear feet of all baseboards, door frames, and window frames. A gallon typically covers 350-400 linear feet of trim. Trim paint is usually glossier (semi-gloss or satin) and applied with a brush, so coverage is slightly less per gallon than wall paint.

Doors: 1 door = ~20 sq ft (front + back + edges + frame). A standard interior door (32x80 inches) is 17.5 sq ft per side, with the frame adding another 5-7 sq ft.

Windows: 1 window = ~15 sq ft for an average 3x4 ft window. Larger picture windows can be 30-50 sq ft each. The calculator subtracts these from the wall area.

Estimating Whole-House Paint Jobs

For a whole-house repaint (most common scenario):

• Interior walls: (Perimeter of house) x (Wall height) - (Doors + Windows)
• Ceilings: (Total floor area) x number of floors
• Trim: ~(Total linear feet of baseboards + door frames + window frames)
• Doors and windows: Subtract from wall area, but if painting them too, add them back

A 2,000 sq ft single-story home typically requires 8-12 gallons of wall paint, 4-6 gallons of ceiling paint, and 2-3 gallons of trim paint, for a total of 14-21 gallons.

Frequently Asked Questions

How much paint do I need for a 12x12 room? A 12x12 room with 8-foot ceilings has 384 sq ft of wall area (4 walls x 12 ft x 8 ft). Subtract one door (20 sq ft) to get 364 paintable sq ft. At 400 sq ft/gallon coverage, one coat needs 0.91 gallons, two coats needs 1.82 gallons. Buy 2 gallons for a refresh (1 coat over similar color) or 3 gallons for a major color change (2 coats over white).

How many square feet does a gallon of paint cover? A gallon of interior latex paint covers 350-400 sq ft on smooth drywall, 250-300 on textured surfaces, and 150-200 on popcorn ceilings. These rates assume proper surface preparation (clean, primed if needed) and standard roller application. Spraying uses 10-20% more paint, and brushing uses 5-10% more than rolling.

Do I need to prime first? Yes, in several cases: (1) New drywall requires primer before paint. (2) Stained surfaces (water stains, smoke damage, tannin bleed) need stain-blocking primer. (3) Dramatic color changes (white to dark or vice versa) benefit from a tinted primer. (4) Previously unpainted surfaces (raw wood, masonry) need a primer compatible with the topcoat. The calculator's output assumes a primed surface; for unprimed surfaces, add 25-50% more paint.

How long does it take paint to dry? Latex paint: 1-2 hours to touch dry, 4 hours to recoat. Oil-based paint: 6-8 hours to touch dry, 24 hours to recoat. Humidity and temperature affect drying time: high humidity slows drying, low temperature slows drying. Plan projects with adequate drying time between coats. Most paint failures are due to insufficient drying time between coats, leading to poor adhesion and visible brush marks.

What is the difference between flat, satin, semi-gloss, and gloss paint? Flat: Matte finish, hides imperfections, no washability. Best for low-traffic ceilings. Satin/Eggshell: Subtle sheen, good washability, most common for living areas. Semi-gloss: Noticeable sheen, excellent washability, ideal for kitchens, bathrooms, trim. Gloss: Highest sheen, very durable, traditional for trim and doors, but shows every surface imperfection. Most walls use eggshell or satin; kitchens, bathrooms, and trim use semi-gloss.

How do I calculate paint for an exterior project? For exterior walls: Length x Height for each wall, minus doors and windows. For trim: linear feet of soffits, fascia, and trim. For siding (lap, board-and-batten): the linear feet times the exposure height. Rough surfaces (stucco, cedar shake) need 30-50% more paint than smooth surfaces. Most exterior projects need 1-2 gallons per 100-150 sq ft of wall area, depending on surface type and number of coats.

Inputs and Their Effects

Each field on the Paint Calculator form plays a distinct part in the calculation.

• Measure each wall's width and height in feet (or meters). Add the areas together - this value feeds the Paint Calculator directly and shows up in the result.
• Subtract areas that won't be painted: doors (~20 sq ft each), windows (~15 sq ft each) - this value feeds the Paint Calculator directly and shows up in the result.
• For ceilings: enter length and width separately - this value feeds the Paint Calculator directly and shows up in the result. Editing one field of the Paint Calculator changes the output in line with the formula, so a misplaced value is visible in the answer.

Common Mistakes to Avoid

The errors that come up most often with the Paint Calculator are easy to spot once you know them:

• Entering a value in the wrong unit for Measure each wall's width and height in feet (or meters). Add the areas together; the Paint Calculator answer is only right when the unit matches the label.
• Mixing conventions, such as percentages and decimals, where the Paint Calculator formula expects one form.
• Rounding the inputs before the Paint Calculator runs; keep the full values and let the tool round the final answer.
• Treating the Paint Calculator result as exact when the inputs themselves were estimates.

When to Use the Paint Calculator

Use the Paint Calculator whenever you need a quick, reliable answer that fits the tool's scope. Common situations for the Paint Calculator include homework and study, on-the-job quick checks, sanity-checking a more complex calculation, or exploring a scenario for personal interest. If the Paint Calculator answer will be used for a decision that has legal, medical, or financial consequences, treat the result as a starting point and verify it with a qualified professional.

How the Math Works

The calculation behind the Paint Calculator follows the standard form for this kind of problem: Total Wall Area = Sum of (Wall Width x Wall Height) for all walls Paintable Area = Total Wall Area - Door Areas - Window Areas Gallons Needed = Paintable Area / Coverage Rate per Gallon x Number of Coats With Waste Buffer = Gallons Needed x The Paint Calculator applies that relationship in the order the algebra prescribes, converting inputs to consistent units first where the formula needs them.

Related Concepts and Where This Fits

The Paint Calculator fits alongside the other tools in its category, and the choice between them usually comes down to which inputs you already have. If the same numbers feed several tools, run them in one pass so the assumptions stay consistent across the comparison, which is where the Paint Calculator earns its place.

Worked Examples

A typical Paint Calculator run takes reasonable inputs, produces a sensible answer, and returns it in a single click. Example: Beyond the worked examples earlier in this page, a few additional cases illustrate how the Paint Calculator behaves at the edges of its input range. Boundary inputs. Entering the smallest sensible value or the largest sensible value for a numeric input should produce a result at the corresponding end of the output range, not a runaway value or a silently clipped resul

References

• Dulux, Paint Calculator, coverage per litre by surface type. https://www.dulux.co.uk/en/paint-calculator
• Home Depot, How Much Paint Do I Need?, the standard coverage estimate of 350 sq ft per gallon for a single coat. https://www.homedepot.com/c/ah/how-much-paint-do-i-need/9ba683603be9fa5395fab90115c4d9d4