Paint Calculator

How to use the paint calculator

This paint calculator answers the question every painting job starts with: how much paint do I need? It works out the area you will paint, takes off the doors and windows, multiplies by the number of coats, divides by how far a gallon or a liter of your paint goes, and then turns the answer into cans you can actually buy. All of the math happens in your browser.

1. Choose what you are painting. Under Paint, pick Whole room (length, width and wall height, with the ceiling if you like), Wall by wall (any number of separate walls), Accent wall (one feature wall), Exterior (the outside walls of a house and its gable ends) or Ceiling only.
2. Count the doors and windows. Enter how many there are on the walls and their size once, under Doors, windows and other openings. Add anything else at its exact size, such as a patio door, a garage door, a fireplace or a skylight. Every length field has its own unit, and you can type 8' 6", 96 in or 245 cm.
3. Set the coats and the coverage. Pick 1, 2 or 3 coats. The coverage starts at about 400 square feet per gallon, a state environmental agency's figure; replace it with the coverage printed on your paint's label. Tick Prime first to add a coat of primer, and Estimate trim paint too for baseboards, casings and doors.
4. Read what to buy. The results show the area to paint, the paint needed, the cans to buy, what will be left over and, if you enter a price, the cost. A shopping list gives one line for each product: wall paint, ceiling paint, trim paint and primer. Copy the results, download them as a CSV file or print them.

Switch Units to Metric to see square meters, liters and metric can sizes. The calculator remembers your last entries on this device.

How to measure a room for paint

A tape measure and a notepad are all you need. Measure in whatever units you like; the calculator converts them.

• Wall widths. Measure each wall along the floor, corner to corner. For a rectangular room the length and the width are enough, because opposite walls match. For an L-shaped room, a room with a sloped ceiling or walls in different colors, use Wall by wall, or work out the floor plan with the square footage calculator first.
• Wall height. Measure from the floor to the ceiling. Paint covers the whole wall, so unlike wallpaper there is nothing to add for trimming.
• Doors and windows. Measure each opening with its trim, because you won't paint the wall behind the casing. If the trim gets its own paint, the calculator can estimate that too.
• The label. Find the coverage on the can or the product sheet, usually given in square feet per gallon or square meters per liter, and the number of coats it recommends. That one figure changes the answer more than anything else.

How to calculate how much paint you need

The calculation takes four steps. The calculator does them for every surface and every product:

wall area   = perimeter × wall height
area to paint = wall area − doors − windows − other openings
paint needed  = area to paint × coats ÷ coverage per gallon
cans          = paint needed, rounded up to cans you can buy

1. Measure the walls. Add the widths of the walls to get the perimeter, and multiply by the wall height to get the wall area.
2. Take off the openings. Multiply the width by the height of each door and window and subtract them from the wall area.
3. Multiply by the coats. Two coats need twice the paint of one, so multiply the area to paint by the number of coats.
4. Divide by the coverage. Divide by the area one gallon or one liter covers, as printed on the label, to get the paint you need.
5. Round up to cans. Paint is sold in fixed sizes, so round up to the quarts, gallons or pails, or the liter cans, that cover what you need.

Example: a 12 × 12 ft room with 8 ft walls, one 36 × 80 in door and two 36 × 48 in windows. The perimeter is 48 ft, so the walls are 48 × 8 = 384 sq ft. The door is 20 sq ft and each window 12 sq ft, 44 sq ft in all, which leaves 340 sq ft to paint. Two coats at 400 sq ft per gallon need 340 × 2 ÷ 400 = 1.70 gallons, so you buy 2 gallons and have 0.30 gallons left for touch-ups. That is the example the calculator opens with.

How much does a gallon of paint cover?

Coverage, also called the spread rate, is the area one gallon or one liter covers in one coat. It depends on the paint and on the surface, so there is no single right number. The calculator starts from a figure published by a state agency: the Pennsylvania Department of Environmental Protection advises, "Measure the area you are painting to determine the amount of paint you need. A typical gallon of paint covers about 400 square feet, although coverage varies by brand and surface to be painted." That advice comes from its guide to painting with less waste.

Treat 400 sq ft per gallon as a starting value, not a promise. Rough, porous or textured surfaces, such as new drywall, bare wood, masonry, stucco or rough siding, soak up more paint, and the label of each paint gives its own figure. Type the label's coverage into the Coverage, one coat field, in square feet per gallon or square meters per liter, and the whole estimate follows it.

Small changes in coverage matter on a big job. With 350 sq ft per gallon instead of 400, the example room needs 1.94 gallons; it is still 2 gallons, but with only 0.06 gallons to spare.

Coats and primer

Every coat covers the whole area again, so the paint needed goes up in step with the coats. The calculator starts at two coats, the common plan for a new color or a wall with patches, and you can choose one or three. Follow the label, which says how many coats the paint needs for an even finish.

• One coat of the example room needs 0.85 gallons: 1 gallon.
• Two coats need 1.70 gallons: 2 gallons.
• Three coats need 2.55 gallons: 3 gallons.

Primer is a separate product, so the calculator gives it its own line. Tick Prime first and it adds one coat of primer over the walls, the gable ends and the ceiling you paint, at the primer's own coverage. Primers often state a different coverage from paint, so check that label too. For the example room, one coat of primer at 400 sq ft per gallon is 0.85 gallons, or 1 gallon. The paint's label usually says which surfaces need priming first, such as bare drywall, bare wood, patched areas or a big change of color.

Gallons, quarts and pails: what to buy

Paint is sold in fixed sizes, and the sizes below are common ones rather than a standard, so check what your store sells. A US gallon is exactly 231 cubic inches, or 3.785411784 liters, and holds four quarts.

A common rule of thumb is to buy another gallon when you need more than half a gallon extra, and quarts when you need less. The calculator does the same thing in a way you can check: it counts in quarts (or half liters), and from every mix of cans that holds enough, it picks the one with the lowest total of paint in the cans plus one quart, or one liter, for each can. That keeps both the leftover and the number of cans low.

• Need 1.20 gallons? Buy 1 gallon + 1 quart.
• Need 1.28 gallons? Buy 1 gallon + 2 quarts.
• Need 1.60 gallons? Buy 2 gallons, not one gallon and three quarts.
• Need 3.20 gallons? Buy 3 gallons + 1 quart rather than a five-gallon pail.

Some paint is left over in almost every job. The U.S. Environmental Protection Agency's advice on indoor air applies here too: "If you use products only occasionally or seasonally, such as paints, paint strippers and kerosene for space heaters or gasoline for lawn mowers, buy only as much as you will use right away." A little left over is still worth keeping, closed tight and labeled with the room, for touch-ups. The Extra paint field starts at 0% because rounding up to whole cans already leaves some over; add a percentage if you want a bigger reserve.

Doors, windows and other openings

Some quick estimates take a fixed allowance off for each door and each window. This calculator uses the sizes you enter instead. The door and window sizes apply to every door and window you count, starting at 36 × 80 in for a door, which is 20 sq ft, and 36 × 48 in for a window, which is 12 sq ft. Change them to match your own.

Anything else goes under Other openings at its exact size, as many as you need: a patio door, a wide pass-through, a fireplace, built-in shelves, or a garage door outside. For example, adding a 6 ft × 80 in patio door to the example room takes another 40 sq ft off, leaving 300 sq ft and 1.50 gallons for two coats. In Ceiling only mode, other openings come off the ceiling instead, which is where skylights go.

Doors and windows are placed on the walls in turn, so the calculator can tell you when they don't fit, and the table under the results shows each wall's area after its openings.

Ceilings and accent walls

A ceiling is its length times its width. In Whole room mode, tick Also paint the ceiling. Ceilings are often painted with a different, flat paint, so by default the ceiling gets its own line: the 12 × 12 ft example ceiling is 144 sq ft, which takes 0.72 gallons for two coats, or 1 gallon of ceiling paint. If the walls and ceiling get the same paint, tick Ceiling in the wall paint: together they are 484 sq ft and 2.42 gallons, or 2 gallons + 2 quarts.

For a ceiling on its own, choose Ceiling only. A 12 × 10 ft ceiling is 120 sq ft, and two coats need 0.60 gallons: 1 gallon.

An accent wall is one wall in its own color. Choose Accent wall and enter its width and height. A wall 14 ft wide and 9 ft high is 126 sq ft; two coats need 0.63 gallons, so 1 gallon is enough. Three quarts would hold less, but in three cans, so the calculator picks the single gallon.

Exterior paint and gable ends

For the outside of a house, choose Exterior. The walls are the perimeter times the wall height, measured from the foundation to the eaves. On a house with a gable roof, the two ends also have a triangle of wall under the roof, the gable. Each gable end is a triangle, so its area is half its width times its rise, the height from the eave line up to the peak:

gable area = ½ × gable width × rise

If you know the roof pitch instead of the rise, the rise is half the width times the pitch over 12: a 6/12 roof over a 30 ft span rises 15 × 6 ÷ 12 = 7.5 ft. The roof pitch calculator can work out a pitch from your measurements.

Example: a house 40 ft long and 30 ft wide with 10 ft walls has 1,400 sq ft of wall. Two gable ends 30 ft wide that rise 6 ft add 180 sq ft. Two doors and ten windows take off 160 sq ft, which leaves 1,420 sq ft. Two coats at 400 sq ft per gallon need 7.10 gallons, so you buy 1 five-gallon pail + 2 gallons + 1 quart. Siding, shingles, stucco and brick soak up more paint than smooth walls, so use the coverage on the label of the exterior paint you choose, and add a garage door under Other openings.

Trim, baseboards and doors

Trim is usually painted in a different paint and sheen from the walls, so the calculator estimates it as its own product. Tick Estimate trim paint too and choose what to include:

• Baseboards run around the room, less one door width for every door.
• Door and window casings are counted as two sides and a top for a door, and all four sides for a window.
• The doors themselves add both faces of every door, width × height × 2.
• More trim length covers crown molding, chair rail or, outside, fascia boards.

The trim length is multiplied by the face width of the trim. In the example room, the baseboards are 48 − 3 = 45 ft, the door casing 16 ft 4 in and the two window casings 14 ft each, 89.33 ft in all. At a 4 in face that is 29.78 sq ft, and two coats need 0.15 gallons: 1 quart of trim paint.

Metric: liters and square meters

Switch Units to Metric to see areas in square meters, paint in liters, and 1, 2½, 5 and 10 liter cans. You can type the coverage in square meters per liter if that is how your label gives it; changing the coverage unit converts the number for you. The starting coverage of 400 sq ft per gallon is about 9.82 m² per liter.

Example: a room 4 × 3.5 m with walls 2.5 m high has 37.5 m² of wall. A 0.9 × 2 m door and a 1.2 × 1.2 m window leave 34.26 m². With a label coverage of 10 m² per liter, two coats need 6.85 liters, so you buy 1 × 5 L + 1 × 2.5 L and have 0.65 liters to spare.

The conversions are exact: an inch is exactly 2.54 centimeters and a foot exactly 0.3048 meters, and a US gallon is exactly 231 cubic inches, as the National Institute of Standards and Technology (NIST) lists in Handbook 44, Appendix C and Special Publication 811. So a square foot is exactly 0.09290304 square meters, and 1 m² per liter is about 40.75 sq ft per gallon.

Lead-safe painting in older homes

Painting over sound paint is one thing; scraping, sanding or removing old paint is another. The U.S. Environmental Protection Agency warns: "If your home was built before 1978, it is more likely to have lead-based paint." Dust and chips from that paint are a hazard, especially for children.

For paid work, "EPA's RRP Rule requires that anyone paid to perform work that disturbs painted surfaces in homes, childcare facilities and preschools built before 1978 be certified" and trained in lead-safe work practices. The rule generally does not cover homeowners working on their own homes, but EPA recommends hiring a lead-safe certified contractor, and if you do the work yourself, using its do-it-yourself lead-safe work practices. Read EPA's Renovation, Repair and Painting Program page, its guide to lead-safe renovations for DIYers and Protect Your Family from Sources of Lead before you sand or scrape anything in a home built before 1978.

Ventilation and VOCs

Paints give off fumes while they go on and while they dry. EPA's indoor air guidance notes that "Paints, varnishes and wax all contain organic solvents" and advises: "Increase ventilation when using products that emit VOCs." Open windows, run a fan that blows air out of the room, take breaks in fresh air, and follow the safety directions on the label. EPA's page on volatile organic compounds and indoor air quality has more steps to reduce exposure, including storing and disposing of leftover paint safely.

How we calculate

Every figure in the tool and on this page comes from the same code, and the examples above are checked against it automatically.

• Lengths. Every measurement is converted to feet first: inches ÷ 12, centimeters ÷ 30.48, meters ÷ 0.3048, the exact NIST definitions.
• Area. Walls = width × height for each wall; in a whole room or an exterior, the four walls are length, width, length, width at one height. Gable ends = ½ × width × rise each. Ceiling = length × width.
• Openings. Doors × door width × door height, windows × window width × window height, and each other opening × its width × height, taken off the walls (off the ceiling in Ceiling only mode).
• Paint needed. Area × coats ÷ coverage × (1 + extra ÷ 100). Primer is one coat over the walls, gable ends and ceiling at the primer's coverage. Trim is (trim length × face width + door faces) × coats ÷ coverage.
• Cans. Counted in whole quarts or half liters, the mix of cans with the lowest paint plus one quart (or one liter) per can, then the fewest cans.
• Left over. Paint in the cans − paint needed.
• Cost. Paint in the cans × your price per gallon or per liter. Nothing is filled in for you, because prices vary from paint to paint, and smaller cans usually cost more per gallon.

The calculator does not adjust for texture, sheen or color by itself, and it does not estimate brushes, rollers, tape or labor. Coverage is the one assumption that matters: set it from the label of the paint you buy.

Your measurements stay on your device

The calculator runs entirely as JavaScript in your browser tab. Nothing you enter is uploaded: the page's Content-Security-Policy allows connections only back to this site, and the tool has no code that sends anything anywhere. It remembers your last measurements and settings in your browser's local storage, on your device only. There is no account and no cookie.

Sources

• Pennsylvania Department of Environmental Protection: "Green" Painting Tips, Protect the Environment While Painting Your House: a typical gallon of paint covers about 400 square feet; coverage varies by brand and surface.
• U.S. Environmental Protection Agency: Protect Your Family from Sources of Lead, on lead-based paint in homes built before 1978.
• U.S. Environmental Protection Agency: Lead Renovation, Repair and Painting Program, the RRP Rule for paid work in pre-1978 homes.
• U.S. Environmental Protection Agency: Lead-Safe Renovations for DIYers.
• U.S. Environmental Protection Agency: Volatile Organic Compounds' Impact on Indoor Air Quality, on paints as a source of VOCs, ventilation and buying limited quantities.
• National Institute of Standards and Technology: NIST Handbook 44 (2026), Appendix C, General Tables of Units of Measurement: 1 foot = 0.3048 meter, 1 inch = 2.54 centimeters and 1 US gallon = 231 cubic inches, exactly.
• NIST Special Publication 811, Appendix B.9, Factors for units listed by kind of quantity: exact factors for the foot, the inch and the gallon.

Reviewed by microplugins · Last updated October 2026

This is an educational tool, not professional painting or construction advice. Coverage, coats and can sizes vary: check the label of the paint you buy and the instructions that come with it, and follow EPA's lead-safe practices in homes built before 1978.