Paint Calculator
Find how many gallons of paint you need for any room. Enter the room size, doors, windows, and coats, and get the paintable area, gallons to buy, and cost in seconds.
A paint calculator estimates how many gallons of paint a room needs, from the wall area, the openings that do not get painted, and the number of coats. It exists because paint math has two traps: people forget that doors and windows subtract real area, and they forget that two coats means buying twice the coverage.
This calculator takes your room’s length, width, and ceiling height, deducts standard door and window openings, multiplies by your coat count, and divides by the paint’s coverage to get whole gallons. If you have already measured the wall area, the Wall Area mode accepts it directly. Coverage defaults to 350 square feet per gallon, the typical figure for interior latex on smooth primed walls, and is editable in the advanced section to match your can’s label.
A paint calculator estimates how many gallons of paint a room needs, from the wall area, the openings that do not get painted, and the number of coats. It exists because paint math has two traps: people forget that doors and windows subtract real area, and they forget that two coats means buying twice the coverage.
This calculator takes your room’s length, width, and ceiling height, deducts standard door and window openings, multiplies by your coat count, and divides by the paint’s coverage to get whole gallons. If you have already measured the wall area, the Wall Area mode accepts it directly. Coverage defaults to 350 square feet per gallon, the typical figure for interior latex on smooth primed walls, and is editable in the advanced section to match your can’s label.
How to Use This Calculator
Step 1: Pick your input method
Choose Room Size to enter the room’s dimensions, or Wall Area if you have already measured the paintable surface, such as a single accent wall or a summed multi-room total.
Step 2: Enter the room
Length, width, and ceiling height, plus the number of doors and windows in the room. The calculator deducts a standard opening for each. For a room with a large opening like an archway or sliding door, count it as two doors.
Step 3: Set the coats
Two coats is the default and the right answer for most jobs. Drop to one for a same-color refresh, raise to three for covering dark or saturated colors.
Step 4: Ceiling decision
Tick Include Ceiling only if you are painting the ceiling with the same product. Ceilings usually get a dedicated flat ceiling paint, in which case leave the box unchecked and run the ceiling as its own quick calculation (its area is just length times width).
Step 5: Match the coverage to your paint
The 350 square feet per gallon default suits typical interior latex on smooth primed walls. Check your can’s label and set the real figure in the advanced section, and lean lower for textured or porous surfaces, which absorb more paint.
Step 6 (optional): Add cost
Enter the price per gallon for a material cost. Interior latex commonly runs about $20 to $60 per gallon depending on line and sheen.
Reading the results
Paintable Area is the surface getting paint; Gallons Needed is what to buy, rounded to whole gallons. If the math lands just past a gallon boundary, a quart (roughly 90 to 100 square feet of coverage) can replace the last gallon. Keep the leftover labeled for touch-ups, since matching a custom tint later is never guaranteed.
How the Calculation Works
The paint estimate is wall area, minus openings, times coats, divided by coverage.
Step 1: Wall area
Wall area = 2 × (Length + Width) × Ceiling height
The perimeter of the room times the wall height. A 10 by 12 foot room with 8 foot ceilings: 2 × 22 × 8 = 352 square feet.
Step 2: Subtract openings
Paintable area = Wall area - (doors × 20) - (windows × 12)
This calculator uses the same standard openings as the rest of the site: a 36 by 80 inch door (20 sq ft) and a 36 by 48 inch window (12 sq ft). 352 – 20 – 12 = 320 square feet.
Step 3: Multiply by coats
Total coverage = Paintable area × Coats
Two coats: 320 × 2 = 640 square feet of coverage to buy.
Step 4: Divide by coverage per gallon
Gallons = ceiling(Total coverage / Coverage per gallon)
At 350 square feet per gallon: 640 / 350 = 1.83, rounded up to 2 gallons.
Cost (optional):
Cost = Gallons × price per gallon
Choosing the coat count: Two coats is the standard recommendation for a color change, new drywall (after primer), and most repaints, because one coat rarely hides evenly. One coat is enough when refreshing the same color over clean, sound paint. Three coats appear when covering very dark or saturated colors, or making a drastic light-over-dark change without a tinted primer.
About primer: Primer is a separate product with its own, lower coverage (often 200 to 300 square feet per gallon). New drywall, bare wood, patches, and stain-blocking all call for primer before paint. Estimate it as a separate pass: paintable area divided by the primer’s labeled coverage.
Worked Example
Scenario: Repainting a 14 by 16 foot living room with 9 foot ceilings
A living room measures 14 by 16 feet with 9 foot ceilings, two doorways, and two windows. You are changing the color, so two coats, walls only, with paint at $45 per gallon and the standard 350 square feet per gallon coverage.
Step 1: Wall area
2 × (14 + 16) × 9 = 60 × 9 = 540 square feet
Step 2: Subtract openings
Doors: 2 × 20 = 40 sq ft. Windows: 2 × 12 = 24 sq ft.
Paintable area = 540 – 40 – 24 = 476 square feet
Step 3: Two coats
476 × 2 = 952 square feet of coverage
Step 4: Gallons
952 / 350 = 2.72, rounded up to 3 gallons
Step 5: Cost
3 × $45 = $135
Final estimate: Buy 3 gallons, about $135 in wall paint. The third gallon is only 72 percent used by the math, which is the right amount of margin: textured spots, touch-ups during cutting in, and the leftover quart-or-so for future repairs all come out of it. If this room’s ceiling were painted too (224 square feet, two coats of ceiling paint), that is a separate 448 square feet of coverage, two gallons of ceiling white as its own line.
Understanding the Variables
Room Length, Width, and Ceiling Height
The room’s footprint sets the wall perimeter, and the ceiling height sets how tall those walls are. Measure height in a corner, floor to ceiling. For vaulted or sloped ceilings, use the average height, or switch to Wall Area mode and measure each wall as its own rectangle or triangle.
Doors and Windows
The openings deducted from the wall area: 20 square feet per door (36 by 80 inches) and 12 square feet per window (36 by 48 inches), the same standard openings used by this site’s drywall and brick calculators. Oversized openings (double doors, picture windows, archways) count as two.
Coats
How many times the walls get painted, which multiplies the coverage you must buy. One coat refreshes the same color over sound paint, two coats is the standard for color changes and new walls, and three handles drastic dark-to-light changes or deep saturated colors. A tinted primer can often replace the third coat at lower cost.
Include Ceiling
Adds the ceiling area (length times width) to the paintable total. Use it only when the ceiling gets the same paint as the walls. Most rooms use a dedicated flat ceiling paint, in which case leave this unchecked and estimate the ceiling separately.
Paintable Wall Area (Wall Area mode)
A directly entered surface, in square feet or square meters, for cases where the room formula does not fit: a single accent wall, a stairwell, an open-plan space, or several rooms summed. Enter the net area after any openings, since no deductions are applied in this mode.
Paint Cost
Price per gallon. Interior latex commonly runs about $20 to $60 per gallon, with contractor lines at the bottom and premium one-coat-hide lines at the top. Optional input, affects only the cost output.
Coverage per Gallon (advanced)
How far a gallon spreads, the figure printed on the can. Typical interior latex covers about 350 to 400 square feet per gallon on smooth, primed drywall; textured, porous, or unprimed surfaces drop real coverage toward 250 to 300, and primer runs lower still. The 350 default is the safe middle; replace it with your product’s labeled number for an accurate count.
Industry Standards & References
Manufacturer coverage specifications
The binding standard for paint quantity is the coverage printed on the can, stated per gallon per coat for a specified surface. Major manufacturers rate typical interior latex at roughly 350 to 400 square feet per gallon on smooth, primed, non-porous surfaces, with the explicit caveat that porous or textured surfaces and drastic color changes reduce coverage. This calculator’s editable coverage input exists to carry that label figure.
MPI (Master Painters Institute) standards
MPI maintains the product and workmanship standards referenced in North American painting specifications, including approved product lists by category and the gloss and sheen level definitions (flat through gloss) that architects specify. Commercial painting contracts typically require MPI-listed products.
ASTM D2486 (Scrub Resistance)
The standard test for a wall paint’s resistance to scrubbing, the property behind washable claims. Higher scrub cycles mean the finish survives cleaning in kitchens, baths, and hallways, which is why sheen and quality matter more in those rooms.
ASTM D16 (Paint Terminology)
The standard terminology for paint and coatings, the reference that pins down what terms like primer, enamel, and sheen designations actually mean across manufacturers.
EPA and regional VOC limits
Volatile organic compound content in architectural coatings is regulated federally by the EPA and more strictly in some states and districts (California’s SCAQMD rules being the strictest). Low-VOC and zero-VOC interior paints are now the residential norm, and the can states compliance.
Surface preparation practice: Industry practice puts primer on new drywall, bare wood, patched areas, and stain-prone surfaces before paint, both for adhesion and because finish paint over unprimed drywall flashes unevenly. Primer carries its own coverage rating, usually lower than paint, and is estimated as a separate pass.
Frequently Asked Questions
A 12 by 12 room with 8 foot ceilings has 384 square feet of wall, about 352 after a door and a window. Two coats is 704 square feet of coverage, which is just past two gallons at 350 square feet per gallon. Buy two gallons plus a quart, or a third gallon if the walls are textured or you want touch-up reserve.
Typically 350 to 400 square feet per coat on smooth, primed drywall, with the exact figure printed on your can. Textured, porous, or unprimed surfaces absorb more and can drop real coverage toward 250 to 300 square feet. A quart covers roughly 90 to 100 square feet.
For a color change, new walls, or worn paint, yes: one coat rarely hides evenly, and the second coat is what produces a uniform color and sheen. One coat is fine when refreshing the same color over clean, sound paint. Very dark or saturated colors can take three coats, or two over a tinted primer.
On new drywall, bare wood, patched areas, and when covering stains or making drastic color changes, yes. Primer seals the surface so the finish coats spread at their rated coverage and dry evenly. It is a separate product with lower coverage (often 200 to 300 square feet per gallon), estimated as its own pass. If you are finishing a freshly built wall, our drywall calculator covers the sheet count that comes before the primer.
Deduct about 20 square feet per standard door and 12 square feet per standard window from the wall area, which the calculator above does automatically. Do not deduct outlets, switches, or small fixtures; normal application losses absorb them.
Only if the ceiling gets the same product, which is uncommon. Ceilings usually use a dedicated flat ceiling paint that resists roller spatter and hides imperfections, so estimate the ceiling separately: its area is the room's length times width, times your coat count, divided by the ceiling paint's coverage.
Interior latex commonly runs about $20 to $60 per gallon: contractor-grade lines at the low end, premium and one-coat-hide lines at the top. Specialty products (ceiling paint, enamels, primers) have their own ranges. Per room, paint is usually a smaller cost than the labor it takes to apply it.
Keep at least a quart of each color, sealed and labeled with the room and date. Custom tints are mixed per can and a later batch may not match exactly, so the leftover from the original purchase is the only guaranteed match. Store it where it will not freeze, since freezing ruins latex paint.