Interior Paint Calculator
Enter your room dimensions, door and window count, and number of coats. The calculator deducts 20 sq ft per door and 15 sq ft per window and rounds up to whole gallons.
Room Dimensions
Openings & Coats
How the math works
Step 1 — gross wall area
gross_wall = 2 × (length + width) × ceiling_height Step 2 — subtract openings
paintable = gross_wall − (doors × 20) − (windows × 15) Each standard 36×80 in door = 20.0 sq ft. Average window = 15 sq ft (industry convention). Paintable area is clamped to ≥0.
Step 3 — gallons
gallons_to_buy = ⌈ (paintable × coats) ÷ coverage ⌉ Coverage defaults to 350 sq ft/gal — the conservative figure used by Sherwin-Williams, Benjamin Moore and Behr field guides. Always round up to whole gallons; running out mid-job risks a dye-lot mismatch.
How the interior paint calculator works
The calculator computes your gross wall area as 2 × (length + width) × ceiling height — the total surface area of all four walls before subtracting openings. It then deducts 20 sq ft per door (standard 36×80 in door) and 15 sq ft per window (industry-average 3×5 ft window) to get the paintable wall area. That area is multiplied by the number of coats and divided by your chosen coverage rate, then rounded up to the next whole gallon using a float-safe ceiling function.
Coverage defaults to 350 sq ft/gal — the conservative figure Sherwin-Williams, Benjamin Moore, and Behr all publish for smooth drywall. Painting over a textured wall or unprimed surface? Switch to the 250 or 300 sq ft/gal option. Working on freshly primed smooth drywall? 400 sq ft/gal is achievable.
Tips for accurate results
- Measure the room's floor perimeter — length and width of the floor, not the diagonal.
- Use the actual ceiling height, not an assumed 8 ft. Vaulted areas need separate treatment.
- Count every door (including closet doors) and every window, even small ones.
- Always add ~10% for waste, touch-ups, and roller tray loss on top of the calculated amount.
Frequently Asked Questions
Coverage per gallon is the area one gallon of paint covers in a single coat. 350 sq ft/gal is the conservative default used by Sherwin-Williams and Benjamin Moore field guides — choose this for most drywall. 400 is optimistic (primed, smooth, ideal conditions). 300 works for light texture or porous surfaces. 250 is for heavy texture or completely unprimed walls.
A standard US interior door is 36 inches wide and 80 inches tall, which equals exactly (36 × 80) / 144 = 20.0 square feet. This is an industry-standard deduction used by Inch Calculator, Benjamin Moore, and Sherwin-Williams. You are not painting that door opening, so it is removed from the paintable wall area.
Enter the average ceiling height for a rough estimate. For precise measurements, measure the wall in sections (treat the rectangular lower portion and the sloped upper triangle separately), add the areas, and enter the total square footage in the paint-by-sq-ft calculator instead.
If your exact gallon need is within 0.25 gallons of the next whole gallon (i.e. you are buying a full gallon but only need 0.25 gal or less of it), the calculator suggests buying a quart instead for that last increment. A quart is 0.25 gallons and costs less than a full gallon.
Yes. The interior calculator covers walls only. Ceiling paint is a different product (flat finish, specific coverage) and uses a different formula (length × width, no opening deductions, texture multiplier). Use the ceiling paint calculator on this site for the ceiling.