Calculate gallons and liters needed to paint walls and ceiling — with automatic deductions for standard doors (20 sq ft) and windows (15 sq ft), and a coat multiplier for two-coat coverage.
What Your Results Mean
Measure the longest wall first for length, then the perpendicular wall for width, and the floor-to-ceiling height. For rooms with vaulted or sloped ceilings, use the average height for the wall measurement and calculate the ceiling separately if you're painting it. The calculator multiplies perimeter by height to get total wall area, then subtracts your door and window counts automatically.
The door and window deductions use industry-standard sizes: 20 square feet per interior door (3'×6'8" minus a small frame allowance) and 15 square feet per window (3'×5'). If you have oversized features — sliding glass doors, bay windows, picture windows — count them as multiple standard windows or measure and subtract the area manually. French doors count as two standard doors.
Coat count matters more than most homeowners realize. A single coat of the same color refreshes an existing wall but rarely achieves full coverage. Two coats are required for any color change, for painting over primer, for deep or saturated colors (reds, yellows, dark blues), and for any premium paint job. The calculator defaults to two coats for this reason — one coat will leave streaks and uneven coverage that's visible in natural light.
Reading the Coverage Number
Premium interior latex paint covers 350 to 400 square feet per gallon on smooth, primed surfaces. Builder-grade paint covers 250 to 300 square feet. Coverage drops on rough surfaces (stucco, brick, textured drywall), on porous surfaces (new drywall without primer, bare wood), and on dark-to-light color changes. The calculator defaults to 350 sq ft per gallon — check your paint can's label and adjust the input if it differs.
Primer vs Paint
Primer is a different product with a different purpose. It seals porous surfaces, blocks stains, provides adhesion for topcoat paint, and creates a uniform base color. Use primer when painting over new drywall, repaired walls, dark existing colors, oil-based paint (with a bonding primer), water stains, or wood paneling. One coat of primer plus two coats of finish paint is the gold standard for any color change. Primer covers slightly more area than paint — about 400 sq ft per gallon — but buy it separately and don't try to use it as a paint substitute.
Pro Tip
Always buy paint from the same batch. Paint color varies subtly between manufacturing runs, even with the same formula. If you must buy multiple batches, mix them together in a 5-gallon bucket ("boxing") to ensure uniform color across the room.
The Math Behind the Calculator
Paint estimating follows a clear sequence: compute wall area, add ceiling if applicable, subtract openings, multiply by coats, and divide by coverage per gallon:
wall_area = (2 × (length + width)) × height
ceiling_area = length × width (if included)
deductions = (doors × 20) + (windows × 15)
paintable_area = wall_area + ceiling_area - deductions
total_coverage_needed = paintable_area × coats
gallons = ceil(total_coverage_needed / coverage_per_gallon)
liters = gallons × 3.78541
Standard Deductions
The 20 sq ft per door and 15 sq ft per window values come from typical interior dimensions:
- Interior door: 3'×6'8" = 20 sq ft (subtracting frame)
- Standard window: 3'×5' = 15 sq ft
- Closet door (double): 5'×6'8" = 33 sq ft — count as 1.5 standard doors
- Sliding glass door: 6'×6'8" = 40 sq ft — count as 2 standard doors
- Picture window: 5'×4' = 20 sq ft — count as 1.3 standard windows
Why Gallons and Liters?
The calculator shows both because US paint is sold in gallons (1 gallon = 3.785 liters) while Canadian, European, and Australian paint is sold in liters (typically 1L, 4L, and 20L containers). The conversion factor is exact, so the same paint job needs 1 gallon or 3.785 liters — round up to the nearest 4L container for purchasing.
Coverage Variations
Manufacturer coverage ratings assume ideal conditions: smooth surface, primed, same-color repaint, 3/8" nap roller, 70°F, 50% humidity. Real-world coverage is typically 10–15% lower due to surface absorption, roller waste, and cutting-in losses. For new construction or dark-to-light repaints, drop the coverage input to 300 sq ft/gal to be safe.
Assumptions & Limitations
Every number above rests on these constants. If your project differs, change the matching input — or read the linked guide for the full reasoning.
- Coverage: 350 sq ft per gallon per coat (typical latex)
- Deductions: 20 sq ft per door, 15 sq ft per window
- Coats: your number multiplies gallons; primer counted separately
- Textured or porous walls can take 10–20% more paint
Worked Example: 14×12 Living Room with 9-ft Ceilings
A standard living room: 14 feet by 12 feet, 9-foot ceilings, two doors, three windows, two coats of premium paint, ceiling not included. Let's walk through the math:
- Perimeter: 2 × (14 + 12) = 52 ft.
- Wall area: 52 × 9 = 468 sq ft.
- Ceiling area: 0 (not included in this example).
- Deductions: (2 × 20) + (3 × 15) = 40 + 45 = 85 sq ft.
- Paintable area: 468 - 85 = 383 sq ft.
- Two coats: 383 × 2 = 766 sq ft total coverage needed.
- Gallons at 350 sq ft/gal: 766 ÷ 350 = 2.19 → 3 gallons.
- Liters: 3 × 3.785 = 11.36 L.
- Order summary: 3 gallons (11.4 L) of finish paint covers two coats on walls and ceiling.
If you also painted the ceiling (168 sq ft), the paintable area would jump to 551 sq ft, total coverage to 1102 sq ft, and you'd need 4 gallons instead of 3. The calculator updates the moment you toggle the ceiling checkbox.
For comparison, if you used paint covering 275 sq ft/gal, you'd need 766 ÷ 275 = 2.78 → 3 gallons — the same count, but lower-coverage paint often needs a third coat on dark-to-light colour changes, pushing the total to 4 gallons versus 3. Coverage per litre is printed on the can: check it before comparing brands.
Reminder
The estimate excludes primer (add 1 gallon / 3.8 L if priming new drywall or making a drastic colour change), brushes, rollers, drop cloths, painter's tape, sandpaper and spackle. List these consumables before you start — they are the classic forgotten trip to the store.
