Short answer
An 11×13 office with 8-foot ceilings, 1 door, and 2 windows needs 1.9 gallons of wall paint for 2 coats. Buy 2 gallons. The ceiling adds another 0.41 gallons (one quart covers it), and trim work requires about 2 quarts of a separate semi-gloss or gloss paint.
How this calculator works
The formula has four moving parts: gross wall area, deductions for openings, number of coats, and coverage rate. Here's how each one works for an 11×13 room.
Step 1 — Gross wall area
The four walls of an 11×13 room form a perimeter of 2 × (11 + 13) = 48 linear feet. Multiply by the 8-foot ceiling height and you get 384 square feet of raw wall surface.
Step 2 — Deduct doors and windows
Painting over a door or window frame just wastes your estimate. The calculator treats a standard interior door as 21 sq ft (roughly 3 ft × 7 ft) and a standard window as 15 sq ft (roughly 3 ft × 5 ft). One door and two windows subtract 21 + 30 = 51 sq ft, leaving 333 sq ft of paintable wall area.
Step 3 — Multiply by coats
Two coats means you're covering that 333 sq ft twice: 333 × 2 = 666 sq ft total.
Step 4 — Divide by coverage rate
Most interior latex paints yield 350 sq ft per gallon on typical drywall. Divide 666 ÷ 350 = 1.9 gallons. That's the number to bring to the store.
Ceiling paint
The ceiling is 11 × 13 = 143 sq ft. At 350 sq ft per gallon, one coat needs 0.41 gallons — a single quart with a small amount left over. Ceiling paint is a separate SKU from wall paint; it's formulated flatter to hide roller texture overhead.
Quarts vs. gallons
The calculator also outputs quart-can equivalents: 1.9 gallons equals 7.6 quarts, so 8 quart cans covers the math. In practice, two 1-gallon cans costs meaningfully less than eight quarts of the same product, so gallons win for anything over 1.5 gallons.
Trim paint
Doors and window casings get a separate, harder finish — typically semi-gloss or gloss. The formula estimates 0.25 quarts per opening plus 1 quart base, rounded up. For this room (3 openings): (3 × 0.25) + 1 = 1.75, rounded to 2 quarts. That's enough for two coats on the door and both window casings with a little left over.
What the output does not include
- Ceiling paint beyond 1 coat (if your ceiling is stained or going from dark to light, add another 0.41 gallons)
- Primer (count it the same as a paint coat if you're doing a full primer pass)
- Accent walls or wainscoting that differ from the main wall color
Coverage rate assumptions
350 sq ft per gallon assumes smooth or lightly textured drywall, a properly primed surface, and a standard-nap roller (⅜ inch). Heavily textured walls — orange peel, knockdown, or skip-trowel — drink more paint. If your walls are heavily textured, knock the coverage rate down to 300 sq ft/gallon and recalculate; for this room that would push you to about 2.2 gallons.
Common mistakes and gotchas
Recommended materials
For a small office, you want paint that covers in two coats without needing a separate primer on already-painted walls. A high-quality roller cover makes a real difference on textured drywall — cheap covers leave stippling and lint. Tape the trim line before you start; pulling it off while the paint is still slightly wet gives a cleaner edge than waiting until it's fully cured.
- Behr Premium Plus interior paint (1 gallon) — solid hide, widely available for color-matching at the store
- Purdy 9-inch roller cover (3-pack) — ⅜-inch nap works on smooth to light-orange-peel texture; having extras means you can swap mid-job if one loads up
- Frog Tape painter's masking tape (1.41-inch x 60yd) — the micro-foam edge seals against bleed-through better than standard blue tape on textured walls
FAQ
How many gallons of paint does an 11x13 room need? With 8-foot ceilings, 1 door, and 2 windows, an 11×13 room needs roughly 1.9 gallons for two coats on the walls. Round up to 2 gallons when buying so you have a little left for touch-ups.
Does the ceiling require separate paint? Yes, ceiling paint has a flatter sheen and better hide than wall paint. An 11×13 ceiling is 143 square feet, which works out to about 0.41 gallons — a single quart handles it comfortably for one coat.
Why does the formula subtract area for doors and windows? Doors and windows are surfaces you won't paint with wall paint, so rolling over them wastes material estimates. The calculator deducts 21 sq ft per door and 15 sq ft per window before dividing by coverage.
What coverage rate does interior paint actually deliver? Most latex interior paints cover 350–400 square feet per gallon on a smooth, primed surface. This calculator uses 350 sq ft/gallon, the conservative end, which is realistic for textured drywall or walls that need good hide.
How much paint do I need for trim and doors? With 1 door and 2 windows, the calculator estimates 2 quarts for trim work. Trim paint is typically a semi-gloss or gloss, sold separately from your wall color.
Should I buy gallons or quarts for a small office? For 1.9 gallons, two 1-gallon cans is cheaper per ounce than buying 8 individual quarts. Buy 2 gallons and keep the remainder sealed for touch-ups — paint in a tightly closed can lasts 2–5 years.
Do I need a primer coat before painting an office? If the walls are already painted a similar color and in good condition, a paint-and-primer-in-one product skips a separate primer step. Drastic color changes — say, dark red to white — typically need a dedicated primer first, which you'd count as an additional coat.
How do paint coats affect the gallon count? The calculator multiplies net wall area by the number of coats before dividing by coverage. One coat on this room is about 0.95 gallons; two coats doubles that to 1.9 gallons. Most colors need two coats for even coverage.
What if my ceilings are higher than 8 feet? Every extra foot of ceiling height adds roughly 2 × (11 + 13) = 48 sq ft of wall area per coat. On 9-foot ceilings with 2 coats you'd need about 2.2 gallons — use the calculator above and change the height input.
Is a 10% waste factor built into these numbers? The raw formula result is 1.9 gallons. A 10% waste buffer would bring that to roughly 2.1 gallons, which is why buying the next full gallon up is the right call anyway.
Can I use the same paint on walls and ceiling? You can, but ceiling-specific paint is formulated to resist spatter and dry without roller marks. Using wall paint on ceilings usually means an extra coat to get uniform coverage, which burns more material.