ClutchCalcs

Construction

Paint Calculator

Painting a room and trying to figure out if 1 gallon covers it or you need 2? Mid-job color-matching is a nightmare — mixing batches differ slightly, and the wall stripe shows. This calculator takes your total wall length (perimeter), ceiling height, door and window count, number of coats, and coverage per gallon (default 350 sq ft for quality interior paint) and returns the exact gallon count. Rounds up to nearest quart to give you safety margin without buying way too much.

Enter the wall total and ceiling height to start.

Coverage and quality rules of thumb

  • Standard door: 21 sq ft (typical 3'x7').
  • Standard window: 15 sq ft (typical 3'x5').
  • Premium interior paint coverage: 350-400 sq ft per gallon.
  • Mid-grade paint: 300-350 sq ft per gallon.
  • Cheap / contractor-grade: 250-300 sq ft per gallon.
  • Textured / sand-finish walls: 20-30% less coverage.
  • Always plan 2 coats minimum. 3 for color changes (dark over light or vice versa).
  • Self-priming paint claims 1-coat coverage but practically still needs 2 for even color.

Worked example

12'x15' bedroom with 8-ft ceilings, 1 door, 1 window, 2 coats at 350 sq ft/gal.

Wall perimeter = (12+15) × 2 = 54 ft. Wall area = 54 × 8 = 432 sq ft. Subtract door (21) + window (15) = 396 sq ft paintable. Two coats = 792 sq ft total coverage. Divided by 350 = 2.26 gallons. Round up to 2.5 gallons (3 quarts above 2 gal). Buy 3 gallons to have safety stock.

How to use this calculator

  1. Wall length total: sum of all wall lengths in feet (perimeter for rectangular rooms).
  2. Ceiling height in feet (typically 8, 9, or 10).
  3. # of doors and windows: subtracts 21 sq ft per door, 15 sq ft per window.
  4. Coats: 1 (touch-up), 2 (standard), 3 (color change).
  5. Coverage per gallon: 350 default; lower for texture or cheap paint.
  6. Output: gallons needed, paintable area, total coverage needed.

Common scenarios

Master bedroom refresh, 14x16, 9-ft ceilings, 1 door, 2 windows, 2 coats, premium paint. Perimeter 60 ft. Wall area 540 sq ft. Subtract 21 + 30 = 489 paintable. 2 coats = 978 sq ft. At 375 sq ft/gal (premium): 2.61 gallons. Round to 2.75 gal = buy 3 gallons. ~$180-210 in premium paint. Allow another $50-80 for primer if changing color significantly.

Whole-house repaint, ~1,800 sq ft floor plan, 8-ft ceilings. Estimated wall perimeter ~600 ft. Wall area ~4,800 sq ft. Subtract typical openings (12 doors × 21 + 18 windows × 15 = ~520) = 4,280 paintable. 2 coats = 8,560 sq ft. At 350 sq ft/gal: 25 gallons. Plus ~1,800 sq ft ceiling = 11 more gallons of ceiling white. Plus trim paint (5 gal of semi-gloss). Total material: $1,500-2,200 for quality paint. Pro labor adds $3,000-5,000 on top.

Color change, navy blue over off-white, 12x12 office, 8-ft ceilings. Perimeter 48 ft. Wall area 384 sq ft. No doors/windows subtraction for this example. 3 coats (color change): 1,152 sq ft. At 350 sq ft/gal: 3.3 gallons. Plus 1 gallon of tinted primer. Total: 5 gallons. Tinted primer is the key — saves a coat of expensive top paint and prevents the off-white bleeding through.

Apartment turnover, white-over-white, 750 sq ft, 8-ft ceilings. ~2,400 sq ft wall area. 1 coat (touch-up) for landlord turnover at 350 sq ft/gal: 7 gallons of contractor-grade flat. ~$200-250 total. Good enough for between-tenant prep; not the right paint for a long-term homeowner's bedroom.

FAQ

What's wall length total vs perimeter? +
For rectangular rooms, they're the same: 2(length + width) of the floor plan. For odd-shaped rooms (L-shape, alcoves), measure each wall segment in feet and add them up. Closet interiors are usually painted same color as the room or with white closet paint; count or skip based on your plan.
Does this include the ceiling? +
No — ceiling is a separate calculation: ceiling area = room length × width. Add it to your order separately. Ceiling paint typically has flatter sheen and may use a different gallon if you're painting ceiling white while walls go a color.
Why round up? +
Paint is sold in fixed sizes: gallon, quart, pint, sample. If you're 0.1 gallon short mid-wall, you have to drive back to the store, hope they can color-match the batch (they usually can, but minor differences happen), and resume. Round up to nearest quart to leave margin for trim touch-ups and future repairs.
Do I need primer? +
For dark color over light, or light over dark: yes — saves a coat of regular paint, blocks bleed-through. For light-to-light changes on previously-painted walls: optional, depending on paint quality. New drywall always needs PVA primer or sealer to control absorption.
Eggshell, satin, semi-gloss, gloss? +
Flat/matte: hides imperfections, hard to clean (use in low-traffic). Eggshell: light sheen, washable, the modern interior standard. Satin: more sheen, more washable, kitchen/bathroom common. Semi-gloss: very washable, durable, trim and doors. Gloss: shiniest, durable, exterior trim. Sheen affects coverage and look — budget for the same gallon count, but flatter sheens hide minor wall issues better.
Do I need to paint trim and doors separately? +
Yes — trim/door paint is typically semi-gloss in a different color (often white). Calculate trim coverage separately: linear feet of trim × typical trim profile area (~6" wide for typical baseboard) = trim sq ft.
How much does paint cost? +
Quality interior paint (Sherwin Williams, Benjamin Moore, Behr): $40-70 per gallon. Premium (Aura, SuperPaint): $70-100 per gallon. Cheap (Glidden, Valspar): $25-40 per gallon. The cost difference is real — cheap paint requires more coats, doesn't cover as well, fades faster. Mid-grade is the value play.
Should I use a paint sprayer? +
For whole-house jobs or new construction: yes — dramatically faster, more even finish. For single rooms with all furniture in place: brush + roller is faster than the masking/prep time required for spraying. Plan based on the scope.
What about ceilings? +
Ceilings need their own gallon count. Area = room length × width. Ceiling paint is typically dead-flat white (hides imperfections, no glare). Coverage is roughly the same as wall paint, but cutting in around crown molding and rolling overhead is slower. Allow extra primer if the ceiling has water stains or yellowed from age — stain-blocking primer (Kilz, Zinsser) is non-negotiable on those spots, or stains bleed through fresh paint.
Can I skip the second coat with "one-coat" paint? +
Marketing claims aside, you almost always still need two coats for an even finish, especially with darker colors or going light over dark. "One-coat" paints have higher solids and more pigment than standard paints, so a single coat looks decent — but side-lighting reveals coverage variation that a second coat fixes. Plan for two coats and treat one-coat as a bonus if the wall looks finished after coat one.
How long should I wait between coats? +
Manufacturer label is gospel — but typical guidance is 2-4 hours for latex/water-based paint at 70°F and 50% humidity. High humidity, low temperature, or thick coats stretch it to 6-8 hours. Recoat too soon and the second coat will lift the first, leaving streaks and a tacky finish that takes days to harden. Touch a hidden corner; if any paint transfers to your finger, wait another hour.