Paint Calculator

How this calculator works

The calculator measures the paintable wall area of each room, subtracts standard allowances for doors and windows, then converts area into gallons using the coverage rate printed on the paint can. Per room:

gross walls = 2 × (length + width) × ceiling height
deductions  = doors × 20 sq ft + windows × 15 sq ft
net walls   = gross − deductions          (never below 0)
gallons     = net walls × coats ÷ coverage → rounded up

Rooms are summed before rounding, so painting three rooms from the same color doesn't triple the round-up penalty. Ceilings, primer, and trim are estimated as separate products, because they usually are: ceilings use their own flat paint over length × width, primer gets one coat at its own (slightly lower) coverage rate, and trim is modeled as a 4-inch-wide band along each room's perimeter and reported in quarts — the size trim paint is usually bought in.

Every allowance is visible and editable: the 20 sq ft door, the 15 sq ft window, the coverage rates, and the coat counts all appear in the assumptions list under your results, so you can match them to your actual home.

Worked example: a 12 × 10 ft bedroom

Take a 12 × 10 ft bedroom with 8 ft ceilings, one door, and two windows, painted with two coats at 350 sq ft per gallon:

• Gross walls: 2 × (12 + 10) × 8 = 352 sq ft.
• Deductions: one door (20 sq ft) + two windows (2 × 15 = 30 sq ft) = 50 sq ft.
• Net walls: 352 − 50 = 302 sq ft.
• Gallons: 302 × 2 coats = 604 sq ft of coverage needed; 604 ÷ 350 = 1.73 → buy 2 gallons.
• Ceiling (if included): 12 × 10 = 120 sq ft; 120 × 2 ÷ 350 = 0.69 → 1 gallon of ceiling paint.

Enter those same numbers above and you'll see the identical arithmetic in the results panel — nothing is hidden in the estimate.

How to measure a room for paint

• Length and width:measure the floor at the baseboard, wall to wall. For L-shaped rooms, split the space into two rectangles and enter them as two "rooms."
• Ceiling height: measure floor to ceiling. For vaulted or sloped ceilings, use the average height (add the low and high points and divide by two) — it slightly underestimates the tall gable walls, so round up when in doubt.
• Count openings, don't measure them: the standard deductions (20 sq ft per door, 15 sq ft per window) are close enough for estimating. Only adjust them for oversized openings like sliding doors or picture windows.
• Closets count:if you're painting closet interiors, add them as their own small room rather than stretching the main room's dimensions.

Our guide How to Measure a Home Project covers tools and technique in more depth.

Buying paint: sheens, primer, and quantities

• Use different sheens for different surfaces.The common combination is flat on ceilings (hides imperfections), eggshell or satin on walls (wipeable without glare), and semi-gloss on trim and doors (durable, scrubbable). That's guidance, not a rule — any quality line offers all three.
• Prime when the surface changes,not by default: new drywall, repaired patches, stains, glossy existing paint, or a big color shift all justify primer. A simple refresh in a similar color usually doesn't need it.
• Buy all gallons of one color at once and have the store shake them together (or box them yourself) so tint variation between cans never shows up mid-wall.
• Quarts cost more per ounce but waste less. Trim on a couple of rooms is usually a quart-scale job; a whole house of trim may justify gallon pricing.
• Keep leftovers labeled with the room and date — touch-ups from the same can beat rebuying and rematching later.

Common estimating mistakes

• Estimating for one coat. Almost every repaint needs two coats for even color, and the label coverage assumes smooth, sealed walls. One-coat claims rarely survive a color change.
• Using floor area instead of wall area. A 120 sq ft bedroom has roughly 350 sq ft of walls. Square footage from a listing tells you almost nothing about paint quantity.
• Ignoring texture and porosity. Fresh drywall, skim coats, and textured walls drink paint — expect the low end of the label range (around 250–300 sq ft per gallon) instead of 350–400.
• Deducting every opening twice.If you shrink the room dimensions to "account for" the closet doors and then also count the doors, you'll run short. Enter true room dimensions and let the deductions do their job.
• Forgetting supplies.Tape, roller covers, brushes, trays, and drop cloths routinely add a meaningful amount to a first-time setup — the shopping list includes a supplies line so it doesn't surprise you.

Limitations

This tool models rectangular rooms with flat walls and uses fixed per-opening deductions; it does not measure stairwells, vaulted gable walls, wainscoting, or accent walls separately, and the trim estimate is a simple perimeter-based allowance rather than a count of actual baseboard, casing, and crown profiles. It also assumes the coverage rate you enter — heavily textured or unpainted surfaces can fall well below label rates. Treat the output as a planning quantity to buy against, and keep receipts for an unopened return-or-exchange buffer on larger jobs.