How Much Paint Do I Need?
The one-number answer to “how much paint do I need?” — measure the room once, subtract the openings, and buy whole gallons from a single batch so you never run short mid-wall or re-order a slightly different color.
This room needs about 3 gallons of paint — the one-number “how much paint do I need” answer, paintable area × coats ÷ coverage, rounded up to whole gallons. Rough or porous walls and a big color change use more, so allow extra for texture, porosity and waste, and buy from one batch for color consistency.
Calculator inputs
This is the flagship starter tool: it turns a tape measure into a shopping list. Paint quantity is plain geometry — the paintable area of the walls, multiplied by the number of coats, divided by how far a gallon spreads. The only judgement calls are how many coats you need and how thirsty the surface is, and both are inputs you control.
Work in this order: measure the room’s length and width, measure the wall height, count the doors and windows, then decide coats and coverage. The tool deducts a standard door (~21 sq ft) and window (~15 sq ft) for you, multiplies by coats, divides by coverage, and rounds up to whole gallons — because you buy paint by the can, not the cupful.
Formula
perimeter = 2 × (length + width)gross wall = perimeter × wall heightnet paintable = gross wall − (doors × 21 + windows × 15)gallons = ceil( net paintable × coats ÷ coverage per gallon )
The deductions and coverage are labeled planning typicals you can override; the geometry never changes.
Worked example
A 12 ft × 15 ft bedroom with 8 ft walls, one door and two windows, two coats, coverage 350 sq ft/gal:
- Perimeter = 2 × (12 + 15) = 54 ft.
- Gross wall = 54 × 8 = 432 sq ft.
- Openings = 1 door (21) + 2 windows (30) = 51 sq ft; net = 432 − 51 = 381 sq ft.
- Gallons = ceil(381 × 2 ÷ 350) = ceil(2.18) = 3 gallons.
Buy all three from the same batch (check the batch/lot number on the can) so the color is dead consistent, and keep the leftover for touch-ups.
Measure first, buy once
Measure first — do not paint from a floor-plan number:
- Take the height in a few spots. Settled or sloped walls vary; use the tallest run so you do not under-buy.
- Do not over-deduct. A standard interior door is ~21 sq ft and a window ~15; a patio/sliding door is closer to 40. Skip the deduction entirely if you are painting the door and casing too.
- Coverage is the big lever. Textured, porous, bare or dark-to-light surfaces drink paint — lower the coverage figure rather than hoping. The paint can’s stated spread rate always wins over a rule of thumb.
- Round up, then add a touch-up quart. Running 5% short means a second store trip and a possible batch mismatch.
Reference table
| Paint × surface | Coverage (sq ft/gal, one coat) |
|---|---|
| Smooth / previously-painted drywall (latex) | 350–400 |
| New / primed drywall | 300–350 |
| Textured or porous interior wall | 250–300 |
| Smooth wood / trim (enamel) | 350–400 |
| Bare / rough wood | 200–300 |
| Exterior lap / vinyl siding (smooth) | 300–400 |
| Stucco / rough masonry | 150–250 |
| Brick (unpainted) | 100–200 |
| Concrete / block | 200–300 |
| Deck / fence (semi-transparent stain) | 200–300 |
| Primer (drywall / general) | 200–300 |
Labeled published planning snapshot — the can’s stated spread rate wins. Rough, porous or thirsty surfaces sit at the low end; a big color change or a light coat pushes you toward the high end. Full matrix: paint coverage by surface.
Frequently asked questions
How much paint do I need for a 12x12 room?
For a 12 ft × 12 ft room with 8 ft walls the perimeter is 48 ft and the gross wall is 384 sq ft; take out a door and a window (~36 sq ft) and you paint about 348 sq ft. At two coats and 350 sq ft/gal that is ceil(348 × 2 ÷ 350) = 2 gallons, plus about a gallon for the ceiling.
Does the calculator include the ceiling?
No — this tool sizes the walls. Add the ceiling with the ceiling paint calculator (length × width, usually one coat), or use the room paint calculator to get walls and ceiling together.
How many coats should I plan for?
Two is the everyday default. Plan on a primer coat plus two finish coats for bare drywall, a dark-to-light change, or a bold color. A same-color refresh on a sound wall can be one. See the coats & drying-time reference.
Should I buy extra paint?
Round up to whole gallons (this tool does), then keep a quart in reserve for touch-ups. Rough or porous surfaces and cutting-in waste push real use higher than the tidy math — allow for it rather than risk a batch mismatch on a mid-project re-order.