Paint Cans Calculator: How Many Gallons Do You Really Need?
The label on a gallon of paint says 400 sq ft per gallon. Your rough, unpainted drywall gets 250. The math between these two numbers is where most home painters end up short — and why buying the wrong number of cans is the single most common home improvement mistake.
Coverage Per Gallon Isn't What the Label Says
Paint manufacturers test coverage on smooth, already-painted surfaces. Real walls — especially new drywall, rough texture, or a dark color being covered by a light one — absorb significantly more paint. A standard 350 sq ft/gallon rate is a reasonable middle ground. For rough, porous, or unpainted surfaces, use 250. For premium paint on already-painted smooth walls, 400 is realistic.
One Coat vs. Two Coats
One coat is only appropriate for touch-ups or when painting the same color over itself with a high-hiding paint. Any color change needs two coats — going from a dark to a light color may need three. This is the variable that catches people off guard most often: they calculate for one coat and buy half what they need.
What to Subtract
A standard interior door is about 20 sq ft. A typical window is 10–15 sq ft. For a room with 4 walls, you'll have 2 doors and 2 windows typically — subtract roughly 70 sq ft from your total. Most people skip this step, which is fine (a little extra is good), but knowing lets you calibrate whether you have just enough or a full can extra.
Quarts vs. Gallons vs. 5-Gallon Buckets
Quarts (0.25 gal) are for small jobs — one accent wall, a closet, trim touch-ups. If you need more than 1.5 gallons, buy gallons. If you need more than 4 gallons of the same color, consider a 5-gallon bucket — it's cheaper per gallon and ensures color consistency across a large area. The calculator shows all three so you can pick based on your project size.
Frequently Asked Questions
How much paint do I need for a 12x12 room?
A 12x12 room with 8-foot ceilings has about 384 sq ft of wall area (perimeter 48 ft × 8 ft). Subtract a door (20 sq ft) and a window (12 sq ft) = 352 sq ft. At 350 sq ft/gallon and 2 coats, you need 2.0 gallons — so buy 2 gallons. Add the ceiling (144 sq ft) and you need roughly 0.4 gallons more, so a 3-gallon total if painting the ceiling too.
Why do I need two coats of paint?
One coat of paint is rarely opaque enough to fully cover the previous color or the bare surface beneath it. Two coats give better adhesion, more even color, and longer-lasting results. For dark-to-light color changes, two coats may still show the old color — a primer coat first is the better solution.
What's the coverage rate for primer?
Primer typically covers 200–300 sq ft per gallon because it's formulated to bond and seal rather than cover efficiently. Use 200 sq ft/gallon for new drywall or highly porous surfaces; 300 for previously painted walls. Always prime new drywall before painting — skipping it causes uneven sheen and poor coverage.
Is it better to buy more paint or less?
Buy slightly more than you need. Leftover paint is useful for touch-ups — matching paint exactly is nearly impossible after the original batch is gone. Most paint stores will mix the same formula from the original record, but it won't be a perfect match after drying time and aging. Keep the leftover in a sealed, labeled can in a cool, dry spot.
This article is for informational purposes only. Results are estimates — actual material needs vary based on site conditions. See our disclaimer.