Paint Calculator
Work out exactly how many gallons or litres of paint you need for a room, based on your actual wall dimensions and number of coats — not a rough "one gallon per room" guess. Add your own paint price for a full cost estimate.
How this calculator works
The calculator totals your wall area, subtracts doors and windows, multiplies by the number of coats, then divides by how far a gallon actually spreads:
gallons = ceil( (wall area × coats) ÷ coverage per gallon )
Coverage varies by paint brand and wall texture — 350 sq ft per gallon is a reasonable default for a smooth interior wall, but check your specific can, since heavily textured walls can run closer to 250–300 sq ft per gallon.
Worked example
A 12 ft × 10 ft room with 8 ft ceilings, one standard door, two coats:
- Wall perimeter: (12 + 10) × 2 = 44 ft. Wall area: 44 × 8 = 352 sq ft.
- Subtract one door (21 sq ft): 352 − 21 = 331 sq ft paintable.
- Two coats at 350 sq ft/gallon: (331 × 2) ÷ 350 ≈ 1.89 → round up to 2 gallons.
A single gallon can looks tempting for a small room, but running short mid-coat on drying paint almost always shows as a visible line — buy the full gallon indicated, even if a little is left over.
Choosing the right number of coats
| Situation | Coats | Why |
|---|---|---|
| Same color, good condition wall | 1 | Existing coat already provides base coverage |
| Standard color change | 2 | Covers roller marks and gives even coverage |
| Light color over dark, or bare drywall | 2–3 | Primer first, then 1–2 topcoats for full opacity |
| Saturated accent color (red, navy, black) | 3 | Pigment-heavy colors need extra coats to look even |
Do you need primer?
Primer is a separate product from paint, and this calculator totals it separately when you turn it on, using the same coverage rate. It's worth adding when you're painting bare drywall or wood, covering a stain or patch, making a big color jump (dark to light or the reverse), or switching from oil-based to latex paint. Skip it for a same-color refresh over paint that's already in good shape.
Walls vs ceilings
Ceilings are usually painted in a flat finish designed to hide imperfections and resist spatter, which is a different product from most wall paint. Coverage per gallon is similar either way, so this calculator lets you total the ceiling separately with its own coats and price if you're using a dedicated ceiling paint.
Frequently asked questions
How many coats of paint do I actually need?
Two coats is standard for most walls, even when the new color looks similar to the old one — a single coat almost always shows roller marks and patchy coverage once it dries. One coat can be enough for a fresh coat of the exact same color in good condition. Three coats are typically only needed for a light color going over a dark one, or a saturated accent color like red.
Do I need primer before painting?
Primer is worth adding as a separate coat when you're painting bare drywall, covering a stain, switching from dark to light (or the reverse), or changing paint types such as oil-based to latex. Painting a similar color over existing paint in good condition usually doesn't need a separate primer coat.
How much paint do I need for a 12x12 room?
A 12×12 ft room with 8 ft ceilings has about 320 sq ft of wall area before subtracting doors and windows. At 350 sq ft of coverage per gallon and two coats, that works out to roughly 2 gallons. Enter your exact wall dimensions and openings in the calculator above for a precise figure.
Is ceiling paint different from wall paint?
Ceiling paint is usually a flat finish formulated to hide imperfections and resist drips and spatter, while wall paint comes in a range of sheens for durability and cleanability. Coverage per gallon is similar for both, so this calculator lets you total the ceiling separately with its own paint if you're using different products.