A paint-coverage calculator estimates how many gallons of paint you need for a room based on wall area, coats, and the paint's spread rate. The Toolenza calculator uses a default of 350 sq ft per gallon for one coat — the manufacturer-quoted spread for most interior latex paints in good condition.

What "350 sq ft per gallon" actually means

That's the first-coat coverage on a smooth, primed, single-colour surface. Real coverage drops on:

• Porous surfaces (raw drywall, bare wood) — often need a separate primer first, then ~280–320 sq ft/gal on the paint.
• Textured surfaces (popcorn ceiling, brick, stucco) — 200–280 sq ft/gal. The extra surface area eats paint.
• Dark colours over light — most need 2 coats, sometimes 3, because high-pigment paints have lower spread.
• Light colours over dark — almost always 2 coats; sometimes 3 with a tinted primer.

Two-coat rule of thumb

For most repaints, plan for 2 coats. The math: room area × 2 / 350 = gallons needed. A 12 × 14 ft room with 9 ft ceilings has ~468 sq ft of wall (less ~40 sq ft for one door and one window) → 428 × 2 / 350 ≈ 2.4 gallons. Round up to 3 — paint stores can't reorder the exact tint match later, so finishing one room with one batch matters.

Buying tips

• Buy all the paint at once. Tint batches vary by a few percent; mid-job replenishment can show on the wall as a streak.
• Save the leftover. Cans last 2–10 years if properly sealed. Label with the room + date.
• Skip the bargain primer. A separate quality primer on bare or stained surfaces saves a coat of finish paint, which costs more per gallon.
• Quart for trim, gallon for walls. Trim takes 50–80 sq ft/quart; a 100-ft baseboard at 4 inches tall is ~33 sq ft per coat, easy on one quart.