Not sure how much paint to buy? Enter the room’s length, width and wall height — or tap a typical room size — then the number of doors and windows. Pick how many coats you’ll roll on and whether you’re painting the ceiling too. The calculator works out the wall area, takes off the doors and windows, and tells you how many gallons and quarts to buy, picking a sensible mix of can sizes. Ceiling paint and primer are shown separately. For a closer estimate, type in the coverage printed on your paint can. Switch to Metric for meters and liters.
How it works: wall area = 2 × (length + width) × height, minus about 20 sq ft per door and 15 sq ft per window (1.9 m² and 1.4 m²). Paint needed = area × coats ÷ coverage. Most interior paint covers 350–400 sq ft per gallon (about 9–10 m² per liter) on smooth walls; use 250–300 for textured, bare or patched walls. The ceiling is length × width, and primer is figured at 300 sq ft per gallon (7.5 m²/L). Can sizes are chosen by typical prices — a quart costs about half as much as a gallon, so a gallon often beats 2–3 quarts. Metric uses 1 L, 2.5 L and 5 L tins.