Enter your room dimensions and get an exact supply list — no guesswork, no wasted trips to the store.
Open full calculator →The most common mistake when buying paint is using the room's floor area instead of its wall area. A 12×12 room has 144 square feet of floor, but if the walls are 8 feet tall and you subtract a door and a window, you're painting closer to 330 square feet. FirstCoat calculates from your actual wall dimensions — length, width, and ceiling height — so the number it gives you reflects what you'll actually be rolling.
Coverage rates vary by finish and brand, but most interior paints cover 350–400 square feet per gallon at a single coat on a primed surface. Darker colours and porous walls tend toward the lower end; light colours over light colours can stretch to the higher end. The calculator uses a conservative 350 sq ft/gallon as its baseline, which means the estimate it gives you accounts for a little real-world variation without sending you back to the store for a second trip.
Two coats is the standard for any colour change or fresh surface. One coat is only reliable when you're applying a very similar colour over a properly primed, sealed wall. If you're going darker, lighter, or painting bare drywall, always plan for two coats — the calculator will handle the math either way.
The guides below cover the most common room sizes and questions in more detail, including finish-by-finish breakdowns and ceiling paint considerations.
Room-by-room guides with exact quantities, finish recommendations, and pro tips.
The most common small bedroom size. Get the exact gallon count for every finish type and coat combination.
Read guide → Room size guideThe standard bedroom and guest room. Includes a finish-by-finish breakdown and ceiling options.
Read guide → Bedroom guideCovers every bedroom size from a 9×9 nursery to a large primary suite, with a size chart and finish tips.
Read guide → Paint quantityThe complete formula — how to calculate exactly how many gallons you need for any room, any ceiling height.
Read guide →Calculate your wall area — not floor area. Add up the perimeter of your room, multiply by ceiling height, then subtract roughly 20 sq ft per door and 15 sq ft per window. Divide by 350 (sq ft per gallon) and multiply by the number of coats. Most rooms need 1–2 gallons for one coat.
Two coats is the standard recommendation for any colour change, new drywall, or a surface that hasn't been painted recently. One coat can work if you're applying a near-identical colour over a recently painted, properly sealed wall. When in doubt, plan for two coats.
Slightly. Matte and flat finishes tend to cover more per gallon (up to 400 sq ft) because they're thicker. Semi-gloss and gloss cover slightly less (300–350 sq ft/gallon). For most rooms the difference is less than a quarter gallon — rarely enough to change how many cans you buy.
Yes — add a 10% buffer to your calculated amount and keep the remainder sealed for touch-ups. Paint batches vary between dye lots, so paint bought later may not match. Leftover paint from the original tin is the best insurance against visible touch-up spots.
Ceiling paint is a separate product — thicker and splatter-resistant. Calculate ceiling area (length × width) and buy dedicated ceiling paint. A standard 12×12 ceiling needs about 0.4 gallons for one coat.
Use primer on bare or new drywall, after repairs, when making a dramatic colour change (especially dark to light), or over glossy surfaces. Primer improves adhesion and often reduces the number of finish coats needed. Paint-and-primer-in-one works for recoats but not bare surfaces.