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Construction & Home
Paint Calculator — How Much Paint Do I Need (Gallons)
Turn room size, ceiling height, doors and windows into gallons of paint
"How much paint do I need?" is the question that decides your hardware-store run — and whether you make a second trip. Buy too little and you run dry mid-wall with no way to match the batch; buy too much and you've got half-used gallons drying out in the garage. This paint calculator turns a room's dimensions, ceiling height, and the number of doors and windows into the exact number of gallons to buy for the walls.
The math is simple once you know the coverage rate. One gallon of interior paint covers about 350 square feet per coat on a smooth, primed wall. The standard industry estimate runs 350–400 sq ft/gallon; this tool uses the conservative 350 so you don't come up short on textured or thirsty drywall.
The formula has three steps:
Find the wall area. Paintable wall area = perimeter (ft) × ceiling height (ft). For a rectangular room, the perimeter is 2 × (length + width). A 12 ft × 12 ft room has a 48 ft perimeter; at an 8 ft ceiling that's 48 × 8 = 384 sq ft of raw wall.
Subtract the openings. You don't paint doors and windows. The rule-of-thumb deductions are 21 sq ft per door (a standard 3 ft × 7 ft door) and 15 sq ft per window. One door and two windows trims 21 + 30 = 51 sq ft, leaving 384 − 51 = 333 sq ft to paint.
Multiply by coats and divide by coverage. Most jobs need 2 coats for even color and full hide, especially over a new or darker base. Gallons = wall area × coats ÷ 350, then rounded up — you can't buy a fraction of a gallon. Here: 333 × 2 ÷ 350 = 1.9 → 2 gallons.
A few real-world notes baked into the tool. Primer is separate — if you're going over bare drywall, a big color change, or stains, plan a primer coat on top of the finish coats. Ceilings and trim aren't included in the wall figure; ceilings add roughly length × width of floor area, and trim/doors are usually a separate quart. And because the 350 sq ft rate already runs conservative, a built-in waste factor (default 10%) covers roller absorption, touch-ups, and the bit left in the tray. Enter your room, pick your coats, and you'll get a buy-it-once gallon count.
📖 See also: How to Calculate a Tip (and Split the Bill)
Calculator
Fill in the fields and click "Calculate" for instant results.
📰 Formula
• Paintable wall area = perimeter (ft) × ceiling height (ft) − openings • Rectangle perimeter = 2 × (length + width) • Door deduction = 21 sq ft each (3 ft × 7 ft door) • Window deduction = 15 sq ft each (average window) • Coverage = 350 sq ft per gallon, per coat • Gallons = round up ( wall area × coats ÷ 350 ), with optional waste % • Default coats = 2
📰 Formula
• Paintable wall area = perimeter (ft) × ceiling height (ft) − openings • Rectangle perimeter = 2 × (length + width) • Door deduction = 21 sq ft each (3 ft × 7 ft door) • Window deduction = 15 sq ft each (average window) • Coverage = 350 sq ft per gallon, per coat • Gallons = round up ( wall area × coats ÷ 350 ), with optional waste % • Default coats = 2
🧪 Worked examples
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Example 2
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Example 3
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Example 4
⚠️ Common mistakes
- Using floor square footage instead of wall area — paint covers the walls, not the floor.
- Forgetting to multiply by 2 coats (most repaints and color changes need two).
- Subtracting nothing for doors and windows, which over-buys paint on rooms with many openings.
- Rounding the gallon count down — always round up, since you can't buy a partial gallon.
- Counting primer in the same total as the finish coats; primer is a separate purchase.
💡 Tips
- Use the perimeter for odd-shaped rooms: just total the length of every wall and multiply by ceiling height.
- Keep a small leftover for touch-ups, and write the color and finish on the lid for later.
- On a big color change (dark to light), plan a primer coat plus 2 finish coats so the old color doesn't ghost through.
- Textured walls, raw drywall, and deep colors drink more paint — lean toward the higher gallon count.
- Buy all your gallons from the same batch (or have the store box-mix them) so the color is consistent across the room.
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❓ Frequently asked questions
How much paint do I need for a room?
Multiply the room's wall perimeter by the ceiling height, subtract about 21 sq ft per door and 15 sq ft per window, then multiply by the number of coats and divide by 350. Round up to whole gallons. A typical 12×12 room with 8 ft ceilings needs about 2 gallons for two coats.
How many square feet does a gallon of paint cover?
One gallon covers roughly 350 to 400 square feet in a single coat on smooth, primed walls. This calculator uses 350 sq ft per gallon to stay on the safe side, since textured or unprimed surfaces absorb more paint.
Do I need one coat or two coats of paint?
Most jobs need two coats for even color and full coverage, especially when changing colors or painting over a darker base. One coat may be enough when repainting the same color over a clean, primed wall in good condition.
How much paint do I need for a 12x12 room?
A 12×12 room has a 48 ft perimeter. At an 8 ft ceiling that's 384 sq ft of wall; subtract one door (21) and two windows (30) for 333 sq ft. Two coats need 666 ÷ 350 = about 2 gallons.
Does this paint calculator include the ceiling?
No — it estimates wall paint only. To paint the ceiling, calculate the floor area (length × width) separately and add it to your wall area, since ceiling paint is often a different product than wall paint.
How much should I subtract for doors and windows?
Use about 21 square feet per standard door (a 3 ft × 7 ft door) and 15 square feet per average window. If your windows are unusually large, deduct their actual height × width instead for a tighter estimate.
Do I need to buy primer separately?
Yes. Primer is a separate gallon from your finish coats. Plan a primer coat when you're covering bare drywall, patched repairs, stains, or making a big color change so the topcoat goes on even and true.
Why does the calculator round up to whole gallons?
Paint is sold in whole gallons (and quarts), so you can't buy a fraction. Rounding up guarantees you have enough to finish the job and leaves a little extra for touch-ups down the road.
How much paint do I need for a whole house interior?
Add up the wall area of every room using this calculator, total the gallons, then add a gallon or two for trim, closets, and touch-ups. As a rough benchmark, a 2,000 sq ft home often takes 12 to 18 gallons for two coats on the walls.