How to Calculate How Much Paint You Need (The Right Way)

Nothing kills a weekend painting project faster than running out of paint halfway through the second wall — or ending up with three half-empty gallons you'll never touch again. The good news: figuring out exactly how much paint you need takes about two minutes with a tape measure and simple math.

Step 1: Measure your walls

For a standard rectangular room, measure the length and width of the floor, plus the height of the walls. Your total wall area is:

2 × (length + width) × height

So a 12 × 12 ft bedroom with 8 ft ceilings gives you 2 × (12 + 12) × 8 = 384 square feet of wall. If your room isn't a simple rectangle, just measure each wall individually (width × height) and add them up.

Step 2: Subtract doors and windows

You aren't painting the doorway. Subtract roughly 21 sq ft for each standard door and 15 sq ft for each average window. Our 384 sq ft bedroom with one door and two windows becomes 384 − 21 − 30 = 333 sq ft of actual paintable surface.

Step 3: Multiply by the number of coats

One coat is fine if you're repainting the same (or a very similar) color. Going from dark to light, or painting over bare drywall? Plan on two coats — which simply doubles your area. Our example: 333 × 2 = 666 sq ft.

Step 4: Divide by your paint's coverage

Check the can: most interior paints cover 350–400 sq ft per gallon. Flat finishes stretch furthest (around 400); satin and semi-gloss cover a bit less (around 350) because they go on thicker. Using 375 sq ft/gal for an eggshell finish: 666 ÷ 375 = 1.78 gallons.

Step 5: Add 10% for waste

Rollers soak up paint, a little spills, and you'll want some left for touch-ups down the road. Add 10%: 1.78 × 1.10 = 1.96 gallons.

Step 6: Round up to what the store sells

Paint is sold in gallons and quarts, so round up to the nearest quart. Our 1.96 gallons becomes 2 gallons — or 1 gallon + 4 quarts if you'd rather keep a small can for touch-ups.

Quick reference

| Situation | Rule of thumb | |---|---| | Average 12×12 bedroom, 2 coats | ~2 gallons | | Living room 16×20, 2 coats | ~3–4 gallons | | Bathroom, 2 coats | ~1 gallon | | Repainting same color, 1 coat | Half the 2-coat amount |

Mistakes to avoid

• Forgetting the ceiling. Ceilings need paint too — measure length × width and treat it as extra wall area (flat white ceiling paint is cheapest).
• Ignoring primer. Bare drywall or dramatic color changes need primer first, which also follows the same math.
• Buying exactly the calculated amount. Always round up. A $45 gallon sitting in your garage beats a second trip to the store mid-project.

Want to skip the math? Use our free paint calculator — enter your room dimensions and it does all six steps instantly, in gallons and liters.