Paint calculator: how much paint do I need?

How much paint do you need for a room?

This paint calculator takes the size of your room, subtracts doors and windows, multiplies by the number of coats, divides by the coverage of the paint and adds a waste allowance. It shows the Paint needed, waste included, then the Suggested cans: the mix of can sizes that leaves the least paint unused. It works in feet and US gallons by default; switch to Metric (m, liters) if your measurements are in meters.

How to use it

1. Under Input, pick Room (length × width × height), or Wall by wall for an accent wall or an L-shaped room.
2. Tick Paint the ceiling too if needed. In room mode, the ceiling area comes from the length and width.
3. With Subtract doors and windows on, adjust the count and sizes. The defaults are a 32 × 80 in interior door and 36 × 48 in windows.
4. Choose a Paint type, which fills in the Coverage, then set the Number of coats (2 by default) and the Waste allowance (10 % by default).
5. Edit the can sizes if your store sells others (quart, gallon and 5-gallon bucket by default), and enter a price per gallon to see the cost.

A worked example

A 12 × 10 ft bedroom with 8 ft ceilings, one 32 × 80 in door, two 36 × 48 in windows, two coats, coverage of 350 ft² per gallon and a 10 % allowance:

• Walls: 2 × (12 + 10) × 8 = 352 ft².
• Door: 32 × 80 = 2,560 in², or 17.78 ft². Windows: 2 × 36 × 48 = 3,456 in², or 24 ft².
• Area to paint: 352 − 41.78 = 310.22 ft².
• Paint: 310.22 × 2 ÷ 350 = 1.77 gal, then + 10 % = 1.95 gal.
• Cans: 2 × 1 gal = 2 gal, with 0.05 gal left over. At $38 a gallon, that is $76.

Add the ceiling (12 × 10 = 120 ft²) and the area grows to 430.22 ft², or 2.70 gal: the tool suggests 2 gallons plus 3 quarts, 2.75 gal in all. The same room in metric (4 × 3.5 m, 2.5 m high, 10 m²/L) needs 7.33 L, which comes as a 5 L and a 2.5 L can.

Coverage by surface

The coverage printed on the can is the figure to trust. The ranges in the tool are typical for a sound, smooth surface:

• Interior walls: 350 to 400 ft² per gallon. At 400, the example drops to 1.71 gal.
• Ceilings: 325 to 400 ft² per gallon; flat ceiling paint is thicker. The example ceiling alone, at 325, needs 0.81 gal: one gallon.
• Primer: 325 to 400 ft² per gallon, usually one coat, on new drywall, patched areas or before a big color change.
• Trim, doors and cabinets: 450 to 550 ft² per gallon on smooth, prepared wood.

Textured, porous or unprimed walls soak up more: lower the coverage or raise the allowance.

How the cans are picked

The tool tries every mix of the sizes you entered and keeps the one whose total goes over your need by the smallest amount. On a tie, it takes the mix with fewer cans. A 5-gallon bucket is often cheaper per gallon than five single gallons, so check the prices before you follow the suggestion to the letter.

Frequently asked questions

How much paint for a 12 × 12 room?

With 8 ft walls, no openings subtracted, two coats at 350 ft² per gallon: 384 × 2 ÷ 350 = 2.19 gal, or 2.41 gal with 10 % extra. That means 2 gallons and 2 quarts.

Do I really need two coats?

Usually yes: the first coat soaks into the surface and the second gives an even color. One coat can do when you repaint the same color over a sound wall.

Why add a waste allowance?

Some paint stays in the tray, the roller and the brushes. 10 % suits smooth walls; allow 15 to 20 % for rough ones.

Should I subtract small windows?

A small window barely changes the result. Untick Subtract doors and windows to keep extra paint in hand.