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Paint Calculator (Gallons)

Work out how many gallons of paint a room needs, and which cans to buy. Every assumption is editable.

Room lengthRoom widthSeen from aboveDoorWindowWall heightOne wall
Key Where each paint measurement is taken.
Units
ft
ft
Typical ceiling. 8 ft is common, and 9 ft is common in newer homes. Editorial rule of thumb.
ft
sq ft

If you fill this in, it is used instead of the room size for the walls.

Typical door. An interior door is usually 80 in tall and 30 to 32 in wide, about 17 to 18 sq ft. A 36 in door is 20 sq ft. Measure yours if it differs. Editorial rule of thumb.
sq ft
Windows vary a lot. The starting figure is only a placeholder. Measure each window you will not paint.
sq ft
sq ft/gal

Check the can. The starting figure is cautious.

%
$

Adds an estimated cost for the suggested sizes.

Paintable area368 sq ft
Paint needed, 2 coats2.10 gal
Paint to buy, with waste2.31 gallons
Suggested cans2 × 1 gal + 2 × 1 qt (2.5 gal)

Walls: 2 × (12 + 14) × 8 ft = 416 sq ft, minus doors and windows 48 sq ft, gives 368 sq ft to paint. × 2 coats = 736 sq ft; ÷ 350 sq ft/gal = 2.10 gal; with 10% waste = 2.31 gal.

Porous, rough or dark surfaces need more paint. Check the coverage on the can.

Fig. 1 Paint for your room.

Every formula is tested against worked examples. How we check

CoatsPaint to buySuggested cans
1 coat1.16 gal1 × 1 gal + 1 × 1 qt
2 coats2.31 gal2 × 1 gal + 2 × 1 qt
3 coats3.47 gal3 × 1 gal + 2 × 1 qt

Fig. 2 The same room at one, two and three coats.

Last checked 11/10/2026. Sources: Sherwin-Williams: Paint calculator (a gallon covers about 350 to 400 sq ft).

Guide

How the amount is worked out

Wall area = 2 × (length + width) × height. Take away the doors and windows, add the ceiling if you are painting it, multiply by the number of coats, then divide by how far a gallon spreads. Finally add a little extra for waste.

Example. A 12 × 14 ft room with 8 ft walls has 416 sq ft of wall. One door (20 sq ft) and two windows (15 sq ft each) leave 366 sq ft. Two coats make 732 sq ft, and at 350 sq ft per gallon that is 2.09 gallons before waste, about 2.3 gallons with 10% extra.

Where the 350 sq ft per gallon comes from

Sherwin-Williams says a gallon typically covers about 350 to 400 square feet. This tool starts at the cautious end, 350, because walls are rarely perfect. Rough, textured or porous surfaces, raw drywall and big color changes all use more paint, so check the coverage printed on the can.

Common mistakes

  • Forgetting the second coat. Most rooms need two, especially over a darker color.
  • Buying exactly the figure shown. Keep a little for touch-ups; a part-used can of the same batch matches better than a new one.
  • Treating the door and window sizes as exact. They are averages; measure big windows and French doors yourself and use the direct wall-area box.
  • Mixing batches. Mix cans of the same color together (boxing) so a slight batch difference does not show on one wall.

Pack sizes

The suggested cans use US sizes: quart, 1 gallon and 5 gallon. The tool picks the least paint that covers the amount and uses the fewest cans. Sizes in stores vary by brand, so treat it as a starting point.

Common questions

How many gallons of paint for a 12 x 14 room?

With 8 ft walls, one door, two windows and two coats, a 12 x 14 ft room needs about 2.1 gallons, or about 2.3 gallons with 10% for waste. Try it in the calculator above.

Do I paint over the door and window area?

No. The calculator subtracts them. If your windows are large or you have lots of trim, change the deduction.

Does the ceiling need the same paint as the walls?

Usually not; ceilings often use a flat ceiling paint. If you tick the ceiling box the tool simply adds its area to the total, so for separate paint run it once for the walls and once for the ceiling.

Do I need primer?

On new drywall, stains, or big color changes, often yes. Primer is a separate product with its own coverage on its can. Enter that figure and run the calculator for one coat.

Worked examples

More calculators

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