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Paint Calculator

Use this paint calculator to find how many gallons of paint a room takes from its walls, ceiling, doors and windows, the number of coats and the coverage of a gallon.

  • Instant answer as you type
  • Nothing you type is sent anywhere
  • Worked example with clear steps

Your measurements

ft
The limit of 10,000 is ours.
ft
The limit of 10,000 is ours.
ft
The limit of 100 is ours.
ft²
36 is an example, a door 3 x 7 ft (21) and a window 3 x 5 ft (15), not a standard. The limit of 1,000,000 is ours.
Price (optional)

The price is yours: we state none, and we store and show no prices.

$ per gallon

Result

Calculated live
Paint to buy 1.81 before rounding up
2 gallons
Area to paint 2 coats, 350 ft² a gallon
316 ft²
Walls 2 x (length + width) x height
352 ft²

Based on NIST Handbook 44, Appendix C Sherwin-Williams data sheet, matte Sherwin-Williams data sheet, satin Full list of sources

Assumptions and constants

You can change

The limits on the other boxes, such as the largest size you can enter, are ours, not from a source.

Coats of paint

Same as the box in the form above

The same data sheets list 2 coats of the paint on drywall. Your surface and your paint may need more or fewer.

Allowed range: 1 to 10 coats. The limits are ours.

Square feet one gallon of paint covers

ft²

Two Sherwin-Williams data sheets give 350-400 square feet per gallon at 4 mils wet. This starts at the low end. Read the coverage on your can.

Allowed range: 50 to 1,000 ft² per gallon. The limits are ours.

Fixed values (exact definitions and references)

These are exact definitions and cannot be edited.

How to use the paint calculator

  1. Enter the Room length, Room width and Wall height, in feet.
  2. Pick Walls only or Walls and ceiling.
  3. Enter Doors and windows, in square feet.
  4. Check the number of Coats. Open Assumptions and constants to change the Square feet one gallon of paint covers.
  5. Read Paint to buy and Area to paint.
  6. Use Copy result or Print, and confirm the amount with your paint seller.

How much paint do I need?

The walls of a room are 2 x (length + width) x height. The ceiling, if you paint it, is length x width. The area to paint is the walls plus the ceiling, less the doors and windows. Gallons = that area x the coats / the coverage of one gallon, rounded up.

Sherwin-Williams' data sheets for its Emerald interior paints give the coverage as 350-400 square feet per gallon at 4 mils wet. [2] They list 2 coats on drywall. [3] This calculator starts at the low end of the coverage, 350.

If your paint also comes in quarts, a gallon is 4 quarts, so the 1.81 gallons of the example are 7.2 quarts. [1]

Those are one maker's figures. Your paint, your surface and the way you apply it change the coverage, so read the can and enter it under Assumptions and constants.

Worked example

Take a room 12 ft long and 10 ft wide, with walls 8 ft high. The walls are 2 x (12 + 10) x 8 = 352 square feet, less 36 square feet for a door and a window, so 352 - 36 = 316 square feet to paint. With 2 coats, 316 x 2 = 632 square feet of paint, and 632 / 350 = 1.81 gallons, which rounds up to 2 gallons.

Room to paint: length 12 ft, width 10 ft A room seen from above, a rectangle a little longer than it is wide. Its length, 12 ft, is marked under it and its width, 10 ft, along the right side. From above Length 12 ft Width 10 ft
Drawn to scale for a room 12 ft long and 10 ft wide, seen from above. The wall height, 8 ft, goes into the page. The calculator adds the walls, and the ceiling if you pick it.

How much paint for a 12x12 room?

Walls only, 8 ft high, with 36 square feet of doors and windows, 2 coats at 350 square feet a gallon. A 12 x 12 ft room takes 348 square feet, which is 1.99 gallons, so 2. The sizes are examples to show the sum.

Gallons of paint for common rooms, walls only
Room (ft) Area to paint (ft²) Gallons before rounding Gallons
10 x 10 2841.622
12 x 12 3481.992
12 x 14 3802.173
14 x 16 4442.543

How do I measure a room for paint?

Measure the length and the width of the room and the height of the walls, in feet. Measure each door and window and add their areas, in square feet. Pick Walls and ceiling if you are painting the ceiling too.

How much does paint cost?

We state no price. Ask local sellers for the price of a gallon, then enter it under Price per gallon, in the section Price (optional) above, to see the total. The site does not store the prices you type.

Common mistakes and limits

  • Counting one coat. Enter the coats you will put on under Coats.
  • Forgetting the ceiling. Pick Walls and ceiling above the doors and windows.
  • Mixing units. Enter the room in feet and the doors and windows in square feet.
  • What this tool does not cover: primer, trim, texture and the paint a rough surface soaks up.

Frequently asked questions

Is paint measured in gallons?

Sherwin-Williams' data sheets give the coverage of its Emerald interior paints as 350 to 400 square feet per gallon. This calculator uses that unit and rounds up to whole gallons. [2][3]

How many coats of paint do I need?

The same data sheets list 2 coats of the paint on drywall, self-priming. Your surface and your paint may need more or fewer, so enter the coats your can calls for. [2][3]

How much paint do I need for a ceiling?

Pick Walls and ceiling. The ceiling is length x width: 12 x 10 = 120 square feet for the example room. With the walls, 436 square feet x 2 coats / 350 = 2.49 gallons, which rounds up to 3.

Can I use this for an outside wall?

We found no source for outside coverage, which depends on the surface. Measure each wall, enter the doors and windows, and enter the coverage printed on your can.

How do I subtract doors and windows from the paint area?

Add the area of each door and window, in square feet, and enter the total under Doors and windows. A door 3 ft by 7 ft is 21 square feet and a window 3 ft by 5 ft is 15.

Sources and references

  1. National Institute of Standards and Technology (NIST). Handbook 44, Appendix C, General Tables of Units of Measurement (2026, accessed November 2026). Used for: the 4 quarts in a gallon.
  2. The Sherwin-Williams Company. Emerald Interior Latex Matte K36 Series, product data (accessed November 2026). Used for: the 350-400 square feet per gallon and 2 coats, from one maker's matte data sheet.
  3. The Sherwin-Williams Company. Emerald Interior Latex Satin K37 Series, product data (accessed November 2026). Used for: the same coverage and coats, from the same maker's satin data sheet.

How we use sources: methodology.

Estimates only. Confirm quantities with your supplier. See the disclaimer. Found a wrong result? Report an error.