How much paint do I need?
By Dexaloop · Updated 27 October 2026 · Checked against the sources listed below
To work out how much paint you need, measure the wall area (height x width of each wall), subtract doors and windows, multiply by the number of coats, and divide by the coverage the maker prints on the can. Sherwin-Williams says a gallon of paint "typically covers about 350-400 square feet" for one coat. A typical 12 x 14 foot bedroom with 8 foot walls needs about 742 square feet of coverage for two coats, so a little over 2 gallons. The sections below explain each step, primer, ceilings, exterior walls and metric tins.
Step 1: measure the walls
For each wall, multiply its width by its height. If the room is a simple box with walls of equal height, there is a shortcut: add up all the wall widths (the perimeter) and multiply by the height once.
A 12 x 14 foot room has a perimeter of 12 + 14 + 12 + 14 = 52 feet. With 8 foot walls the wall area is 52 x 8 = 416 square feet.
Step 2: subtract doors and windows
Dulux's advice is to measure each window and door the same way and deduct them from the total. Measure your own openings rather than using a standard size, because they vary. In the example room:
- One door, 3 x 7 feet = 21 sq ft
- Two windows, each 3 x 4 feet = 24 sq ft
Net wall area = 416 minus 45 = 371 square feet. Skipping this step would overstate the room by about 12%.
Step 3: multiply by the number of coats
Most jobs need two coats for even color. The Dulux Matt product page says "Apply 2 coats with a brush or roller," and Dulux's exterior walls guide says two coats are usually all you need on bare walls, adding that "if you're painting over a strong shade, you might need a third." Some products are rated differently. Sherwin-Williams's Duration exterior paint, for example, calls for 2 coats on new, bare surfaces and 1 coat for a repaint. Use what your label says.
Example: 371 x 2 coats = 742 square feet of coverage.
Step 4: divide by the coverage figure
Coverage is printed on the can or the product data sheet. It varies by product and by surface, so use your product's figure when you have it.
| Product or item | Maker's stated coverage |
|---|---|
| Interior paint, general (Sherwin-Williams) | About 350 to 400 sq ft per gallon |
| Quart of paint (Sherwin-Williams) | 100 sq ft |
| Primer (Sherwin-Williams) | About 200 to 300 sq ft per gallon |
| Duration Exterior Acrylic Satin (Sherwin-Williams) | 250 to 300 sq ft per gallon |
| Dulux Matt (UK) | Up to 14 m² per liter |
| Weathershield All Weather Smooth Masonry (UK) | About 15 m² per liter |
| Weathershield textured masonry (UK) | About 9 m² per liter |
For the example room at 350 to 400 sq ft per gallon:
- 742 / 400 = 1.86 gallons
- 742 / 350 = 2.12 gallons
Two gallons plus one quart gives 700 + 100 = 800 sq ft even at the low rate, which covers both ends of the range. Two gallons alone would only be enough if you get the best-case coverage.
Primer
Primer is a separate coat with its own coverage. Dulux's guide to painting exterior walls recommends priming, and says this matters most for "shiny or porous surfaces so that your first coat of paint adheres properly." The same exterior guide says to prime new or bare wood, and to use a stabilizing primer on chalky masonry. Some paints are labeled self-priming on certain surfaces; Duration is one example on its data sheet.
For one coat of primer over the example's 371 square feet at 200 to 300 sq ft per gallon: 371 / 300 = 1.24 and 371 / 200 = 1.86 gallons. Two gallons covers it.
Ceilings
Treat the ceiling as its own job, since you may use a different paint there. A 12 x 14 ceiling is 168 square feet. Two coats is 336 square feet, so one gallon at 350 to 400 sq ft is enough.
Worked example in metric
A room 4 m by 3.5 m with walls 2.5 m high:
- Perimeter: 4 + 3.5 + 4 + 3.5 = 15 m. Wall area: 15 x 2.5 = 37.5 m².
- Door 0.8 x 2.0 = 1.6 m². Window 1.2 x 1.2 = 1.44 m². Net: 37.5 minus 3.04 = 34.46 m².
- Two coats: 34.46 x 2 = 68.92 m².
- Dulux Matt at up to 14 m² per liter: 68.92 / 14 = 4.92 liters.
Because "up to" is a best case, a single 5 liter tin leaves almost no margin. If the walls are porous or the color change is large, plan for a little more. To compare with US cans, NIST lists one US gallon as about 3.785 liters.
Rough and exterior surfaces
Texture soaks up paint. Dulux warns that a rough surface can "double the amount of paint you need," and its own masonry figures show it: about 15 m² per liter for smooth masonry paint against about 9 m² per liter for the textured version. Exterior products can be rated lower too: the Duration data sheet gives 250 to 300 sq ft per gallon, against the general 350 to 400. Use the exterior product's own figure for siding or brick.
Common mistakes
- Using floor area instead of wall area. A 12 x 14 floor is 168 sq ft; its walls are 416.
- Forgetting the second coat. It doubles the amount.
- Using best-case coverage on a hard surface. Porous, textured or new surfaces use more.
- Skipping primer in the estimate when the surface needs it.
- Not deducting openings, or deducting a guessed size instead of the measured one.
- Mixing units. Keep feet with square feet per gallon and meters with m² per liter.
Once you have the wall sizes, openings and your product's coverage, the paint calculator below does the sums for walls, ceilings and coats and rounds to whole cans.
Work out your paint: tools.dexaloop.com/paint-calculator/. Free, no sign-up, and it runs on your device.
Questions
How many square feet does a gallon of paint cover?
Sherwin-Williams says a gallon typically covers about 350 to 400 square feet. That is for one coat, and the exact figure for your product is on its label or data sheet. Some exterior paints are rated lower, such as 250 to 300 square feet per gallon.
Do I really need two coats?
Usually. Dulux Matt is labeled for 2 coats, and Dulux's exterior walls guide says two coats are usually enough, with a third sometimes needed over a strong color. A few products, and some repaints in the same color, are rated for one coat. Follow the label.
Do I need primer?
Prime bare, new, shiny or porous surfaces unless the paint is labeled self-priming for that surface, as Dulux's exterior walls guide and the Sherwin-Williams data sheets describe. Sherwin-Williams says a gallon of primer covers about 200 to 300 square feet.
How many liters are in a US gallon?
About 3.785. NIST lists one US gallon as 3.785 411 784 liters.
Should I deduct doors and windows?
Yes. Dulux advises measuring each window and door the same way as the walls and subtracting them from the total. For a room with one door and two windows this can cut the area by more than 10%.
Sources
- Sherwin-Williams: Paint calculator and coverage guide
- Sherwin-Williams: Duration Exterior Acrylic Satin product data sheet
- Dulux UK: How much paint do you need?
- Dulux UK: Dulux Matt product page
- Dulux UK: How to paint exterior walls
- NIST Handbook 44, Appendix C: General tables of units of measurement
This guide is general information, not legal, tax or financial advice. Rules change; check the official source for your own case. Found a mistake? Email support@dexaloop.com.