How much paint do I need for this room?
Measure once, buy once. Here is the arithmetic painters use, a calculator that does it for walls, ceilings and trim, and a chart for the room sizes most people are painting.
By Painter Hotline · Published
Multiply the room’s perimeter by the wall height, subtract about 21 square feet per door and 15 per window, multiply by the number of coats, and divide by the coverage on the can. Angi puts a gallon of interior paint at 300 to 400 square feet. A 12 × 12 bedroom with 8-foot walls takes about 2 gallons for two coats.
Paint calculator for one room
Change any number. The door and window sizes are typical defaults, not rules; measure yours if they are unusual.
How do you measure a room for paint?
You need four measurements and a tape measure. Precision to the nearest half foot is plenty.
1. Wall area
Add up the length of every wall (the perimeter) and multiply by the height from floor to ceiling. A 12 × 14 room with 8-foot walls has a perimeter of 52 feet and 416 square feet of wall. In an open plan, measure only the walls getting the color, and treat an opening into another room as a big doorway.
2. Take out doors and windows
Subtract what you will not paint. A standard interior door is about 3 by 7 feet, or 21 square feet; a typical window is around 3 by 5, or 15. Large picture windows and sliding doors are worth measuring. If a room has very few openings, skip this step and you will simply have a little extra paint for touch-ups.
3. Decide on the number of coats
- One coat can be enough when you are refreshing the same color over sound paint.
- Two coats is the norm for most repaints and for any color change. It gives even color and sheen.
- Primer plus two coats for new drywall, large patches, water stains and big jumps from dark to light. Angi’s Denver interior guide puts primer at about 100 square feet per quart, which is 400 per gallon.
4. Divide by coverage
Coverage is printed on the can, usually as a range. Angi’s guide gives 300 to 400 square feet per gallon as typical for interior paint. Use the low end for textured walls, porous surfaces and deep colors, the high end for smooth walls in the same color. Many Front Range homes have textured drywall, which soaks up more paint than a smooth wall.
Put together, the formula is: (perimeter × height − openings) × coats ÷ coverage = gallons.
How much paint do common room sizes need?
These figures assume 8-foot walls, one door, two windows, two coats and 350 square feet per gallon, the middle of Angi’s range. They are our arithmetic, not a manufacturer table, so round up when you buy.
| Room | Wall area | Walls, 2 coats | Ceiling, 1 coat | Buy |
|---|---|---|---|---|
| 10 × 10 ft | 269 sq ft | 1.5 gal | 0.3 gal | 2 gal walls + 1 gal ceiling |
| 10 × 12 ft | 301 sq ft | 1.7 gal | 0.3 gal | 2 gal walls + 1 gal ceiling |
| 12 × 12 ft | 333 sq ft | 1.9 gal | 0.4 gal | 2 gal walls + 1 gal ceiling |
| 12 × 14 ft | 365 sq ft | 2.1 gal | 0.5 gal | 3 gal walls + 1 gal ceiling |
| 14 × 16 ft | 429 sq ft | 2.5 gal | 0.6 gal | 3 gal walls + 1 gal ceiling |
| 16 × 20 ft | 525 sq ft | 3.0 gal | 0.9 gal | 3 gal walls + 1 gal ceiling |
How much paint for ceilings, trim and doors?
Ceilings are easy: the area is the room’s length times its width. A 12 × 12 ceiling is 144 square feet, well under half a gallon per coat. Ceiling paint is usually flat to hide seams and roller marks, and it is often one coat when you are repainting white over white.
Trim is measured in linear feet. Multiply by the width in inches and divide by 12 to get square feet: 100 feet of 4-inch baseboard is about 33 square feet. That is why a quart of trim enamel goes a long way, and why the calculator above usually shows a fraction of a gallon.
Doors are about 21 square feet per side. Paneled doors take a little more than flat slab doors because of the profiles.
Buying it
- Round up to the next whole gallon for walls, and buy quarts for small amounts.
- Buy all of one color at once from the same batch, and pour the cans together in a bucket before you start so the color is identical from wall to wall.
- Keep a labeled quart of every color for touch-ups.
Why do people run out of paint, or end up with too much?
The arithmetic is simple, so when the numbers go wrong it is almost always one of these:
- Counting the floor instead of the walls. A 12 × 12 room is 144 square feet of floor but over 380 square feet of wall before openings. Paint is for the walls.
- Forgetting the second coat. One coat of a new color over an old one almost always looks patchy. Plan for two from the start, then keep the spare gallon if you do not need it.
- Using the top of the coverage range on a thirsty surface. Fresh drywall mud, flat old paint, textured finishes and bare wood all drink paint. Primer evens them out and is cheaper per gallon than finish paint.
- Deep colors. Saturated reds, yellows and navies often need an extra coat or a gray-tinted primer to cover evenly. Read the can; many deep bases say so.
- Rolling too thin. Stretching paint to make a gallon reach saves a few dollars and costs a coat. Hold the roller loaded, and let the coverage figure be what it is.
Ending up with extra is the better problem. Unopened gallons can often be returned if they were not tinted; tinted paint goes to a PaintCare site once you have kept a touch-up quart.
Exterior walls are a different calculation
Siding, stucco and masonry use the same formula, but coverage changes a lot with the surface. Rough-sawn wood, stucco and block soak up far more than smooth interior drywall, and thick masonry coatings are applied at a fraction of the coverage of wall paint. For outside work, use our exterior paint calculator, which handles gables, doors, garage doors and stucco coatings, or let us measure it with the estimate.
What does the paint itself cost?
Angi’s Denver interior guide lists paint at about $15 to $40 per gallon depending on brand and finish, with typical prices of:
| Product | Per gallon |
|---|---|
| Primer | $25 |
| Flat paint | $30 |
| Semi-gloss interior paint | $38 |
| High-gloss interior paint | $60 |
| VOC-free paint | $70 |
Paint is the small part of a professional job. Most of the cost of having a room painted is labor and preparation, which is why Angi’s Denver figure for a 12 × 12 room is $360 to $790 while the paint for it is two gallons. The full picture is on our what it costs page.
What should you do with leftover paint in Colorado?
The EPA’s advice for paints and other products that release VOCs is to buy only as much as you will use, and to ventilate while you use it. A careful measurement does the first part for you.
For what is left over, Colorado has a paint stewardship program run by PaintCare. According to PaintCare, the Colorado program began in July 2015; its drop-off sites accept house paint, primers, stains, sealers and clear coatings; and there is no charge to drop off, because the program is funded by a fee added to new paint sold in the state. PaintCare also lists a free pickup service for households with more than ten gallons in certain counties, including the Denver-area counties, offered for a limited time.
Keep a quart of each color for touch-ups, label it with the room and the date, and recycle the rest.
When is it worth hiring a painter?
A single bedroom in the same color is a reasonable weekend project. These are the jobs where homeowners most often call us instead: high stairwells and vaulted ceilings, rooms with a lot of trim, big color changes, water-stained or patched walls, and anything in a house built before 1978, where federal lead-safe rules apply to paid work. In winter, our winter interior painting page explains how we handle heat and fresh air.
Paint quantity questions
How much paint do I need for a 12 by 12 room?
With 8-foot ceilings, one door and two windows, a 12 × 12 room has about 333 square feet of wall. Two coats at 350 square feet per gallon comes to about 1.9 gallons, so buy 2 gallons. The ceiling adds about 0.4 gallons per coat.
Does primer count as a coat?
No. Primer seals and evens out the surface so the finish coats look uniform and bond well. New drywall, big patches, stains and dramatic color changes usually need primer and then the finish coats on top.
How much paint does a gallon cover?
Angi puts a gallon of interior paint at 300 to 400 square feet. The exact figure is printed on the can and changes with the product and the surface; textured and porous walls take more.
Is one gallon enough for an accent wall?
Usually. A 12-foot wall with an 8-foot ceiling is 96 square feet, so two coats need about 192 square feet of coverage, roughly half a gallon at 350 square feet per gallon. A dark color over a light one may need a third coat or a tinted primer.
What do I do with leftover paint in Colorado?
Keep a labeled quart for touch-ups, and take the rest to a PaintCare drop-off site. Colorado’s program has run since July 2015, accepts house paint, primers, stains and clear coatings, and charges nothing to drop off; it is funded by a fee on new paint.
Sources
Coverage and prices come from the sources listed; room figures are our arithmetic from them. Checked October 22, 2026.
- How much does it cost to paint a room in Denver, CO?, Angi, updated July 8, 2026 (coverage per gallon, paint prices).
- How much does it cost to paint the interior of a house in Denver, CO?, Angi, updated August 4, 2026 (primer coverage, typical prices by paint type).
- Colorado paint recycling and drop-off locations, PaintCare, program details.
- Volatile organic compounds’ impact on indoor air quality, U.S. Environmental Protection Agency, buying only what you will use, ventilation.
Rather have it done for you?
Send room sizes or a few photos and we will send back a written estimate, or come measure in person. Both are free.
Prefer to talk it through? 720-208-5645