How to use the paint calculator
Enter the room's length, width and ceiling height, and count the doors and windows. The calculator works out the paintable wall area, multiplies by the number of coats and divides by the coverage on your can. The ceiling is figured separately, since it's usually a different paint, and so is primer. Several rooms the same size? Set "Rooms this size".
The formula
wall area = 2 × (length + width) × height − 21 × doors − 15 × windows
gallons = area × coats ÷ coverage per gallon
Worked example: a 14 × 12 ft bedroom, 8 ft ceiling
- Walls: 2 × (14 + 12) × 8 = 416 ft².
- Openings: one door (21 ft²) and two windows (30 ft²) leave 365 ft².
- Two coats at 400 ft² per gallon: 365 × 2 ÷ 400 = 1.83 gallons, so buy 2.
- Ceiling: 168 ft² × 2 coats ÷ 400 = 0.84 gallons, so buy 1 gallon of ceiling paint.
How many coats?
| Situation | Primer | Top coats |
|---|---|---|
| Same color, sound existing paint | No | 1-2 |
| New color, similar depth | No | 2 |
| Dark to light, or light to deep color | Yes (tinted) | 2 |
| New drywall or patches | Yes (drywall primer) | 2 |
| Stains, water marks, smoke | Stain-blocking primer | 2 |
Why coverage varies
The coverage on the can assumes a smooth, sealed surface and the film thickness the maker tested with. Real walls drink more paint in several situations:
- New drywall and fresh patches are porous until primed.
- Textured walls and ceilings (orange peel, knockdown, popcorn) have more surface than their flat area. Allow 10 to 25% more.
- Masonry and rough exterior siding can take half again as much paint.
- Deep and bright colors often need an extra coat for full hide.
If any of these apply, lower the coverage figure in the calculator rather than guessing extra gallons.
Tips
- Buy per color. Accent walls and trim are separate colors; run the calculator for each.
- Trim and doors take little paint: a quart usually covers the trim in an average room.
- Box your paint. Pour several gallons of the same color into one bucket and mix so slight batch differences don't show.
- Keep a little back for touch-ups, labeled with the room and date.
Questions people ask
How much paint do I need for a 12x12 room?
With 8 ft ceilings the walls are 384 square feet. Take off one door (about 21 square feet) and two windows (about 15 each) and you have 333 square feet; two coats at 400 square feet per gallon is 1.7 gallons, so buy 2 gallons. The ceiling adds 144 square feet per coat.
How many square feet does a gallon of paint cover?
Most interior paints are rated for about 350 to 450 square feet per gallon on smooth, primed walls. Benjamin Moore lists 400 to 450 for Regal Select. Rough, porous or unpainted surfaces take more paint, so check your can.
Do I need two coats of paint?
Usually. One coat can work when repainting the same color with a good paint, but color changes, bare drywall and dark-to-light changes need two coats, often over primer.
How much paint for a ceiling?
Ceiling area is just length × width. A 12 × 14 room's ceiling is 168 square feet: about half a gallon per coat.
Should I buy gallons or a 5-gallon bucket?
If the total is more than about 3 gallons of one color, a 5-gallon bucket is usually cheaper per gallon and ensures the color is consistent. Mix multiple gallons together (boxing) if you buy separate cans.
Sources and assumptions
- Benjamin Moore Regal Select Interior (N549) technical data sheet: spread rate 400-450 ft² per gallon
- Door (21 ft², a 3 × 7 ft door) and window (15 ft², 3 × 5 ft) deductions are estimating conventions. Enter your actual sizes for unusual openings.
Last reviewed 27 October 2026. Every formula on this page is checked against the worked example above in our automated tests. Found a problem? Tell us.