Buying paint is a balance between not running out halfway through a wall and not storing three half-empty gallons in the garage. This calculator measures a room’s walls (and ceiling, if you like), subtracts doors and windows, applies the number of coats and the coverage printed on the can, and turns the result into gallons and quarts — or liters if you work in metric.
How to use the paint calculator
- Choose US or Metric units.
- Select Room size to enter length, width and wall height, or Wall area if you have measured the walls already.
- Tick Paint the ceiling too to include it.
- Enter the number of doors and windows and the area of each.
- Set the number of coats and the coverage from the paint label.
- Read the amount needed and a practical purchase on the tape.
Paint formulas
The calculator rounds the result up to whole gallons plus quarts. When you would need two or more quarts, it also offers the extra gallon, which usually costs about the same as three quarts.
Worked example
A 14 × 12 ft bedroom with 8 ft walls: 2 × (14 + 12) × 8 = 416 ft².
One 21 ft² door and two 15 ft² windows: 416 − 21 − 30 = 365 ft² to paint.
Two coats at 350 ft² per gallon: 365 × 2 ÷ 350 = 2.09 gallons — buy 2 gallons and 1 quart.
In metric, a 4.3 × 3.7 m room with 2.4 m walls and similar openings has 33.7 m² to paint; two coats at 9 m² per liter need about 7.5 liters.
Typical coverage rates
| Surface | Approximate coverage per gallon |
|---|---|
| Smooth, previously painted drywall | 350 – 400 ft² |
| New drywall (after primer) | 300 – 350 ft² |
| Light texture (orange peel, knockdown) | 300 – 350 ft² |
| Heavy texture, popcorn ceilings | 250 – 300 ft² |
| Primer on bare drywall | 200 – 300 ft² |
Rough and porous surfaces soak up more paint, and thick-nap roller covers load more paint onto the wall. If you are unsure, enter a lower coverage number — running short costs a second trip, while a little leftover is useful for touch-ups.
Coats, primer and color changes
Two coats is the standard for an even finish. Primer is worth adding on new drywall, patched areas, stained surfaces and when moving from a dark color to a light one; a tinted primer can save a third coat on strong colors. The calculator shows a separate primer estimate at about 300 square feet per gallon. “Paint-and-primer” products help with coverage on previously painted walls but do not replace a dedicated primer on bare drywall.
Buying tips
- Paint mixed in one batch matches best. For large jobs, buy all the gallons at once or pour them together in a bucket (called boxing) before you start.
- A 5-gallon bucket is economical when you need four or more gallons of the same color.
- Trim, doors and baseboards are usually painted in a different sheen. A quart typically covers the trim of an average room.
- Keep a labeled leftover for touch-ups, sealed tightly and stored away from freezing temperatures.
For flooring estimates in the same room, try the square footage calculator or the tile calculator.
Frequently asked questions
How much paint do I need for a 12 × 12 room?
With 8-foot ceilings the walls total 2 × (12 + 12) × 8 = 384 ft². Subtracting a standard door and two windows (51 ft²) leaves 333 ft². Two coats at 350 ft² per gallon need about 1.9 gallons, so two gallons will do.
How many square feet does a gallon of paint cover?
Most interior wall paints cover about 350 to 400 square feet per gallon per coat on smooth, primed drywall. Rough, porous or textured surfaces can drop that to 250 to 300 square feet. The exact figure is printed on the can.
Do I really need two coats?
Usually. A second coat evens out the color and sheen and hides roller marks. One coat can be enough when repainting in a similar color with a high-quality paint, while a big color change, especially light over dark, may need primer plus two coats.
Should I subtract doors and windows?
Yes for doors and large windows, since they can add up to 50 square feet or more in a typical room. Many painters ignore very small windows, which simply leaves a little extra paint for touch-ups.
How much paint does a ceiling need?
A ceiling's area is just length × width. A 12 × 14 ft ceiling is 168 ft², about half a gallon per coat at 350 ft² per gallon. Tick the ceiling option to add it, or estimate it separately if you are using a dedicated ceiling paint.