Paint Calculator by Square Footage
Enter the wall area you want to paint. We'll tell you how many gallons you need for one or two coats at the standard 350 sq ft per gallon coverage.
Last updated:Tip: If you only have the diameter, divide by 2 to get the radius.
L-shape = two rectangles sharing a corner.
Add one row per rectangle and we'll sum them.
How to Calculate Wall Square Footage
- Measure each wall's length and height.
- Multiply length × height for each wall.
- Subtract the area of any door (typically 21 sq ft) and each window (typically 15 sq ft).
- Sum all walls — that's your paintable area.
For a 12 × 15 ft room with 8 ft ceilings: perimeter = (12 + 15) × 2 = 54 ft; wall area = 54 × 8 = 432 sq ft. Subtract one door (21) and two windows (30) = 381 sq ft of paintable area.
Paint Coverage Rules of Thumb
| Product | Coverage per Gallon |
|---|---|
| Interior latex (smooth drywall) | 350–400 sq ft |
| Interior latex (textured walls) | 300 sq ft |
| Primer | 200–300 sq ft |
| Exterior acrylic | 250–300 sq ft |
| Stain (deck) | 150–250 sq ft |
Example Gallons Needed
- 381 sq ft paintable, 2 coats: 381 × 2 / 350 = 2.18 gal → 3 one-gallon cans (always round up).
- 1,000 sq ft paintable, 2 coats: 2,000 / 350 = 5.7 gal → 6 one-gallon cans or 2 five-gallon buckets.
Don't Forget Primer
For new drywall, over dark colors, on stains, or changing from glossy to matte, apply primer first. Primer covers about 250 sq ft per gallon — roughly 25% less than finish paint. A good rule of thumb is to add one gallon of primer for every two gallons of topcoat on bare surfaces.
Ceiling and Trim Separately
Walls usually get eggshell or satin latex; ceilings use flat white; trim uses semi-gloss or gloss enamel. Keep the three orders separate.
| Surface | Typical sheen | Coverage |
|---|---|---|
| Walls | Eggshell / satin | 350 sq ft / gal |
| Ceilings | Flat / matte | 350 sq ft / gal |
| Doors and trim | Semi-gloss / gloss | 400 sq ft / gal |
| Cabinets | Gloss enamel | 400 sq ft / gal |
Wall Area for Common Room Sizes (8 ft Ceiling)
| Room size | Wall area (gross) | Minus 1 door + 1 window |
|---|---|---|
| 10 × 10 | 320 sq ft | ~284 sq ft |
| 10 × 12 | 352 sq ft | ~316 sq ft |
| 12 × 12 | 384 sq ft | ~348 sq ft |
| 12 × 15 | 432 sq ft | ~396 sq ft |
| 15 × 20 | 560 sq ft | ~524 sq ft |
Paint Project Checklist
- Use the wall square footage calculator for rooms with unusual ceiling heights.
- Buy all paint from the same batch (check the lot number) to avoid visible color shifts.
- Transfer paint to a 5-gallon bucket before starting to keep color consistent across walls.
- Plan two coats as the default — single-coat claims rarely hold up on tinted paints.
- Order 10% extra for touch-ups later and store it cool and dark.
Frequently Asked Questions
How many gallons of paint for a 12x15 bedroom?
About 1 gallon per coat for roughly 380 sq ft of paintable wall. For two coats, buy 2 one-gallon cans plus a quart for touch-ups.
How much paint do I need per square foot?
Divide the wall square footage by 350 for one coat of standard interior latex on smooth drywall. On textured walls use 300 sq ft per gallon, and on exterior acrylic use 275.
Do I need to subtract trim?
Only if trim is a different color. Subtract about 15 sq ft of baseboard per 100 ft of perimeter and buy trim paint separately.
How much extra paint for touch-ups?
Keep about 10% of the original order unopened and labeled with the room, color name, and lot number. Stored cool and dark it will last about 2 to 3 years.
How many square feet does a 5 gallon bucket cover?
About 1,750 sq ft per coat at 350 sq ft per gallon coverage. That is enough for an entire small house interior in one coat.
Is primer the same as paint?
No. Primer is formulated to bond to the substrate and block stains. It covers only 200 to 300 sq ft per gallon, so plan for about 25% more primer than finish paint on bare surfaces.
Can I use one gallon for ceiling and walls?
Only for very small rooms. Most ceilings use flat white paint while walls use eggshell or satin, so order separate cans for each.
Does paint price depend on square footage?
Yes — most contractors quote 1.50 to 3.50 USD per sq ft of wall for interior work, materials included. Use the cost per sq ft calculator to compare bids.