CalculateY
How much paint do I need? Formula and room-by-room examples
The formula for paint coverage is simple: divide the area you are painting by what one gallon covers, then multiply by the number of coats. One gallon of standard interior paint covers about 350 square feet on a primed wall. A 12 ft × 14 ft bedroom with 8 ft ceilings has roughly 380 paintable square feet after subtracting a door and window. Two coats need about 2.2 gallons, so buy 3.
\[\text{Gallons} = \frac{\text{Paintable Sq Ft}}{\text{Coverage per Gallon}} \times \text{Coats}\]Coverage per gallon by paint type
Paint manufacturers print coverage on the label, but ranges vary by surface and conditions:
| Paint type | Coverage (sq ft per gallon) |
|---|---|
| Standard interior latex | 350-400 |
| Standard exterior latex | 300-400 |
| Primer (on bare drywall) | 200-300 |
| Primer (on bare wood) | 250-350 |
| Stain-blocking primer | 200-300 |
| Textured walls | 250-300 |
| Rough exterior siding | 200-300 |
| Smooth metal | 400-450 |
| Ceiling paint | 350-400 |
Use 350 sq ft per gallon as a baseline for any standard interior wall and adjust for surface type. Dark colors often need more coats than light colors, even at the same per-coat coverage.
Calculating wall area
For a rectangular room, wall area is the perimeter times the ceiling height:
\[\text{Wall Area} = (2 \times \text{Length} + 2 \times \text{Width}) \times \text{Height}\]A 12 × 14 room with 8 ft ceilings:
\[\text{Wall Area} = (24 + 28) \times 8 = 416 \text{ sq ft}\]Then subtract the area of openings (doors and windows):
| Opening | Standard area |
|---|---|
| Interior door | 21 sq ft (3 × 7 ft) |
| Exterior door | 24 sq ft (3 × 8 ft) |
| Standard window | 12-15 sq ft |
| Picture window | 24-30 sq ft |
| Closet door (sliding) | 30-40 sq ft |
For the 12 × 14 bedroom with one door and two windows:
\[\text{Paintable} = 416 - 21 - 30 = 365 \text{ sq ft}\]Worked examples
Master bedroom (12 × 14, 8 ft ceilings)
- Wall area: 416 sq ft
- Subtract door + 2 windows: 51 sq ft
- Paintable: 365 sq ft
- 2 coats: 365 / 350 × 2 = 2.09 gallons
- Buy: 3 gallons (rounds up; covers touch-ups)
Living room (16 × 20, 9 ft ceilings)
- Wall area: 72 × 9 = 648 sq ft
- Subtract 2 doors + 3 windows: 21 + 21 + 45 = 87 sq ft
- Paintable: 561 sq ft
- 2 coats: 561 / 350 × 2 = 3.21 gallons
- Buy: 4 gallons
Two-story house exterior (1,800 sq ft)
Exterior siding area is approximately 2.5 to 3 times floor area for a two-story home:
- Estimated siding area: 1,800 × 2.5 = 4,500 sq ft
- Subtract trim, doors, windows: roughly 15% reduction = 3,825 sq ft
- 2 coats on standard siding (350 sq ft per gallon): 3,825 / 350 × 2 = 21.9 gallons
- Buy: 22-25 gallons (or 5 five-gallon buckets)
A more accurate exterior estimate measures actual wall faces. For a rectangular two-story 30 × 40 ft house with 18 ft total wall height: perimeter (140 ft) × height (18 ft) = 2,520 sq ft of wall, minus 15% for openings = 2,142 sq ft of paintable surface. Add 200-400 sq ft for gable ends if present.
Ceiling only (12 × 14 room)
- Ceiling area: 168 sq ft
- 1 coat (ceiling paint covers well, 1 coat usually enough): 168 / 350 = 0.48 gallons
- Buy: 1 gallon (1 quart is rarely sold; a gallon gives margin)
Number of coats: when one is enough, when you need three
One coat is enough when:
- Painting the same color over an existing coat in good condition
- Using high-quality paint-and-primer products on minor color shifts
- Touching up specific areas
Two coats is the standard when:
- Repainting a different color
- Painting fresh primer on drywall
- Restoring a worn surface
Three coats may be needed when:
- Going from dark to light colors
- Painting bold accent colors
- Covering stains or water damage (use a stain-blocking primer for the first coat)
- Painting onto raw wood without prior priming
The cost of one extra coat is one extra gallon and a few hours. The cost of underestimating coats is a streaky finish and a second trip to the store.
Primer: when you need it and how much
Primer is required on any unfinished surface (raw drywall, bare wood, fresh patches), when changing from oil-based to latex paint, when covering stains, or when going from dark to light colors. Primer coverage is lower than finish paint: plan on 250-300 sq ft per gallon.
For a 12 × 14 bedroom with new drywall:
- Paintable area: 365 sq ft
- Primer: 365 / 275 = 1.33 gallons
- Buy: 2 gallons primer, then 3 gallons of finish paint
Self-priming paints save a step on previously-painted walls in good condition. They do not replace true primer on raw drywall, stains, or major color changes.
Common estimation mistakes
Forgetting the second coat. A single gallon of paint covering 350 sq ft does not paint a 350 sq ft room properly. It gives one thin coat that usually shows brush marks and requires a second coat anyway.
Over-counting deductions. Subtracting more than 15% of total wall area for openings is unusual. Most rooms have one door and 1-2 windows, totaling 30-50 sq ft of openings, which is a small fraction of total wall area.
Mixing up wall area and floor area. A 12 × 14 room is 168 sq ft of floor but 416 sq ft of wall. Use floor area only for ceiling paint.
Ignoring trim, baseboards, doors. These need separate paint (often a different finish like semi-gloss). Plan on 1 quart per 100 linear feet of trim for two coats. A 12 × 14 room has 52 linear feet of baseboard plus door and window casings, totaling roughly 80 linear feet, needing about 1 quart.
Buying just one gallon when math says 1.1 gallons. Always round up. The unused portion is essentially free insurance against running out mid-job.
Quick reference: paint per room size
For interior walls, two coats, average door/window deductions:
| Room size (ft) | Paintable wall sq ft | Gallons (2 coats) |
|---|---|---|
| 8 × 10 | 250 | 2 |
| 10 × 12 | 310 | 2 |
| 12 × 14 | 365 | 3 |
| 14 × 16 | 430 | 3 |
| 16 × 20 | 560 | 4 |
| 20 × 25 | 700 | 4 |
For ceilings, one coat, no deductions: divide floor area by 350 to get gallons.
The paint calculator handles different ceiling heights, coverage rates, and number of coats. Pair it with the room sqft calculator for irregularly-shaped rooms.