Wall Paint Calculator
Run out of paint halfway through the job and the new gallon never quite matches. Buy two extra gallons and they sit in the garage for a decade. This wall paint calculator gives you the exact number of gallons (and liters) for any room or exterior wall, factoring in the number of coats, coverage rate, and the square footage of doors and windows you are not painting.
How the Wall Paint Calculator Works
Paint needs come down to one formula: gallons = (paintable area x number of coats) / coverage rate per gallon. The calculator does two things before that:
- Computes wall area. From room dimensions: perimeter x ceiling height. In direct-entry mode, the wall paint calculator square feet you enter is used as-is.
- Deducts openings. Each standard door = 21 sq ft, each standard window = 15 sq ft. Small windows and large picture windows should be measured individually.
Then it divides by the coverage rate: 350 sq ft/gal for standard interior latex on smooth drywall, 300 sq ft/gal for exterior wall paint on primed siding, and 250 sq ft/gal on textured or rough masonry.
Exterior Wall Paint Calculator Coverage
An exterior wall paint calculator has to account for a rougher surface and higher UV / weather load. Coverage drops from 350 to about 300 sq ft/gal on primed lap siding, 250 on rough stucco or brick, and as low as 200 on split-face block. Exterior latex also needs a thicker mil build, meaning you often apply 2 full coats even when the color barely changes. Buy 10-15% extra for exterior jobs.
Wall Paint Calculator Square Feet Reference
If you know the wall square feet already and just need gallons, use the direct square feet mode. Quick reference for common rooms at 8 ft ceilings:
| Room | Dimensions | Wall Sq Ft | Gallons (2 coats, no openings) |
|---|---|---|---|
| Small bedroom | 10 x 10 | 320 | 2 |
| Master bedroom | 14 x 16 | 480 | 3 |
| Living room | 16 x 20 | 576 | 4 |
| Kitchen | 10 x 14 | 384 | 3 |
| Hallway | 3 x 20 | 368 | 3 |
Exterior Wall Paint Example
A 2,000 sq ft two-story home has roughly 2,500-3,000 sq ft of exterior wall area after deducting windows and doors. At 300 sq ft/gal coverage and 2 coats, that is 17-20 gallons of exterior paint. Add 1 gallon for touch-ups and trim, call it 20 gallons at $50/gal = $1,000 in paint alone, before primer, caulk, and labor.
Coverage Rate Tips
- Primer counts separately. If you are painting over dark walls, stains, or new drywall, add a primer gallon on top of the topcoat count. Primer covers about 200-300 sq ft/gal.
- Low-sheen and matte finishes cover slightly less than semi-gloss and gloss on the first coat because they are thicker.
- Dark-to-light color changes often need 3 coats, not 2. Use a tinted primer to cut this to 2 coats.
- Ceiling paint is separate from this wall paint calculator. One gallon covers about 350-400 sq ft of ceiling.