How the calculator works
Paint is bought by the surface you cover, not the floor size. For each room the calculator finds the wall area as the perimeter times the wall height: 2 × (length + width) × height. It subtracts each door and window, then adds the ceiling (length × width) if you tick it. Trim is estimated as linear feet times the trim's width.
Gallons needed = area × coats ÷ coverage per gallon. The calculator rounds up to whole quarts, then finds the cheapest combination of 5-gallon pails, gallons and quarts at the prices you enter. If two mixes cost the same, it picks the one that leaves less paint over. Walls, ceilings, trim and primer are counted separately because they are usually different products.
Worked example: two rooms, two coats
| Step | Living room 16 × 13 × 9 ft | Bedroom 12 × 11 × 9 ft |
|---|---|---|
| Gross wall area | 2 × 29 × 9 = 522 sq ft | 2 × 23 × 9 = 414 sq ft |
| Doors (21 sq ft each) | 2 → −42 | 2 → −42 |
| Windows (15 sq ft each) | 2 → −30 | 1 → −15 |
| Net wall area | 450 sq ft | 357 sq ft |
| Ceiling | 208 sq ft | 132 sq ft |
- Walls: 807 sq ft × 2 coats ÷ 350 sq ft per gallon = 4.61 gallons, rounded up to 19 quarts. At the example prices, one 5-gallon pail ($200) is cheaper than five single gallons ($225), so buy the pail.
- Ceilings: 340 sq ft × 2 ÷ 350 = 1.94 gallons: two gallons ($90).
- Trim: 120 linear ft × 4 in (0.33 ft) = 40 sq ft × 2 ÷ 350 = 0.23 gallons: one quart ($18).
- Estimated total: $308 for paint, before brushes, rollers, tape and drop cloths.
The numbers you should change
- Coverage. Every can lists its spread rate in square feet per gallon. We start at 350 only so the tool shows an answer; replace it. Primer, textured walls, bare drywall and dark-to-light color changes all change how far a gallon goes.
- Openings. The 21 sq ft door (3 × 7 ft) and 15 sq ft window (3 × 5 ft) are planning assumptions. Measure a big picture window or sliding door and enter its real area, or count a double door as two.
- Coats. Follow the label. If a can claims one-coat coverage, check the surface and color conditions it lists.
Edge cases
- Stairwells, gable walls and vaulted ceilings are not simple rectangles. Find their area with the square footage calculator (triangle and trapezoid shapes), then apply the same rule: area × coats ÷ coverage.
- A single accent wall in a different color is its own product: its area is just wall length × height, minus any openings in it. Subtract it from the main wall total if you are not painting it twice.
- New drywall: prime first. The drywall calculator estimates sheets, compound and tape.
Renting in New York City?
You may not need to buy paint at all. NYC rules on repainting apartments are explained in Does my NYC landlord have to paint every three years? Buying or selling an older home? Read about lead paint disclosure, and plan upkeep with the home maintenance budget calculator.