How this is calculated
Wall area starts from the room's perimeter (2 × (length + width)) multiplied by ceiling height, then subtracts a standard allowance for each door and window — painters call this the 'gross-to-net' area, and skipping the deduction is the most common reason people over-buy paint.
Paint coverage is quoted per coat, so the net wall area is multiplied by the number of coats before converting to gallons or litres using a standard coverage rate. Textured walls, unprimed drywall, and strong color changes all reduce real coverage below the can's rated figure.
Formula & methodology
Net area = (2 × (Length + Width) × Height) − (Doors × 21 ft² + Windows × 15 ft²). Paint needed = Net area × Coats ÷ 350 ft² per gallon.
Worked example
A 12 ft × 10 ft room, 8 ft ceilings, 1 door, 2 windows, 2 coats
- 1.Perimeter = 2 × (12 + 10) = 44 ft
- 2.Gross wall area = 44 ft × 8 ft = 352 ft²
- 3.Deductions = (1 × 21) + (2 × 15) = 51 ft²
- 4.Net area = 352 − 51 = 301 ft²
- 5.Paint needed = 301 × 2 coats ÷ 350 ft²/gal = 1.72 gal → round up to 2 gallons
Frequently asked questions
How many coats of paint do I actually need?
Two coats is standard for a color change or a wall in average condition. One coat can work over a very close existing color with a high-quality paint-and-primer product; three coats are common when covering a dark color with a light one.
Should I round up or down on the gallon count?
Always round up. Running short mid-wall means a delay while the exact color re-mixes, and re-mixed batches can vary slightly ("batch variation"), which sometimes shows as a visible line.
Does this include the ceiling?
No — this calculator estimates wall paint only. Ceilings are typically a different product (flat, stain-blocking) and should be calculated separately using the room's floor area.
Why does the estimate not match the coverage number on my paint can?
Can labels quote coverage for a smooth, primed surface. Textured walls, unprimed drywall, and porous surfaces like new plaster all absorb more paint — budgeting an extra 10% is reasonable in those cases.
Last updated 2026-06-20. Formulas are planning methods — confirm with a supplier before you order.