Paint Calculator

Paint quantity estimation requires calculating the total paintable surface area, then dividing by the paint’s coverage rate. The key is accurately measuring wall area while subtracting the area taken up by doors and windows, which are not painted with wall paint.

Core formulas (Imperial):

Wall Area = Room Perimeter × Wall Height

Paintable Area = Wall Area − (Number of Doors × 21 sq ft) − (Number of Windows × 15 sq ft)

Gallons Needed = (Paintable Area × Number of Coats) / Coverage per Gallon

Core formulas (Metric):

Wall Area = Room Perimeter × Wall Height

Paintable Area = Wall Area − (Doors × 1.95 m²) − (Windows × 1.40 m²)

Liters Needed = (Paintable Area × Coats) / Coverage per Liter

Variable definitions:

• Perimeter — total length of all walls (length + width + length + width)
• Wall Height — floor to ceiling height, typically 8–9 ft (2.4–2.7 m)
• Coverage per gallon — standard interior latex: 350–400 sq ft; primer: 250–300 sq ft
• Coverage per liter — standard: approximately 10–12 m²

Worked example: Room dimensions: 14 ft × 12 ft, 8 ft ceilings, 2 doors, 3 windows.

Perimeter = (14 + 12) × 2 = 52 ft Wall area = 52 × 8 = 416 sq ft Subtract doors: 2 × 21 = 42 sq ft Subtract windows: 3 × 15 = 45 sq ft Paintable area = 416 − 42 − 45 = 329 sq ft

Two coats: 329 × 2 = 658 sq ft total At 380 sq ft/gallon: 658 / 380 = 1.73 gallons → round up to 2 gallons

Practical tips:

• Always round up to the nearest gallon — running out mid-wall means a visible seam
• Textured walls (orange peel, knockdown) absorb 10–15% more paint
• Going from dark to light requires a tinted primer plus 2 coats — add a third coat’s worth to your estimate
• One gallon of ceiling paint at 400 sq ft/gal covers the ceiling of this 14×12 room in one coat
• Buy paint from the same batch (same lot number) to avoid color variation between cans