Paint Coverage Calculator: Square Feet Per Gallon
Buying too little paint means a second trip to the store; buying too much wastes money. The key is accurately calculating your wall area and knowing the coverage rate of the paint you're using. This guide walks through the math for any room size, surface type, and coat count — including metric liter coverage for international projects.
How to Calculate Paint Coverage
Standard coverage rate: Most interior latex paints cover 350–400 square feet per gallon on a smooth, properly primed surface for a single coat. Rough, textured, or highly porous surfaces absorb more paint and will reduce coverage to 250–300 sq ft/gal.
Coats needed: When painting over a similar color in good condition, one coat is often sufficient. For dramatic color changes (dark to light or light to dark), plan on two coats. Bare drywall requires a primer coat plus at least one topcoat.
Calculating wall area: Measure the perimeter of the room (add all four wall lengths) and multiply by ceiling height. Subtract the area of doors (roughly 20 sq ft each) and windows (roughly 15 sq ft each). The result is your net paintable area.
Metric reference: In metric countries, coverage is stated as liters per square meter or square meters per liter. Standard interior paint covers 9–11 m² per liter on smooth surfaces.
Paint Coverage Formulas
Wall Area = (Perimeter × Height) − Door/Window Areas
Perimeter = 2 × (Room Length + Room Width)
Gallons needed = Wall Area (sq ft) ÷ Coverage Rate (sq ft/gal)
For 2 coats: multiply single-coat gallons × 2
Add 10–15% to your total for touch-ups and uneven absorption.
Room Sizes to Gallons Needed
Estimates assume one standard door (20 sq ft) and two windows (30 sq ft total) subtracted from wall area. Coverage rate: 350 sq ft/gal. Round up to the nearest quart or gallon when purchasing.
| Room Size | Net Wall Area | 1 Coat (gal) | 2 Coats (gal) |
|---|---|---|---|
| 10 × 10 ft (8 ft ceiling) | 282 sq ft | 0.8 | 1.6 |
| 12 × 12 ft (8 ft ceiling) | 334 sq ft | 1.0 | 2.0 |
| 12 × 15 ft (8 ft ceiling) | 382 sq ft | 1.1 | 2.2 |
| 14 × 18 ft (8 ft ceiling) | 454 sq ft | 1.3 | 2.6 |
| 14 × 18 ft (9 ft ceiling) | 511 sq ft | 1.5 | 3.0 |
| 20 × 20 ft (9 ft ceiling) | 668 sq ft | 1.9 | 3.8 |
Metric Coverage Reference (Liters per m²)
| Surface Type | Liters per m² | m² per Liter |
|---|---|---|
| Smooth / primed | 0.09–0.11 | 9–11 |
| Lightly textured | 0.11–0.14 | 7–9 |
| Rough / masonry | 0.14–0.20 | 5–7 |
| Previously unpainted wood | 0.16–0.22 | 4.5–6 |
Related Conversion Tools
Frequently Asked Questions
How much paint do I need for a 12×12 room?
A 12×12 room with 8-foot ceilings has approximately 384 square feet of wall area (perimeter 48 ft × 8 ft). Subtracting a door (20 sq ft) and two windows (30 sq ft) leaves about 334 sq ft. At 350 sq ft/gallon, one coat requires roughly 1 gallon; two coats require about 2 gallons. Add 10–15% for touch-up.
How many square feet does a gallon of paint cover?
Most interior paints cover 350–400 square feet per gallon on a smooth, primed surface for a single coat. Textured, rough, or highly porous surfaces reduce coverage to 250–300 sq ft/gal. Exterior paints typically cover 250–350 sq ft/gal depending on the substrate.
How do I calculate wall area for a room?
Calculate wall area with: (Perimeter × Wall Height) − Door and Window Areas. Perimeter = 2 × (Length + Width). A standard interior door is about 20 sq ft (2.5 ft × 8 ft) and a standard window is about 15 sq ft (3 ft × 5 ft). Subtract these openings from the total wall area before calculating paint needed.