Paint Calculator
Calculate how much paint you need for your room based on wall dimensions, doors, windows, and number of coats. See also Wallpaper Calculator and Flooring Calculator.
Standard: 21 sq ft each
Standard: 15 sq ft each
How to Calculate Paint Needed for a Room
Calculating the right amount of paint prevents costly over-purchasing or frustrating mid-project store runs. The process involves measuring your room's walls, subtracting areas that won't be painted (doors and windows), accounting for the number of coats, and dividing by the paint's coverage rate. Most interior paints cover 350-400 square feet per gallon on smooth surfaces.
Paint Calculation Formula
Wall Area = 2 x (Length + Width) x Height
Paintable Area = Wall Area - (Doors x 21 sq ft) - (Windows x 15 sq ft)
Total Area = Paintable Area x Number of Coats
Gallons Needed = Total Area / Coverage per Gallon
Example Calculation
Room: 15 ft x 12 ft x 9 ft, 1 door, 2 windows, 2 coats
Wall Area: 2 x (15 + 12) x 9 = 486 sq ft
Subtract: 1 door (21) + 2 windows (30) = 51 sq ft
Paintable: 486 - 51 = 435 sq ft
With 2 coats: 435 x 2 = 870 sq ft
Gallons: 870 / 350 = 2.5 gallons (buy 3 gallons)
Paint Coverage by Type
| Paint Finish | Coverage (sq ft/gal) | Best For | Avg Price/Gallon |
|---|---|---|---|
| Flat/Matte | 350-400 | Ceilings, low-traffic areas | $25-$45 |
| Eggshell | 350-400 | Living rooms, bedrooms | $30-$50 |
| Satin | 350-400 | Kitchens, bathrooms, hallways | $30-$55 |
| Semi-Gloss | 350-400 | Trim, doors, cabinets | $30-$55 |
| High Gloss | 300-350 | Trim, furniture, accents | $35-$60 |
| Primer | 300-350 | New drywall, stain blocking | $20-$40 |
Factors That Affect Paint Coverage
- Surface texture — rough or textured walls require 10-20% more paint.
- Color change — going from dark to light (or vice versa) may need an extra coat.
- Paint quality — premium paints often have better coverage per coat.
- Application method — rollers are more efficient than brushes for large areas.
- Surface porosity — new drywall or bare wood absorbs more paint (use primer first).
- Temperature and humidity — paint in 50-85 degrees F with low humidity for best results.
Frequently Asked Questions
How many coats of paint do I need?
Most projects require 2 coats for even coverage. You may need 3 coats when covering dark colors with light ones, painting over stains, or using low-quality paint. One coat may suffice for touch-ups or same-color refreshes.
Should I buy extra paint?
Yes, buy 10-15% more than calculated to account for waste, touch-ups, and uneven surfaces. Keep leftover paint for future touch-ups — store it sealed in a cool, dry place.
Do I need primer before painting?
Primer is recommended for new drywall, bare wood, stain coverage, and dramatic color changes. Many modern paints include primer (paint-and-primer-in-one), which works well for repainting similar colors.
How long does paint last once opened?
Latex paint lasts 2-10 years when properly sealed and stored. Oil-based paint can last up to 15 years. If paint smells bad, has lumps that won't mix out, or has separated beyond remixing, it should be discarded.
What is the standard door and window size for calculations?
Standard interior doors are approximately 3 ft x 7 ft (21 sq ft). Standard windows average about 3 ft x 5 ft (15 sq ft). Adjust these values if your doors or windows are significantly different.
How do I calculate paint for a ceiling?
For ceilings, simply multiply room length by room width to get the area. A 15 x 12 ft room has a 180 sq ft ceiling, requiring about 0.5 gallons per coat.