Quick Answer: Calculate wall area as Perimeter (ft) × Height (ft), then subtract 21 sq ft per door and 15 sq ft per window. Divide paintable area by coverage rate: interior latex covers 400 sq ft/gallon, exterior 350, primer 300. Multiply by number of coats (usually 2). A 12×12 ft room with 8 ft ceilings, 1 door, and 2 windows needs about 2 gallons for 2 coats of interior latex. Add 10% for waste.
Calculate how much paint you need for any room, wall, or project. Enter your dimensions, number of doors and windows, and choose your paint type to get an accurate gallon count and cost estimate.
How to Calculate Paint
The formula for calculating paint needed for a rectangular room:
- Measure the room's length and width. Calculate the perimeter: 2 × (length + width).
- Multiply the perimeter by the ceiling height to get the total wall area in square feet.
- Subtract 21 sq ft for each door and 15 sq ft for each window.
- Multiply the paintable area by the number of coats (2 is standard).
- Divide by the coverage rate for your paint type (350-400 sq ft per gallon for most paints).
- Round up to the nearest whole gallon and add 10% for waste.
Paint Coverage by Room Size
Quick reference for common room sizes (8-foot ceilings, 1 door, 2 windows, 2 coats of interior latex):
| Room Size | Wall Area | Paintable Area | Gallons (2 coats) |
|---|---|---|---|
| 8 x 10 ft | 288 sq ft | 237 sq ft | 2 gallons |
| 10 x 12 ft | 352 sq ft | 301 sq ft | 2 gallons |
| 12 x 12 ft | 384 sq ft | 333 sq ft | 2 gallons |
| 12 x 16 ft | 448 sq ft | 397 sq ft | 2 gallons |
| 14 x 18 ft | 512 sq ft | 461 sq ft | 3 gallons |
| 16 x 20 ft | 576 sq ft | 525 sq ft | 3 gallons |
Paint Coverage Rates by Type
Different paint types have different coverage rates per gallon:
| Paint Type | Coverage/Gallon | Price Range | Best For |
|---|---|---|---|
| Interior Latex | 350-400 sq ft | $25-60 | Walls and ceilings |
| Exterior Latex | 300-350 sq ft | $30-70 | Siding, trim, fences |
| Primer | 250-300 sq ft | $20-40 | Bare drywall, stain blocking |
| Ceiling Paint | 350-400 sq ft | $25-45 | Ceilings (spatter-resistant) |
| Semi-Gloss | 300-350 sq ft | $30-55 | Kitchens, bathrooms, trim |
| High-Gloss | 250-300 sq ft | $35-65 | Doors, cabinets, furniture |
Choosing the Right Paint Finish
- Flat / Matte: Zero sheen, hides wall imperfections and nail pops. Best for ceilings and low-traffic adult bedrooms. Not washable — marks and scuffs are permanent.
- Eggshell: Slight soft sheen that is easy to clean with a damp cloth. The most popular choice for living rooms, dining rooms, and hallways. Good balance of appearance and durability.
- Satin: Smooth, velvety finish with moderate sheen. More durable and washable than eggshell. Excellent for kids' rooms, family rooms, and high-traffic areas.
- Semi-Gloss: Noticeable sheen that resists moisture and cleans easily. Standard choice for kitchens, bathrooms, laundry rooms, and all trim and molding.
- High-Gloss: Maximum sheen and hardness. Extremely durable and easy to clean but shows every wall imperfection. Use for doors, cabinets, railings, and accent furniture.
Tips for Buying and Using Paint
- Buy all at once: Paint colors can vary slightly between production batches. Buy all the paint you need in one trip and have the store shake each can.
- Box your paint: If using multiple cans of the same color, pour them all into a 5-gallon bucket and stir thoroughly. This eliminates any slight color differences between cans.
- Primer matters: Always prime bare drywall, new wood, stain-covered surfaces, and any time you are making a dramatic color change. Paint-and-primer-in-one products work for minor color changes only.
- Save a quart: Keep leftover paint sealed tightly for touch-ups. Label the can with the room name and date. Properly stored latex paint lasts 2-10 years.
- Temperature: Paint between 50-85°F with humidity below 85%. Cold temperatures prevent proper adhesion; high humidity causes runs and slow drying.
- Coverage killers: Textured walls use 20-30% more paint than smooth surfaces. Dark-to-light color changes may need 3 coats. Porous surfaces like bare wood and new drywall absorb the first coat.
Formula Summary
Wall Area Formula: Total Wall Area = Perimeter × Ceiling Height = 2 × (L + W) × H
Paintable Area: Paintable = Total Wall Area - (Doors × 21 sqft) - (Windows × 15 sqft)
Gallons = ⌈(Paintable × Coats × (1 + W%)) / Coverage⌉
| Paint Type | Coverage (sq ft/gal) |
|---|---|
| Interior Latex | 400 |
| Exterior Latex | 350 |
| Primer | 300 |
| Ceiling Paint | 400 |
| Semi-Gloss | 350 |
| High-Gloss | 300 |
Standard Assumptions: Standard door 3×7 ft (21 sq ft). Standard window 3×5 ft (15 sq ft). Default 2 coats. 10% waste factor for roller/brush application. 1 gallon = 4 quarts.
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Estimates are for planning purposes only. Consult a qualified contractor for critical projects. Actual material requirements may vary based on site conditions, waste, and installation methods.
Pricing last verified: February 2026. Material prices vary by region, season, and supplier. Prices shown are national averages for planning purposes.