Paint Quantity Calculations: Pro Guide
Master the exact formulas and methods professionals use to calculate paint quantities. Never run short or overbuy again. Part of our comprehensive painting quote guide.
What is the Formula for Paint Quantity?
The paint quantity formula is: (Total Square Footage ÷ Coverage Rate per Gallon) × Number of Coats × 1.1 (10% waste factor) = Gallons Needed. For example: (400 sq ft ÷ 350 sq ft/gallon) × 2 coats × 1.1 = 2.5 gallons.
The Master Paint Calculation Formula
Formula Components
- Area: Total square footage to paint
- Coverage: Sq ft per gallon (varies by surface)
- Coats: Number of paint applications
- Waste: 1.1 (10%) to 1.2 (20%) multiplier
Example Calculation
// Living room walls
Area: 385 sq ft
Coverage: 350 sq ft/gal
Coats: 2
Waste: 1.1 (10%)
// Calculation
(385 ÷ 350) × 2 × 1.1 = 2.42 gal
Round up = 3 gallons
Paint Coverage Rates by Surface
| Surface Type | Primer Coverage | Paint Coverage | Recommended Coats | Waste Factor |
|---|---|---|---|---|
| Smooth Drywall | 300-350 sq ft | 350-400 sq ft | 2 | 10% |
| Textured Walls | 250-300 sq ft | 300-350 sq ft | 2 | 15% |
| Stucco | 150-200 sq ft | 200-250 sq ft | 2-3 | 20% |
| Brick | 125-175 sq ft | 150-200 sq ft | 2-3 | 20% |
| Wood Siding | 250-300 sq ft | 300-350 sq ft | 2 | 15% |
| Concrete Block | 100-150 sq ft | 150-200 sq ft | 2-3 | 25% |
| Metal | 350-400 sq ft | 400-450 sq ft | 2 | 10% |
Pro Tip: Always check the specific paint manufacturer's coverage rates as premium paints often cover more area. Spray application typically uses 30-40% more paint than brush/roll.
Special Surface Calculations
Ceiling Calculations
Formula
Ceiling Area = Length × Width
// 12×15 room example
12 × 15 = 180 sq ft
180 ÷ 350 = 0.51 gal
× 2 coats = 1.03 gallons
Special Considerations
- • Textured ceilings use 25% more paint
- • White ceiling paint hides better
- • Usually needs only 1-2 coats
- • Add 5% for light fixtures/fans
Trim & Door Calculations
Quick Estimates
- • Standard door: 20 sq ft per side
- • Door + frame: 30 sq ft total
- • Window trim: 10-15 sq ft
- • Baseboard: Height × 0.5 × Length
- • Crown molding: Height × Length
Coverage Rates
Trim paint coverage:
• 1 quart = 100 sq ft
• 1 gallon = 400 sq ft
Average room trim:
• 150-200 sq ft total
• Needs 2 quarts (2 coats)
Color Change Calculations
Light to Light
2 coats
Standard calculation
Dark to Light
3-4 coats
Or use primer + 2 coats
Red/Dark Colors
3+ coats
Tinted primer recommended
Paint Calculation Mistakes to Avoid
Common Errors
- ✗Forgetting to multiply by number of coats
- ✗Using same coverage for all surfaces
- ✗Not accounting for surface porosity
- ✗Ignoring waste factor completely
- ✗Measuring room perimeter wrong
Pro Best Practices
- ✓Always round up to nearest gallon
- ✓Keep detailed notes on coverage rates
- ✓Factor in primer separately
- ✓Add extra for touch-ups (5%)
- ✓Document actual vs estimated usage
Quick Reference Cheat Sheet
Room Sizes
- Small bedroom: 1-2 gallons
- Master bedroom: 2-3 gallons
- Living room: 2-3 gallons
- Open concept: 4-6 gallons
Quick Math
- 1 gallon = 350 sq ft
- 5 gallons = 1,750 sq ft
- Average room = 300-400 sq ft
- Always add 10% minimum
Sheen Coverage
- Flat: Best coverage
- Eggshell: -5% coverage
- Satin: -10% coverage
- Semi-gloss: -15% coverage