Paint Gallons Calculator Formula
Understand the math behind the paint gallons calculator. Each variable explained with a worked example.
Formulas Used
Gross Wall Area
gross_sqft = gross_areaDeductions (Doors/Windows)
deductions = door_area + window_areaNet Paintable Area
paintable_area = net_areaGallons Needed
gallons_needed = ceil(net_area * num_coats / coverage_rate)Variables
| Variable | Description | Default |
|---|---|---|
wall_perimeter | Total Wall Perimeter(feet) | 56 |
wall_height | Wall Height(feet) | 8 |
num_doors | Number of Doors | 2 |
num_windows | Number of Windows | 4 |
num_coats | Number of Coats | 2 |
coverage_rate | Coverage Rate(sq ft/gallon) | 350 |
gross_area | Derived value= wall_perimeter * wall_height | calculated |
door_area | Derived value= num_doors * 21 | calculated |
window_area | Derived value= num_windows * 15 | calculated |
net_area | Derived value= gross_area - door_area - window_area | calculated |
How It Works
How to Calculate Paint Gallons with Deductions
Formula
Net Area = (Perimeter × Height) - (Doors × 21 sq ft) - (Windows × 15 sq ft)
Gallons = (Net Area × Coats) / Coverage Rate
A standard door opening is about 3 ft × 7 ft = 21 sq ft. A standard window is about 3 ft × 5 ft = 15 sq ft. One gallon of paint covers approximately 350 sq ft.
Worked Example
A 14×14 ft room (56 ft perimeter), 8 ft ceiling, 2 doors, 4 windows, 2 coats.
- 01Gross wall area = 56 × 8 = 448 sq ft
- 02Door deductions = 2 × 21 = 42 sq ft
- 03Window deductions = 4 × 15 = 60 sq ft
- 04Net paintable area = 448 - 42 - 60 = 346 sq ft
- 05Gallons = 346 × 2 / 350 = 1.98 → 2 gallons
Frequently Asked Questions
What size do you assume for doors and windows?
This calculator uses industry-standard estimates: 21 sq ft per door (3 ft × 7 ft) and 15 sq ft per window (3 ft × 5 ft). For non-standard sizes, adjust the count accordingly.
Should I buy extra paint?
Yes. It is wise to keep a small amount of extra paint for touch-ups. The ceiling function in the calculation already rounds up to the next whole gallon.
Does primer count as a coat?
No. If you need primer (e.g., for new drywall or drastic color changes), apply it first, then add your finish coats on top.
Learn More
Guide
How to Estimate Paint Needed for Interior and Exterior Projects
Learn how to calculate the amount of paint needed for walls, ceilings, trim, and exteriors. Covers coverage rates, surface types, primer needs, and multi-coat calculations.
Ready to run the numbers?
Open Paint Gallons Calculator