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.
wall_perimeter = 56wall_height = 8num_doors = 2num_windows = 4num_coats = 2coverage_rate = 350
- 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
Ready to run the numbers?
Open Paint Gallons Calculator