Paint Quantity Calculator for Walls, Ceilings & Rooms
🛒 Recommended Materials & Supplies
Compare pricing and in-stock inventory for your project:
9-Piece Complete Paint Roller & Tray Kit with 3/8" Nap
High-density roller covers and sturdy tray for lint-free wall coating.
ScotchBlue Original Multi-Surface Painter's Tape (3-Pack)
Delivers sharp, crisp paint lines without residue for up to 14 days.
Heavy-Duty Canvas Drop Cloth 9x12 Ft
Absorbs paint splatters and protects wood, carpet, and furniture.
How This Estimator Works
Our paint calculator determines precisely how many gallons or liters of interior or exterior paint and primer you need to buy for any room project. It calculates the gross perimeter wall area, automatically subtracts typical door openings (21 sq ft / 2 m² each) and window openings (15 sq ft / 1.4 m² each), and factors in optional ceiling painting. By applying standard manufacturer spread rates (350 sq ft per US gallon on smooth drywall, or 8.6 m² per liter in metric countries to match 350 sq ft/gal), the tool eliminates wasteful over-purchasing and multiple frustrating trips to the paint store.
Formulas & Calculation Principles
Gross Wall Area = 2 × (Length + Width) × Height
Openings Deductions = (Doors × 21 sq ft) + (Windows × 15 sq ft)
Net Wall Area = Gross Wall Area - Deductions
Total Coverage Required = (Net Wall Area + Ceiling Area) × Number of Coats
Gallons Needed = Ceiling of (Total Coverage Required ÷ 350 sq ft per gallon)
Liters Needed = Ceiling of (Total Coverage Area (m²) ÷ 8.6 m² per liter)
Step-by-Step Worked Example
Painting a 15 ft long by 12 ft wide living room with 9 ft ceilings, 1 entry door, 2 windows, and 2 coats of paint (excluding ceiling):
1. Gross wall area: 2 × (15 + 12) × 9 = 54 × 9 = 486 sq ft.
2. Deductions: 1 door (21 sq ft) + 2 windows (30 sq ft) = 51 sq ft.
3. Net paintable area: 486 - 51 = 435 sq ft.
4. Two coats required: 435 × 2 = 870 sq ft of total coverage.
5. Gallons required: 870 ÷ 350 = 2.48 -> Buy 3 gallons (or one 1-gallon can + two quarts, but 3 single gallons or a 5-gal pail is more cost effective).
Frequently Asked Questions (FAQ)
How many square feet does one gallon of paint cover?
One US gallon covers approximately 350 to 400 square feet on smooth, pre-primed drywall. On rough, unprimed, or heavily textured surfaces, coverage drops to 250 to 300 square feet per gallon.
Do I really need two coats of paint?
Yes. Even "paint and primer in one" formulas require two coats for rich depth of color, maximum durability, scrubbability, and true uniform sheen.
When is a separate primer necessary?
A separate primer is essential when painting over fresh drywall, unpainted wood, masonry, stained or smoke-damaged walls, or when making a drastic transition from dark to light colors.
How many liters of paint cover a standard room in the UK or Australia?
Based on standard spread rates (8.6 to 10 square meters per liter, equivalent to 350 sq ft per US gallon), a standard 4m × 3m bedroom with 2.4m walls requires approximately 6 to 7 liters for two coats.
What paint finish should I choose for each room?
Flat/Matte for living room and bedroom ceilings; Eggshell/Satin for bedrooms and living rooms; Semi-Gloss for bathrooms, kitchens, trim, and doors due to high moisture and washability.
How do I calculate paint for trim and baseboards?
Measure total perimeter length in feet and multiply by trim height in inches divided by 12. As a rule of thumb, one quart covers all baseboards and trim in an average 12x14 room.
How long should I wait between coats?
Water-based latex or acrylic paint typically dries to the touch in 1 hour and is ready for recoating in 2 to 4 hours. Oil-based paints require 16 to 24 hours between coats.
Technical & Trade Disclaimer
Material quantities and volume estimates provided by this calculator are for planning and estimation purposes. Always consult local building codes, structural engineering drawings, or professional trade contractors before placing bulk orders.