How Much Paint Do I Need? Room Calculator & Coverage Guide

Buying too much paint wastes money. Buying too little means an extra trip to the store and potential color-matching issues between batches. With a simple calculation, you can figure out exactly how many gallons you need for any room, accounting for doors, windows, and multiple coats.

The Paint Coverage Formula

Standard interior latex paint covers approximately 350–400 square feet per gallon on smooth surfaces. Use 350 sq ft for textured walls and ceilings, 400 sq ft for smooth drywall.

Gallons Needed = (Total Wall Area - Unpainted Areas) × Number of Coats / Coverage Rate

Step 1: Calculate total wall area

Wall Area = Perimeter × Ceiling Height
Perimeter = 2 × (Length + Width)

Example: A 14 ft × 12 ft room with 9 ft ceilings:

• Perimeter: 2 × (14 + 12) = 52 feet
• Wall area: 52 × 9 = 468 sq ft

Step 2: Subtract doors and windows

Opening	Standard Deduction
Standard door (3 × 7 ft)	21 sq ft
Double/French door	42 sq ft
Small window (3 × 3 ft)	9 sq ft
Medium window (3 × 4 ft)	12 sq ft
Large window (4 × 5 ft)	20 sq ft
Sliding glass door	40 sq ft

Our room has 1 door and 2 medium windows:

• Deductions: 21 + 12 + 12 = 45 sq ft
• Paintable area: 468 - 45 = 423 sq ft

Step 3: Calculate gallons needed

For 2 coats at 375 sq ft/gallon:

Gallons = 423 × 2 / 375 = 2.26 gallons

You’d need 3 gallons (round up to the nearest available size).

When You Need Primer (And How Much)

Primer coverage is typically 200–300 sq ft per gallon (lower than paint). You need primer when:

• Painting over dark colors with a lighter shade
• Covering bare drywall, new patches, or repairs
• Switching from oil-based to latex paint
• Painting over stains (water, smoke, marker)
• The surface is porous (bare wood, masonry)

For our 423 sq ft room needing primer:

Primer gallons = 423 / 250 = 1.69 → Buy 2 gallons

Many premium paints now include primer (paint-and-primer-in-one), which can eliminate this step for most color-over-color scenarios. These products won’t replace dedicated stain-blocking or adhesion primers for problem surfaces.

Multiple Coats: When and Why

Most paint jobs require 2 coats for full, even coverage. Plan for additional coats when:

• Going from dark to light (may need 3 coats even with primer)
• Using reds, yellows, or bright oranges (poor hiding power)
• Painting over patched or repaired areas
• Using bargain paint with lower pigment concentration

Quick room estimates (2 coats, standard coverage):

Room Size	Wall Area (8 ft ceiling)	Gallons Needed
8 × 10 ft (bathroom)	288 sq ft	1.5 → 2 gal
10 × 12 ft (bedroom)	352 sq ft	2 → 2 gal
12 × 14 ft (bedroom)	416 sq ft	2.2 → 3 gal
14 × 16 ft (living room)	480 sq ft	2.6 → 3 gal
16 × 20 ft (large room)	576 sq ft	3.1 → 4 gal

Estimates include typical door/window deductions and 2 coats.

Always buy an extra quart or half-gallon for touch-ups later. Leftover paint stored properly lasts 5+ years and saves you from color-matching headaches.

Calculate instantly with our Paint Calculator.