How Much Paint Do I Need? Calculator Guide

If you've ever stood in the paint aisle guessing how many gallons to grab, you're not alone. Most homeowners overbuy paint by 20–30%, and a few underbuy and end up with a second trip mid-project. This guide gives you the contractor formula plus a free paint calculator so you order the exact amount the first time.

The simple paint coverage formula

One gallon of interior latex paint covers roughly 350 square feet on a smooth, primed wall. Two coats are standard for true color and durability. The core formula is:

Gallons = (Wall Area × Number of Coats) ÷ 350

Step 1: Measure your wall area

For a rectangular room, add up the length of all four walls and multiply by the ceiling height:

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

Example: a 12 ft × 12 ft room with an 8 ft ceiling = 2 × (12 + 12) × 8 = 384 sq ft of wall.

Step 2: Subtract doors and windows (optional)

A standard door is about 21 sq ft and a standard window is about 15 sq ft. Subtracting them is a nice optimization, but most pros skip it because the extra paint covers touch-ups.

Step 3: Multiply by coats and divide by 350

Using the 384 sq ft example with two coats: (384 × 2) ÷ 350 ≈ 2.2 gallons. Always round up — buy 3 gallons.

Paint coverage cheat sheet

• Small bedroom (10×10 ft) — 2 gallons for 2 coats
• Average bedroom (12×12 ft) — 3 gallons for 2 coats
• Living room (16×20 ft) — 4 gallons for 2 coats
• Hallway (4×20 ft) — 1–2 gallons for 2 coats

Try the free paint calculator

Skip the math — punch your room size into our paint calculator on the home page and see gallons and cost update instantly.