Paint calculator: how much paint do I need?

Buying the right amount of paint before starting a room project is one of those calculations that seems simple but frequently catches people out. Buy too little and you're back at the hardware store hoping they can match the batch number from the original can — a gamble, since paint color can vary slightly between production runs. Buy too much and you're storing half-empty cans in your basement for years. Getting it right the first time requires knowing your wall area, the coverage rate of your chosen paint, and how many coats your situation calls for.

Most interior house paints marketed in the United States advertise coverage of 350 to 400 square feet per gallon on a smooth, properly prepared surface. This is the theoretical maximum. In practice, especially on new drywall, bare concrete block, or any porous surface, coverage can drop to 250–300 sq ft per gallon. On previously painted walls of similar color in good condition, you may get the full 400 sq ft/gal. Understanding where your specific project falls on this spectrum prevents the most common paint-buying mistake.

How to calculate how much paint you need

The formula has three inputs:

• Paintable area (sq ft): total wall area minus doors and windows (see the Room Area Calculator for help computing this)
• Coverage (sq ft/gallon): found on the paint can label, typically 350–400 sq ft/gal for quality interior paints
• Number of coats: typically 2 for most applications; 1 for same-color touch-ups; 3 for dramatic color changes

Formula: gallons needed = paintable area (sq ft) ÷ coverage (sq ft/gal) × number of coats. Always round up to the nearest available container size, and add a 10% buffer for edges, cutting in, and small repaint touch-ups.

Detailed example with purchasing recommendation

Using the bedroom from our room area example: paintable wall area = 380 sq ft. Paint chosen: a quality interior eggshell at 350 sq ft per gallon coverage. Applying 2 coats.

Gallons needed = 380 ÷ 350 × 2 = 2.17 gallons. With a 10% safety buffer: 2.17 × 1.10 = 2.39 gallons. Round up to the next available size: 3 quarts (0.75 gal each = 2.25 gal) is slightly short — buy 2 gallons + 1 quart = 2.25 + 0.75 = 3.0 gallons total. The extra 0.6 gallons gives a comfortable margin and future touch-up reserve.

For the ceiling of the same room (180 sq ft), using flat ceiling white at 400 sq ft/gal, 2 coats: 180 ÷ 400 × 2 = 0.9 gallons. With buffer: 0.9 × 1.10 = 0.99 gallons. Buy 1 gallon — just enough with minimal leftover.

If you're making a dramatic color change (dark to light or light to dark), plan for 3 coats. Same wall area: 380 ÷ 350 × 3 = 3.26 gallons + 10% = 3.58 gallons → buy 2 gallons + 2 quarts = 2.5 gallons. Wait — that's 3.5 gallons. Alternatively, use a tinted primer for the first coat (it's cheaper than finish paint) and only 2 coats of finish: primer coverage is typically 300–350 sq ft/gal, so 380 ÷ 325 = 1.17 gal primer + 380 ÷ 350 × 2 = 2.17 gal finish = 3.34 gal total, similar cost but much better final result. For deep-color walls, this tinted-primer approach consistently delivers better coverage with fewer finish coats.

Frequently asked questions