Paint Coverage Calculator
Calculate the exact number of paint gallons required for any surface area, factoring in multiple coats and a configurable waste allowance. Useful for both DIY painters and professional contractors bidding jobs.
About this calculator
This calculator computes how many gallons of paint to purchase based on surface area, the number of coats planned, a waste allowance percentage, and the paint's coverage rate. The waste factor accounts for paint lost in roller trays, brushes, and overspray. The formula is: Gallons = ⌈(surfaceArea × coatsNeeded × (1 + wasteAllowance ÷ 100)) ÷ paintTypeCoverage⌉. The ceiling function ensures you always round up to a whole gallon, since partial gallons cannot be purchased separately at most retailers. Typical coverage rates: interior latex ~350–400 sq ft/gal, exterior paint ~300–350 sq ft/gal, and primer ~200–300 sq ft/gal. A 10% waste allowance is a reasonable default for brush-and-roller work; spray application may warrant 15–20% due to overspray losses.
How to use
Say you need to paint 800 sq ft of exterior siding with 2 coats of paint, a 10% waste allowance, and a paint that covers 350 sq ft per gallon. Step 1 — apply coats and waste: 800 × 2 × (1 + 10 ÷ 100) = 800 × 2 × 1.10 = 1,760 sq ft equivalent. Step 2 — divide by coverage: 1,760 ÷ 350 = 5.03 gallons. Step 3 — round up: ⌈5.03⌉ = 6 gallons. Purchase 6 gallons of exterior paint to complete the job.
Frequently asked questions
What waste allowance percentage should I add when calculating paint for a project?
For standard brush-and-roller interior work, a 10% waste allowance is typically sufficient. Exterior projects or rough-textured surfaces like stucco or brick absorb more paint and benefit from a 15% buffer. Spray application loses the most material to overspray—especially in windy outdoor conditions—so 15–20% is appropriate in those cases. It's better to buy a little extra than to run short mid-coat, since color-matching a new batch of paint to an existing one is difficult.
How does the number of coats affect total paint needed?
Each additional coat requires essentially the same amount of paint as the first, so the total paint needed scales linearly with the number of coats. Two coats means double the paint of one coat; three coats means triple. However, certain high-hide paints claim one-coat coverage—these typically have a coverage rate closer to 250–300 sq ft/gal because more pigment is packed into each gallon. Always verify coverage claims against real-world conditions, especially on new or porous surfaces.
How do I calculate paint needed for a room with multiple wall colors?
Measure and calculate each wall section separately if you are using different colors, then run the calculator once for each color zone. For example, if you're painting an accent wall a different color from the remaining three walls, note the square footage of the accent wall independently and calculate its gallons separately from the main color. This ensures you buy the right quantity of each color without overpurchasing. Many paint stores sell quarts (covering ~100 sq ft) for accent walls or touch-up work, which can reduce waste on smaller sections.