Gallons per Square Foot Calculator

Free Online Gallons per Square Foot Calculator

This gallons per square foot calculator turns a product's coverage rate and the size of your surface into a straight answer: how many gallons do you actually need to buy? It works for interior and exterior paint, primer, stain, sealants, epoxy floor coatings, roof coatings, waterproofing membranes, and any other liquid product sold by the gallon and rated for a certain square footage of coverage.

Every can or bucket has a coverage rate printed on the label, but that number alone doesn't tell you how much to order once you factor in multiple coats, an uneven surface, and the fact that stores only sell full containers, not partial gallons. This tool does all of that math in one step and shows exactly how it got there.

What Does 'Gallons per Square Foot' Mean?

Gallons per square foot is a coverage rate — it tells you how much liquid product is needed to cover one square foot of surface at the manufacturer's recommended thickness. Most paint and coating labels don't print it this way directly; instead they usually list 'square feet per gallon' (like '350–400 sq ft per gallon'), which is the same information flipped upside down. Some sealants, roof coatings, and waterproofing products instead list coverage as 'gallons per 100 square feet,' which is common because their coverage rate is a much smaller number than paint.

This calculator accepts either format, so you can enter the coverage rate exactly as it's printed on your product's label without doing any manual conversion first.

Gallons per Square Foot Formula

Here's the math this calculator runs, in plain terms:

• If coverage is given as sq ft per gallon: Gallons per Sq Ft = 1 ÷ (Sq Ft per Gallon)
• If coverage is given as gallons per 100 sq ft: Gallons per Sq Ft = (Gallons per 100 Sq Ft) ÷ 100
• Raw Gallons Needed = Area (sq ft) × Gallons per Sq Ft × Number of Coats
• Gallons with Waste = Raw Gallons Needed × (1 + Waste % ÷ 100)
• Containers to Buy = Round Up (Gallons with Waste ÷ Container Size)
• Total Cost = Containers to Buy × Container Size × Price per Gallon

How to Use This Calculator

Start by measuring or calculating the total area you're covering in square feet. Find the coverage rate printed on your product's label — most paints list it as square feet per gallon, while sealants and coatings often list gallons per 100 square feet — and choose the matching option. Enter how many coats the job needs, since two coats simply double the gallons required. Add a waste allowance to cover roller loss, overspray, and touch-ups, and set the container size the product is sold in (1 gallon and 5 gallon buckets are the most common). If you know the price per gallon, enter it to see your total material cost. The calculator updates instantly and rounds your final order up to a whole number of containers, since you can't buy a partial can.

Worked Example

Suppose you're painting a room with 500 square feet of wall area, using a paint rated for 400 square feet per gallon, and applying two coats. First, flip the coverage rate: 1 ÷ 400 = 0.0025 gallons per square foot. Multiply by the area and coats: 500 × 0.0025 × 2 = 2.5 gallons of raw paint needed. Add a 10% waste allowance for rollers and touch-ups: 2.5 × 1.10 = 2.75 gallons. Since paint is sold in 1-gallon cans, round up to 3 gallons. At $40 per gallon, the total cost is 3 × $40 = $120.

Typical Coverage Rates by Product

Coverage varies a lot between product types and even between brands of the same product, so always check your specific label first. These figures are common ranges to sanity-check your calculation:

• Interior wall paint: 350–400 sq ft per gallon per coat on primed drywall.
• Exterior house paint: 250–400 sq ft per gallon per coat, lower on rough or unprimed siding.
• Primer: 200–300 sq ft per gallon, since primer soaks into porous surfaces more than finish paint.
• Deck and fence stain: 150–300 sq ft per gallon depending on wood porosity and stain thickness.
• Concrete sealer: 150–300 sq ft per gallon for a single coat on broom-finished concrete.
• Epoxy floor coating: 150–200 sq ft per gallon for the base coat, often less for the second coat.
• Roof coating: commonly rated in gallons per 100 sq ft, often 1 to 2 gallons per 100 sq ft per coat.

Why Coats and Waste Both Matter

Skipping either of these two adjustments is the most common reason people run short mid-project. Coverage rates on a label almost always describe a single coat, but most paint jobs use two coats for even color and durability, and some coatings call for three. If you only calculate for one coat and then paint twice, you'll come up roughly half a can short on your second trip to the store.

Waste is separate from coats — it accounts for paint left in the roller tray, overspray that misses the surface, cutting-in around edges and trim, and touch-ups after the main coat dries. A 10% allowance is a reasonable default for roller and brush application; spray application, textured surfaces, or first-time DIY painters often benefit from bumping that up to 15–20%.

Rounding to Full Containers

Paint, sealant, and coating products are sold in fixed container sizes — usually quarts, 1-gallon cans, and 5-gallon buckets — not in the exact decimal amount your calculation produces. This calculator rounds your final gallons-with-waste figure up to the next full container so the number you see is something you can actually walk into a store and buy. For large jobs, buying a 5-gallon bucket is almost always cheaper per gallon than buying several 1-gallon cans, so it's worth comparing both container sizes if your total is close to a 5-gallon threshold.

Common Mistakes to Avoid

A few errors come up again and again when estimating coverage by hand:

• Forgetting to multiply by the number of coats, which is the single biggest cause of running short on paint.
• Using the coverage rate for a smooth, primed surface when the actual surface is rough, porous, or unprimed, which soaks up far more product than the label rate assumes.
• Not subtracting doors, windows, and other openings from wall area, which slightly overstates the paint needed (a small buffer that usually just becomes part of the waste allowance).
• Buying the exact calculated amount with zero waste allowance, leaving no room for touch-ups or a second coat on thin spots.
• Mixing up 'sq ft per gallon' and 'gallons per 100 sq ft' when reading two different product labels, which throws off the whole calculation by a factor of 100.

Where This Calculator Is Used

Coverage-rate math like this comes up across a wide range of trades and DIY projects:

• Painters and homeowners estimating how many cans of interior or exterior paint a room or house needs.
• Contractors ordering primer, stain, or sealer for decks, fences, and concrete surfaces.
• Flooring installers calculating epoxy or polyurethane coating for garage floors and industrial floors.
• Roofing crews estimating elastomeric or silicone roof coating for flat and low-slope roofs.
• Waterproofing crews figuring out membrane or sealant quantities for foundations and basements.