Gallons per Square Foot Calculator
Enter your surface area and product coverage rate to find exactly how many gallons you need. Three calculation modes cover everyday paint and sealers, technical coatings specified in mils, and irrigation or liquid applications. Add a waste factor, number of coats, and material cost to get a complete project estimate.
Formula
Worked example
A 500 sq ft room, 2 coats of paint rated at 350 sq ft/gal, 10% waste: (1/350) x 2 x 1.10 = 0.006286 gal/sq ft. Total = 0.006286 x 500 = 3.14 gallons. Buy 4 gallons.
What is a gallons per square foot calculation?
Any liquid coating project requires knowing how much material to buy before you start. The gallons per square foot figure is simply the inverse of the coverage rate printed on your paint can or product data sheet: if a paint covers 350 square feet per gallon, you need 1/350 gallons for every square foot you coat. Multiply that rate by the total area and by the number of coats, add a waste factor for spills and overspray, and you have your purchase quantity. This calculator automates that math across three distinct modes - everyday paint and coatings, technical industrial coatings specified in mils thickness, and liquid or irrigation applications - so the formula is always correct for your product type.
Coverage rate mode vs. mils mode vs. depth mode
Most consumers choose coverage rate mode: find the square footage per gallon on your paint can label, enter it, and the calculator converts it to a per-square-foot consumption rate. Technical coating mode is for products like epoxy floor coatings and protective industrial finishes whose data sheets specify a dry film thickness (DFT) in mils and a volume-solids percentage. The wet film thickness (WFT) you must apply equals DFT divided by the volume-solids fraction. At 100% solids, 1 mil of wet coating across 1 square foot requires exactly 1/1,604.17 gallons, a physical constant derived from gallon-to-cubic-inch conversion. Depth mode is for liquid volumes applied to a horizontal surface - irrigation, concrete curing, or roof waterproofing - where 1 inch of depth over 1 square foot equals 0.62338 gallons (another fixed conversion: 1 gallon covers 231 cubic inches; 1 square foot is 144 square inches; 231/144 = 1.604 in, and its inverse gives the gallon-per-in-depth factor).
Why waste factor matters
Real-world paint jobs always use more material than the theoretical rate. Roller nap holds paint that never reaches the wall, spray guns produce overspray, uneven surfaces have peaks that absorb more than expected, and you need some left over for touch-ups. A 10% waste factor is a good baseline for roller application on smooth interior walls. Spray application typically needs 15-20% because overspray can be significant. Very rough or porous surfaces like raw brick or weathered wood may warrant 20-25%. When in doubt, round up to the next full gallon rather than making a second trip to the store.
How to read your product data sheet
For standard paints, the coverage rate is printed directly on the can, usually as a range like "350-400 sq ft per gallon." Use the lower number for rough or porous surfaces and the higher number for smooth, previously painted surfaces. For technical coatings, locate the volume solids percentage and the recommended dry film thickness in the product data sheet (PDS) rather than the label. The PDS is available on the manufacturer website; the information on the can often omits these values. Industrial contractors always verify DFT with a wet film thickness gauge during application to ensure specifications are met.
Typical coverage rates by surface type
| Surface / Product type | Coverage (sq ft / gal) | Notes |
|---|---|---|
| Smooth drywall / plasterboard | 400 | Best-case interior; new primed surface |
| Standard interior latex | 350-400 | Most common household paint |
| Textured interior walls | 300-350 | Orange peel or knockdown finish |
| Wood siding (exterior) | 250-350 | Varies with grain and porosity |
| Concrete / masonry sealer | 200-300 | Depends on porosity; seal first |
| Brick (exterior) | 150-200 | Highly porous; two coats typical |
| Stucco (exterior) | 150-175 | Rough texture absorbs heavily |
| Epoxy floor coating (50% solids) | 160 | Based on 4 mil DFT target |
| 100% solids epoxy | 250-320 | High-build; self-leveling floors |
| Waterproofing membrane | 40-80 | Heavy mil build; check TDS |
Manufacturer label rates vary; always verify against your product data sheet. These are industry averages for one coat.
Frequently asked questions
How many gallons of paint do I need for 500 square feet?
At a standard coverage rate of 350 sq ft per gallon and 2 coats with a 10% waste factor, you need about 3.14 gallons - so buy 4 gallons. At a higher-quality rate of 400 sq ft per gallon, 2 coats with 10% waste comes to about 2.75 gallons, so 3 gallons. Always round up to the next full gallon.
What does "coverage rate" mean on a paint can?
The coverage rate (or spreading rate) tells you how many square feet one gallon will coat in a single application at the recommended wet film thickness. It is almost always listed on the label and in the product data sheet. Manufacturers test it on smooth primed surfaces; expect 10-20% lower coverage on textured or porous surfaces.
What is dry film thickness (DFT) and why does it matter?
Dry film thickness is the cured depth of a coating measured in mils (thousandths of an inch). For structural and protective coatings like epoxies, polyurethanes, and industrial finishes, the DFT determines corrosion protection, abrasion resistance, and adhesion. Under-applying (too thin a DFT) causes premature failure; over-applying wastes material and can cause adhesion problems. Always target the DFT range in the product specification.
How do I calculate gallons for a concrete floor?
For a standard floor epoxy at 50% volume solids targeting a 4-mil DFT, the wet film thickness is 4 / 0.50 = 8 mils. Using the mils mode in this calculator with DFT = 4, solids = 50%, 1 coat gives about 0.00499 gallons per square foot, or about 200 sq ft per gallon. A 500 sq ft garage floor needs roughly 2.5 gallons per coat. Most epoxy systems use 2 coats, so plan for about 5 gallons before waste.
What waste factor should I use for spray painting?
Airless sprayers typically waste 15-20% due to overspray, especially near edges and corners. HVLP (high volume, low pressure) sprayers are more efficient at around 10-15%. Conventional air sprayers can waste 20-30% or more. For roller and brush application, 10% is usually sufficient for smooth surfaces and 15% for textured or rough ones.
How many gallons of water does it take to cover 1 acre to 1 inch depth?
One acre is 43,560 square feet. At 0.62338 gallons per square foot per inch of depth, 1 acre-inch of water equals 43,560 x 0.62338 = approximately 27,154 gallons. This is the well-known agronomic figure of roughly 27,000 gallons per acre-inch used in irrigation planning.
Does surface texture affect how many gallons I need?
Yes, significantly. A rough or porous surface like raw brick, stucco, or open-grained wood can cut effective coverage by 30-50% compared to smooth drywall. When painting brick or stucco, use the lower end of any coverage range and add a 20% waste factor. Applying a masonry primer or sealer coat first fills the pores and dramatically improves coverage efficiency for subsequent color coats.