How this instrument works
Epoxy coatings — for garage floors, countertops, tabletops and industrial flooring — are specified by two numbers: the area to cover, and how thick a layer to apply. A thin sealer coat and a thick self-leveling flood coat use dramatically different volumes of material to cover the identical floor area, which is why thickness, not just area, drives the gallons figure.
This instrument converts both measurements into a common unit and multiplies them into a volume. Area in square feet times thickness in inches, times 144 to convert square feet to square inches, gives a volume in cubic inches; dividing by 231 — the exact number of cubic inches in one US gallon — converts that volume to the gallon figure epoxy is actually sold and mixed in.
Epoxy is a two-part product, mixed from a resin and a hardener in a fixed ratio set by the manufacturer, and this instrument returns the total combined volume needed, not a split between the two components. Check the product's data sheet for its mix ratio (commonly 1:1 or 2:1 by volume) to work out how much of each part to buy once you know the combined gallons figure here.
- Enter Area to coat (sq ft) — the total surface area the epoxy needs to cover.
- Enter Coating thickness (in) — convert from mils first if that's how your product specifies it: divide the mil figure by 1000 (20 mils = 0.02 in).
- Read Epoxy needed (US gallons) directly; it recalculates the instant either field changes.
- Check the product's mixing ratio on its data sheet to split this combined figure between resin and hardener components before ordering.
Worked example — 200 sq ft at 0.125 in (1/8 in) thick
Enter 200 into Area to coat (sq ft) and 0.125 into Coating thickness (in) — a 200 sq ft garage floor getting a thick 1/8 in self-leveling flood coat. Volume works out to 200 × 0.125 × 144 = 3,600 cubic inches, since 144 converts the square-foot area term into square inches.
Epoxy needed (US gallons) reads 15.5844 — 3,600 cubic inches divided by 231, the exact number of cubic inches in a US gallon. That combined figure covers both parts of a two-part epoxy; check the product's mix ratio to know how many gallons of resin versus hardener to actually buy.
Questions
Why does the formula multiply by 144 and then divide by 231?
Because area is entered in square feet but thickness is entered in inches, so 144 (the number of square inches in one square foot) converts the area term into square inches, matching thickness's units and giving a volume in cubic inches. Dividing that cubic-inch volume by 231 — the exact number of cubic inches in one US gallon — then converts it to the gallon figure epoxy is sold in.
My product specifies thickness in mils — how do I convert that?
Divide the mil figure by 1,000 to get inches, since 1 mil is defined as exactly 0.001 inch. A common 20 mil coating becomes 0.02 in, and a thick 125 mil flood coat becomes 0.125 in — the figure this instrument's default already represents.
Does this account for waste, spillage or a porous surface soaking up extra?
No — this is the theoretical volume for a perfectly flat, non-absorbent surface at exactly the thickness you specify. Porous concrete, uneven surfaces, and normal mixing or application spillage all use more material in practice; many installers add 10% or more to the calculated figure, especially for a first coat on bare concrete.
Is the gallons figure the resin, the hardener, or both combined?
Both combined — this instrument returns the total volume of mixed, applied epoxy, not a breakdown between the resin and hardener components of a two-part system. Check your specific product's data sheet for its mix ratio by volume (often 1:1 or 2:1) to divide this total into the correct amount of each component to purchase.
Does coating thickness really change the gallons needed that much?
Yes, proportionally — doubling the thickness exactly doubles the gallons needed for the same area, since the formula is a straight multiplication. A thin 0.02 in (20 mil) sealer coat over 200 sq ft needs about 2.5 gallons, while the same 200 sq ft at a thick 0.125 in flood-coat thickness needs over 15.5 gallons — more than six times as much epoxy from thickness alone.