Epoxy Coverage Calculator
Estimate gallons of epoxy for a garage floor or countertop.
Rule of thumb: one gallon covers 1,604 sq ft at 1 mil thickness. Porous concrete may absorb extra material \u2014 5% allowance included.
Epoxy is unforgiving arithmetic: mix too little and you can’t just blend a new batch into a half-cured pour; buy too much and the leftover hardens in its cans within months. Our epoxy calculator above converts surface area and coating thickness into gallons, using the industry’s universal coverage constant — with a 5% allowance for absorption and mixing loss.
The Formula
Coverage (sq ft/gal) = 1,604 ÷ Thickness (mils)
Gallons = Area ÷ Coverage × 1.05
The magic number 1,604 comes from geometry: one gallon spread at exactly 1 mil (a thousandth of an inch) covers 1,604 square feet. Everything in epoxy estimating flows from it. At 16 mils — a typical garage floor build — one gallon covers just over 100 sq ft, so a 400 sq ft garage needs about 4.2 gallons.
How Thick Should Each Coat Be?
| Application | Typical thickness |
|---|---|
| Thin primer coat | 3–5 mils |
| Garage floor color coat | 8–12 mils |
| Full flake system (per coat) | 10–16 mils |
| Countertop flood coat | ~1/8 in (125 mils) |
| Self-leveling floor epoxy | 30–125 mils |
Most floor systems are multi-coat: primer + color + clear top. Run the calculator once per coat at each coat’s thickness and add the results — a 400 sq ft garage at 5 + 10 + 12 mils needs roughly 1.3 + 2.6 + 3.1 ≈ 7 gallons across the system.
Countertop flood coats at 1/8 inch work out to about 12.8 sq ft per gallon — dramatically less coverage than people expect, and the reason countertop kits are sold by the square foot.
Why Concrete Drinks Your First Coat
Bare concrete is porous, and porosity varies wildly with age, finish, and how the slab was cured. A thirsty slab can absorb 20–30% more primer than the math suggests. Two defenses:
- Buy the primer generously — round the primer coat up, not the top coat.
- Test porosity: pour a little water on the slab. Soaks in within a minute → very porous, expect extra absorption. Beads up → you may have a sealer that must be ground off before any epoxy will stick at all.
Surface prep — grinding or etching, then cleaning — determines whether epoxy bonds for a decade or peels in a year. Budget more time for prep than for coating.
Mixing Rules That Save Projects
- Mix only what you can spread within the pot life — typically 20–40 minutes for floor epoxy. Mixed epoxy in a bucket generates heat and cures faster than epoxy spread thin.
- Respect the ratio exactly (1:1 or 2:1 by volume, per product). Eyeballing ratios is the top cause of soft, sticky floors.
- Mix A and B for a full 2–3 minutes, scraping sides, then pour out immediately — never let mixed epoxy sit in the container.
- Watch the recoat window: most products want the next coat within 12–24 hours; miss it and you must sand for adhesion.
- Temperature matters: most epoxies want 60–85°F and low humidity. Cold slabs slow curing and thicken the product, cutting real-world coverage.
Frequently Asked Questions
How much epoxy for a 2-car garage (~450 sq ft)? A typical 2-coat system (10 + 14 mils): about 3 + 4.4 ≈ 7.5 gallons with the 5% allowance. With a separate primer, add ~1.5 gallons.
How much epoxy for a 40 sq ft countertop? A 1/8-inch flood coat: 40 ÷ 12.8 ≈ 3.1 gallons, plus a thin seal coat (~0.3 gal). Kits rated “covers 40 sq ft” already assume this.
Do flakes change coverage? Full broadcast flake absorbs some of the top coat — add 10–15% to the clear coat quantity when going full-flake.
Can I return unopened kits? Often yes at big-box stores, but specialty epoxy is frequently final-sale. Since Part A and B have shelf lives of about a year once opened, buy accurately rather than hugely over.
Estimates are for planning purposes. Coverage varies with surface porosity and temperature — check your specific product’s technical data sheet.