How the epoxy estimate works
gallons = floor area × coats × (1 + extra) ÷ coverage per gallon
wet film (mils) = 1,604 ÷ coverage (sq ft per gallon)
Coverage and thickness are two sides of the same number. One US gallon spread over 1 sq ft would be 1,604 mils (thousandths of an inch) thick, so spreading it over 250 sq ft gives a film of about 6.4 mils. Spread it thinner and it goes further, but the floor gets less protection.
Worked example
A 24 × 20 ft two-car garage is 480 sq ft. Two coats at 250 sq ft per gallon, plus 10 percent for absorption into the concrete and material left in the bucket, need about 4.2 gallons (16 liters). With kits that mix to 1 gallon, buy 5. Each coat goes on at about 6.4 mils wet. If the data sheet allows 300 sq ft per gallon, the total drops to 3.5 gallons and 4 kits, with each coat about 5.3 mils.
Coverage and film thickness
| Coverage per gallon | Metric | Wet film per coat |
|---|---|---|
| 100 sq ft | ≈ 2.5 m²/L | 16 mils (407 µm) |
| 150 sq ft | ≈ 3.7 m²/L | 10.7 mils (272 µm) |
| 200 sq ft | ≈ 4.9 m²/L | 8 mils (204 µm) |
| 250 sq ft | ≈ 6.1 m²/L | 6.4 mils (163 µm) |
| 300 sq ft | ≈ 7.4 m²/L | 5.3 mils (136 µm) |
| 400 sq ft | ≈ 9.8 m²/L | 4 mils (102 µm) |
Many 100% solids epoxies list something in the 250 to 300 sq ft per gallon range for a typical coat, while thick self-leveling or decorative-flake systems go on much heavier. These are product decisions, so start from the data sheet, not a generic number.
Solids content changes the dry thickness
Water-based and solvent-based epoxies lose part of their volume as they cure. A coat applied at 6 mils wet from a product with 50 percent solids dries to about 3 mils. A 100% solids epoxy keeps nearly all of its thickness. That is why low-cost kits often need more coats to build the same protection.
How many coats?
A basic DIY kit is often applied as one or two coats of colored epoxy. Many longer-lasting systems use three layers: a primer that soaks into the concrete, a colored body coat, often with decorative flakes broadcast into it, and a clear topcoat for wear and chemical resistance. Each layer has its own coverage rate, and flake broadcasts change how much topcoat you need. Enter each product separately rather than averaging them.
Choosing a coverage figure: if the data sheet gives a range, the low end of the range gives a thicker, more durable coat. Rough or porous concrete and the first coat over bare slab tend to land at the low end too.
Typical garage sizes
Two coats at 250 sq ft per gallon with 10 percent extra:
| Garage | Floor area | Epoxy | 1 gal kits |
|---|---|---|---|
| One car, 12 × 20 ft | 240 sq ft | 2.1 gal | 3 |
| Two car, 20 × 20 ft | 400 sq ft | 3.5 gal | 4 |
| Large two car, 24 × 24 ft | 576 sq ft | 5.1 gal | 6 |
Tips and safety
- Primers, base coats and clear topcoats often have different coverage rates. Run the calculator once per product.
- Mix only what you can spread within the pot life. Mixed epoxy left in the bucket heats up, and a large mass can get hot enough to smoke.
- Ventilate well and follow the safety data sheet. Uncured epoxy can sensitize skin, so wear nitrile gloves, long sleeves and eye protection, and use the respirator the data sheet calls for.
- Keep ignition sources away from solvent-based products, including water heaters and furnaces in the garage.
Surface preparation decides whether the coating lasts. Patch cracks first, and for new slabs, the concrete calculator helps plan the pour.
Frequently asked questions
How much epoxy do I need for a two-car garage?
A 24 × 20 ft garage, 480 sq ft, needs about 4.2 gallons for two coats at 250 sq ft per gallon with 10 percent extra. A 20 × 20 ft garage needs about 3.5 gallons.
How thick is one coat of epoxy?
At 250 sq ft per gallon a coat is about 6.4 mils wet. At 300 sq ft per gallon it is about 5.3 mils. For 100% solids epoxy the dry film is about the same.
Why add extra for concrete?
Bare concrete soaks up some of the first coat, and some epoxy is always left in the bucket and roller. About 10 percent covers it on sound concrete; very porous floors may need more.