To find how much coating a job needs, start from one number: at 100% solids, one US gallon covers about 1,604 square feet at one mil of dry film. From there, coverage is just that constant scaled by the coating's percent solids and divided by your target thickness in mils, then padded with a loss factor for overspray and waste. Get those four inputs right — solids, target mils, area, and loss — and the order quantity falls straight out. Below is the whole calculation, every worked example checked by hand.
First, one definition that trips people up: a mil is one thousandth of an inch (0.001 in), not a millimetre. Exactly, 1 mil = 25.4 µm = 0.0254 mm. If a datasheet says 60 mils, that is 0.060 in — a substantial build, not a hair's width.
Where the 1,604 constant comes from
A US gallon is 231 cubic inches (exact). Spread that over an area at a given thickness and the coverage is volume ÷ thickness. Convert to square feet and mils:
- 231 in³ ÷ 144 in² per ft² = 1.6042 ft² covered at one inch thick.
- One inch is 1,000 mils, so at one mil you cover 1,000 × that = 1,604 ft² (231,000 ÷ 144 = 1,604.17, rounded to the nearest whole number).
That 1,604 assumes a perfect 100%-solids film with zero loss — the theoretical ceiling. Real coatings contain solvent or carrier that evaporates, so you scale by percent solids by volume:
Theoretical coverage (ft²/gal) = 1,604 × (% solids by volume) ÷ (dry mils)
Metric works the same with a cleaner constant: one litre covers 1,000 m² at one micron, so coverage (m²/L) = 1,000 × (% solids) ÷ (microns of dry film).
Wet film vs dry film
You apply wet film; you get paid for dry film. As the solvent leaves, the layer shrinks by the non-solids fraction, so:
Dry film thickness (DFT) = wet film thickness (WFT) × % solids by volume
Rearranged, the wet film you must lay down to hit a target dry mil is WFT = DFT ÷ % solids. A 100%-solids coating has WFT = DFT (nothing evaporates); a 60%-solids coating needs noticeably more wet film to reach the same dry number.
Reference table: coverage at 100% solids
| Dry film (mils) | Coverage (ft²/gal) at 100% solids |
|---|---|
| 1 | 1,604 |
| 5 | 320.8 |
| 10 | 160.4 |
| 20 | 80.2 |
| 40 | 40.1 |
| 60 | 26.7 |
| 80 | 20.1 |
For any other solids level, multiply by that fraction — e.g. a 70%-solids coating at 10 mils covers 160.4 × 0.70 = 112.3 ft²/gal. Values rounded to one decimal.
Worked example 1: a thick, high-solids coating
Say you are applying a 100%-solids coating (many polyurea and polyurethane systems are near this) at a target 60 mils DFT over a 500 ft² floor.
- Coverage: 1,604 × 1.00 ÷ 60 = 26.73 ft²/gal.
- Theoretical gallons: 500 ÷ 26.73 = 18.7 gal.
- Add a loss factor. Spray overspray, uneven surfaces, and waste commonly run 15–40%; use 20% here. Divide by (1 − 0.20 = 0.80): 18.7 ÷ 0.80 = 23.4 gal.
Check: 23.4 gal × 26.73 ft²/gal = 625 ft² of theoretical coverage for a 500 ft² job — 25% more material than bare area, which is the ~20% loss expressed as extra material. If your padded number covers less than the area, you divided the wrong way.
Because this coating is 100% solids, WFT = DFT: you spray 60 mils wet to land 60 mils dry.
Worked example 2: a lower-solids coating
Now a 60%-solids coating at a 5 mil DFT over 300 ft², with a 25% loss factor.
- Coverage: 1,604 × 0.60 ÷ 5 = 192.48 ft²/gal.
- Theoretical gallons: 300 ÷ 192.48 = 1.56 gal.
- With loss: 1.56 ÷ 0.75 = 2.1 gal.
- Wet film to apply: WFT = 5 ÷ 0.60 = 8.33 mils wet to cure to 5 mils dry.
Check: 8.33 mils wet × 0.60 solids = 5.0 mils dry. The arithmetic closes.
The gotchas that produce wrong answers
- Solids by volume, not by weight. Coverage math uses volume solids; datasheets sometimes also quote weight solids, which is a different (usually higher) number. Use the volume figure.
- Wet vs dry mixed up. Comparing a wet film thickness to a dry target overstates your coverage every time. Convert first.
- Forgetting the loss factor. Ordering exactly the theoretical gallons guarantees you run short. Pad for overspray, surface profile, and waste — more for spray and rough concrete, less for a roller on smooth steel.
- Mil ≠ millimetre. A 10× or 25× error hides here. 40 mils is about 1 mm, not 40 mm.
The same "which unit is this really in?" trap runs through every conversion — our guide to unit conversions and their gotchas covers the metric-and-imperial mix-ups that cause most of these.
To run the numbers, you need two inputs from the product itself: percent solids by volume and recommended dry-film thickness. Reputable manufacturers publish both — for instance, ArmorThane lists solids and recommended DFT for its polyurea and polyurethane coatings — which is exactly the data these formulas need, so pull them from the datasheet rather than guessing.
FAQ
What is the 1,604 rule for coating coverage? It is the theoretical coverage of a perfect 100%-solids film: one US gallon covers 1,604 ft² at one mil dry (231 in³ ÷ 144 in² × 1,000 mils). Scale it by percent solids and divide by your target mils, then add a loss factor.
How do I convert wet film to dry film? Multiply wet film by the percent solids by volume: DFT = WFT × % solids. A 50%-solids coating laid at 8 mils wet cures to 4 mils dry. To go the other way, divide: WFT = DFT ÷ % solids.
Isn't the theoretical coverage number optimistic? Yes — deliberately. It assumes zero waste and perfect thickness, which never happens. That is why you always divide by (1 − loss). Real coverage runs 15–40% below theoretical depending on method and surface, so treat 1,604-based figures as a ceiling, not an order quantity.
How many microns is a mil? Exactly 25.4, because 1 mil = 0.001 in and 1 in = 25.4 mm = 25,400 µm. So 60 mils = 1,524 µm ≈ 1.5 mm.
Take your numbers to the supplier
Work out coverage and mil thickness once by hand so you can sanity-check any quote against figures you trust. When you are ready to order for a real project, take your target DFT and area to a manufacturer who can confirm solids and coverage for the specific product. ArmorThane makes polyurea and polyurethane coating systems and supports a certified applicator network — a sensible place to request a quote or find an applicator once your coverage math is done.
