Spread Rate, Wet Film and Dry Film: The 1604 Number

Why this matters

Every coverage figure on a data sheet, every gallon count in a takeoff, and every warranty a coating maker writes is the same quantity stated three ways: square feet per gallon, mils of wet film, and mils of dry film. A crew that can convert between them can tell from the empty pails whether the wall got the film it was sold, and can explain to an owner why a porous wall ate a third more paint than the label promised. A crew that cannot ends up arguing with the store about "bad coverage" when the paint did exactly what its own numbers said it would.

Where 1604 comes from

A mil is one thousandth of an inch (not a millimeter; one mil is 25.4 microns). One US gallon is 231 cubic inches. Spread that volume into a layer one mil thick and ask how much area it covers.

ONE US GALLON SPREAD ONE MIL THICK
  1 gallon                       = 231 cu in
  1 mil                          = 0.001 in
  area = 231 cu in / 0.001 in    = 231,000 sq in
  231,000 sq in / 144 sq in/sf   = 1,604 sq ft

THIS PRODUCT  (illustrative sheet values)
  volume solids                  = 38 percent
  spread rate                    = 400 sq ft per gal
  wet film  = 1,604 / 400        = 4.0 mils
  dry film  = 4.0 x 0.38         = 1.5 mils

SAME PRODUCT, WORKED BACKWARDS
  coverage = 1,604 x 0.38 / 1.5  = 406 sq ft per gal
  the sheet rounds 1.52 mils to 1.5 and 406 to 400

The exact figure is 1,604.2; the trade rounds to 1,604. Nothing about it depends on the paint. It is pure geometry, which is why it works for primer, enamel, elastomeric and stain alike.

The three conversions

Spread rate to wet film: wet film in mils = 1,604 / spread rate in sq ft per gallon. Lay a gallon over 400 sq ft and the film is 4.0 mils wet; over 300 sq ft it is 5.3 mils; over 500 sq ft it is 3.2 mils. Double the area, halve the film.

Wet film to dry film: dry film = wet film x volume solids as a fraction. Volume solids is the share of the can's volume that stays on the wall after the water or solvent leaves. At 38 percent, 4.0 mils wet dries to 1.5 mils. Use volume solids, never weight solids; pigments are dense, so weight solids reads higher, and a dry film computed from it is flattering by a wide margin.

Target dry film to theoretical coverage: sq ft per gallon = 1,604 x volume solids / target dry film. This is the line to run when the sheet gives a dry film per coat and you want to know what coverage that implies. It is a ceiling: it assumes every drop lands on the wall and stays on the surface.

Metric sheets state the same relationship as square meters per liter = volume solids percent x 10 / dry film in microns. Check it against the box above: 38 x 10 / 38.1 microns (1.5 mils) = 9.97 square meters per liter, which is 107.3 sq ft over 0.264 gallon, about 406 sq ft per gallon. The two systems agree.

Two cans with the same coverage claim

Volume solids is why two products that both print "400 sq ft per gallon" are not the same purchase. Both lay 4.0 mils wet at that rate, because the 1,604 geometry does not care what is in the can. What stays on the wall does:

• Product A, 32 percent volume solids: 4.0 x 0.32 = 1.28 mils dry per coat.
• Product B, 45 percent volume solids: 4.0 x 0.45 = 1.80 mils dry per coat.

At identical coverage, B leaves 1.80 / 1.28 = 1.41 times the dry film of A, about 40 percent more, per coat. Two coats of A come to 2.56 mils dry, under the 2 x 1.80 = 3.60 of B and even under one and a half coats of it, 1.5 x 1.80 = 2.70 mils. When an estimator is weighing a cheaper line that "covers the same", this is the comparison to make: dry film per gallon, 1,604 x volume solids, not the coverage number on the label. A data sheet that does not publish volume solids at all has withheld the one figure that makes its coverage claim checkable; ask the maker's rep for it.

What the sheet's spread rate assumes

The printed range is theoretical or near it: the recommended film, on a smooth, sealed, non-absorbent surface, with nothing lost. A real job loses paint in four places, and each one pushes actual square feet per gallon below the sheet:

• Absorption. A porous substrate (bare drywall paper, raw wood, chalky old flat, stucco, masonry) pulls part of the wet film into itself. That paint is spent, but it is not film on the surface.
• Texture. Knockdown, rough-sawn siding and split-face block carry more true surface than the flat area you measured, so the same gallons spread over more real area.
• Application loss. Overspray and bounce-back from an airless gun, drips, paint left in the roller nap, tray, cut bucket and pump. On a spray job this is usually the largest of the four.
• Over-application. A heavy nap rolled loaded on a smooth wall lays more film than the target. That film is on the wall, and it is still gallons the takeoff did not plan for.

The first three cost paint without adding surface film. The fourth adds film. That distinction is the whole point of the worked case below.

Thinning, and why it does not change coverage per gallon of paint

Thinning adds volume and no solids. Thin 38 percent solids paint by 10 percent and the mix runs 0.38 / 1.10 = 0.345 solids. To hit the same 1.5 mils dry, the crew now has to lay 1.5 / 0.3455 = 4.34 mils wet instead of 3.95. Each gallon of mix covers 1,604 x 0.345 / 1.5 = 369 sq ft, but there are 1.10 gallons of mix per gallon of paint, so a gallon of original paint still covers 369 x 1.10 = 406 sq ft. Thinning moves the wet film target the crew has to read on the comb; it does not stretch the paint.

Worked case: the family room that drank the third gallon

A family room, 600 sq ft of walls, last painted twenty years ago in a builder flat that now chalks on a dark cloth. Several patched areas were spot primed. The new finish is an eggshell whose sheet reads 38 percent volume solids, 350 to 400 sq ft per gallon, 4.0 to 4.6 mils wet, 1.5 to 1.75 mils dry. The estimator planned two coats at 400: 1,200 / 400 = 3.0 gallons, boxed in one pail.

The first coat took 2.0 gallons. That is 600 / 2.0 = 300 sq ft per gallon, so by consumption the crew laid 1,604 / 300 = 5.3 mils wet, more than the sheet's top of 4.6. The crew lead's comb gauge, read right behind the roller, showed the last wetted tooth at 4 mils, inside the band.

Both numbers are true. The comb reads what is on the surface a few seconds after the roller passes; consumption counts everything that left the pail. The gap of about 1.3 mils (5.3 by consumption minus 4.0 on the comb) went somewhere other than the surface film. Two candidates, and the room shows both:

• The chalky flat drank it. Old flat that chalks is porous and loosely bound. Part of every wet mil soaked in.
• The roller and tray held it. The crew was using a 3/4 in nap on smooth drywall to "get coverage", which loads more paint than the wall needs and leaves more in the nap at the end.

The second coat went on a surface the first coat had sealed. It took 1.6 gallons: 600 / 1.6 = 375 sq ft per gallon, inside the sheet's range, which confirms the substrate was the main thief on coat one. Total used, 2.0 + 1.6 = 3.6 gallons against 3.0 planned. The crew ran out with about 0.6 gallon of wall to go, 600 x 0.6 / 1.6 = about 225 sq ft of the second coat, and the store run produced a can tinted on a different day that had to be dab-tested before it could be boxed in.

What would have predicted it: the chalk on the cloth. A chalky or porous surface either gets a sealing primer on its own line at the primer's rate, or the first finish coat gets planned at well below the sheet's low end. Either choice puts the gallons on the truck. The shop's reconciliation from this job, 300 sq ft per gallon on chalky flat, becomes the estimator's own number for the next one.

Reading it backwards: over-coverage is a thin film

The same arithmetic catches the opposite failure. If a crew reports 500 sq ft per gallon from the product above, the film was 1,604 / 500 = 3.2 mils wet, 3.2 x 0.38 = 1.2 mils dry, below the sheet's 1.5 minimum on that coat. "Great coverage" on a reconciliation is not a compliment; it is a thin film, usually from rolling dry or spraying fast, and it shows up as early fading and chalking. Any actual spread rate above the sheet's range means the film is short somewhere, whatever the wall looks like on handover day.

What a crew does with this on site

• Before the pail opens: turn the sheet's dry film target into a wet film target, adjusting for any thinning, and set the comb step that will gate it.
• During the first pass: read the wet film with a comb (notch) gauge, the method in ASTM D4414, per Film Build and Wet Film Thickness Verification, which owns the gauge procedure and the stop rules.
• On the next bid: use the shop's own reconciled spread rate for that substrate where it is lower than the sheet, and say on the estimate which surfaces were figured below the sheet and why. An owner who sees the chalky family room priced at more gallons than the label implies deserves the one-line reason.
• At the truck: divide measured area times coats by gallons used, for each product. Inside the range is a pass, below it means losses worth a note for the estimator, above it means thin film to fix before the job closes.

References

• See related: Film Build and Wet Film Thickness Verification
• See related: How to Figure Gallons for an Interior Room
• See related: Reading a Paint Data Sheet for Coverage, Thinning and Recoat
• ASTM D4414, Standard Practice for Measurement of Wet Film Thickness by Notch Gages, in the edition your specification references
• Coating manufacturer product data sheets for volume solids, film thickness and spread rate on the product in use