Learn & Understand

What's Actually in the Can: The Chemistry of Paint

A gallon of paint covers a predictable amount of wall because of chemistry, not luck. Paint is a carefully balanced mixture of ingredients — some to color, some to stick, some to carry — and the reason one coat sometimes shows through while two do the job comes down to a single physical property called hiding power.

Four Ingredients, Four Jobs

Nearly every paint is built from four components. Pigments provide color and opacity; the binder (or resin) glues everything to the wall and to itself; the solvent or vehicle — water in modern latex paints — keeps the mix fluid enough to spread; and additives adjust everything from drying time to mildew resistance. As the vehicle evaporates, the binder locks the pigment into a solid film.

The White That Hides

The workhorse of opacity is titanium dioxide, a brilliant white pigment with an exceptional ability to scatter light. Its power comes from a high refractive index: light striking the film bounces off countless tiny particles rather than passing through to the surface below. This scattering is what “hides” the old color. Cheaper paints skimp on titanium dioxide, which is why they often need an extra coat to cover.

Why Coverage Is a Fixed Number

Paint is formulated to be applied at a certain film thickness — spread too thin and the pigment cannot fully hide; too thick and it sags and drips. Manufacturers publish a coverage figure, commonly around 350 square feet per gallon, that reflects the correct film thickness for good hiding. That number is why paint estimation works: it assumes the film is laid down properly, not stretched.

The Coat Count

Whether a wall needs one coat or two depends on the contrast between old and new color and on the paint's hiding power. Covering a dark wall with a light color, or vice versa, defeats a single film's ability to scatter away the old shade, so a second coat is needed. The number of coats is really a chemistry question about how much pigment must accumulate to block what lies beneath.

Estimating the Gallons

To convert wall area, openings, and coats into gallons, use the Paint Quantity Calculator. Choose the color first with the Room Color Scheme Calculator, and if you are papering instead of painting, the Wallpaper Roll Calculator handles rolls.