One Coat vs. Two Coats: Why the Paint Quantity Roughly Doubles
Each additional coat of paint covers the same total wall area again, so paint needed for two coats is approximately double the amount needed for one coat, all else equal — two coats are standard practice for most projects because a single coat often leaves uneven coverage, visible brush or roller marks, or inconsistent color, especially over a different existing wall color.
Understanding exactly why two coats are the norm — not just accepting it as a rule of thumb — makes it clearer when it's genuinely necessary and when a single coat might reasonably suffice.
Why paint quantity doubles for a second coat
Each coat covers the same total wall area, so the paint required for a second full coat is essentially the same amount as the first — this is why doubling the single-coat estimate, as shown in the worked-example guide, gives an accurate total for a standard two-coat project.
Why two coats are usually necessary
A single coat often doesn't fully hide the underlying wall color or surface, especially when painting a noticeably different or darker color over a lighter one (or vice versa) — the second coat evens out any inconsistency from the first, producing a more uniform, professional-looking finish across the whole wall.
When a single coat might reasonably suffice
Repainting a wall the identical or very similar color to its current color, using a high-quality, high-coverage paint, can sometimes achieve acceptable, even results with a single coat — though checking the result carefully before deciding a second coat isn't needed avoids an uneven finish discovered only after the project is considered complete.