Paint Coverage Explained: Why One Gallon Never Covers 400 sq ft

What the can actually promises

Coverage claims from manufacturers we checked:

• Sherwin-Williams SuperPaint Interior: “350–400 sq. ft. per gallon @ 4 mils wet, 1.5 mils dry” (product data sheet).
• BEHR PREMIUM PLUS Interior: 250–400 sq ft per gallon.
• KILZ 2 All-Purpose primer: 300–400 sq ft per gallon.

The Sherwin-Williams sheet is the most revealing, because it states the conditions: the coverage applies to a film exactly 4 mils (four thousandths of an inch) thick when wet. Behr's wider 250–400 range points the same way: how far a gallon goes depends on the surface and how the paint goes on.

The math: why 400 sq ft is the ceiling

A US gallon is 231 cubic inches. If you spread that volume into a layer 4 mils (0.004 in) thick, it covers 231 ÷ 0.004 = 57,750 square inches — which is 401 square feet. In other words, 400 sq ft per gallon is simply what a gallon of liquid physically covers at 4 mils. There is no paint left over for anything else.

The dry figure checks out too. SuperPaint is about 38% solids by volume, and 4 mils × 0.38 ≈ 1.5 mils — the rest is water that evaporates. So the coverage number assumes every drop ends up on the wall at exactly the recommended thickness.

Change the thickness and the coverage changes in proportion:

Why real walls get less

• Heavier application. Rolling paint on thicker to hide the old color in fewer passes uses it up faster. As the table shows, going from 4 to 5 mils cuts coverage by a fifth.
• Thirsty surfaces. New drywall, joint compound patches and bare wood soak up paint before a film forms. Sherwin-Williams' own spec for new drywall is either two coats of the paint as a self-primer or a coat of primer followed by two coats.
• Texture. Orange-peel, knockdown or rough plaster walls have more real surface area than their flat measurement, and the valleys fill with paint.
• Loss. Paint left in the tray, soaked into the roller cover and brushed onto trim edges never makes it onto the wall area you measured.
• Color changes. Big jumps from dark to light or light to dark often need an extra coat, which means another full coat's worth of paint for the whole room.

How to estimate paint realistically

Plan at the low end of the label's range. Our paint calculator defaults to 350 sq ft per gallon for that reason, and lets you lower it for textured or unprimed walls.

Wall area = 2 × (length + width) × height
minus about 20 sq ft per door and 15 sq ft per window
Gallons = area × coats ÷ coverage per gallon

For a 12 × 12 ft bedroom with 8 ft ceilings, one door and two windows, the walls come to 2 × 24 × 8 = 384 sq ft, minus 20 and 30 = 334 sq ft. Two coats at 350 sq ft per gallon is 1.91 gallons — so two gallons, with a little left for touch-ups. At 400 sq ft per gallon the math says 1.67 gallons; it's the same two cans, but with no margin at all.

The door and window allowances are common estimating values (Dummies and McCormick Paints use 20 and 15 sq ft); measure and subtract large openings like patio doors at their real size.

Ceilings, trim and multiple rooms

A ceiling is simply length × width: 144 sq ft for that 12 × 12 ft room, or 0.82 gallons for two coats at 350 sq ft per gallon. Ceiling paint is usually a different product and sheen from the walls, so estimate it separately rather than adding it to the wall total. Trim, doors and cabinets are almost always a separate enamel in a quart or gallon; a gallon goes a long way on trim.

When you paint several rooms the same color, add their wall areas together before dividing. Leftovers from each room pool into one shared margin, which often saves a can compared with rounding up room by room — and makes the 5-gallon bucket math more likely to work in your favor.

Does “paint and primer in one” change the math?

Coverage labels for self-priming paints still assume the stated film thickness. On a wall that's already painted and in good shape, a paint-and-primer product can let you skip a separate primer coat. On bare drywall, the Sherwin-Williams data sheet still calls for either two coats of the paint used as its own primer or a coat of primer and then two coats — so a new room with fresh drywall should be estimated as three coats' worth of work either way. Planning the extra coat up front is cheaper than a second trip to the store.

Buying: gallons, buckets and primer

At the median prices we collected in September 2026, a gallon of mid-range interior paint (Behr Premium Plus or Glidden Diamond class) cost $35.98 and a 5-gallon bucket $154.00, or $30.80 per gallon. Once you need about 4.3 gallons of one color, the bucket is the cheaper buy. Keep leftover paint sealed and labeled with the room: it's the only reliable color match for touch-ups later.

Primer spreads less far than finish paint on bare surfaces, and it's worth using on new drywall, patches, stains and dramatic color changes. The calculator's primer option assumes 300 sq ft per gallon, the low end of KILZ 2's stated range.

Sources

• Sherwin-Williams SuperPaint Interior data sheet accessed 2026-09-17
• BEHR PREMIUM PLUS Interior — product page accessed 2026-09-16
• KILZ 2 All-Purpose primer — product page accessed 2026-09-16
• Dummies — Estimating how much paint to buy accessed 2026-09-16
• McCormick Paints — How to calculate how much paint accessed 2026-09-16
• The Home Depot — BEHR PREMIUM PLUS Flat 1 gal accessed 2026-09-16
• The Home Depot — BEHR PREMIUM PLUS Flat 5 gal accessed 2026-09-16
• The Home Depot — Glidden Diamond Flat 1 gal accessed 2026-09-16