Every paint can in the store makes the same promise: 400 square feet per gallon. And every experienced painter quietly plans around 350 — because the label number describes a laboratory wall, not yours. Here’s what each paint type actually covers, and why.

The honest coverage table

Imperial figures are per US gallon, metric per litre — the metric column is the honest conversion (1 gal = 3.785 L, 1 sq ft = 0.093 m²), not a relabeling.

Paint type	sq ft/gal	m²/L	Notes
Standard interior wall (flat–eggshell)	350–400	8.6–9.8	400 on smooth primed drywall, 350 in practice
Budget / contractor grade	300–350	7.4–8.6	Thinner film, weaker hide — often needs 3 coats
Premium / one-coat lines	350–450	8.6–11.0	The “one-coat” claim holds only over similar colors
Ceiling paint (flat)	350–400	8.6–9.8	Extra-flat and splatter-resistant; same spread rate
Primer (PVA drywall)	250–350	6.1–8.6	Meant to soak in — the low number is the feature
Stain-blocking primer	200–300	4.9–7.4	Shellac/oil bases run lowest and still beat paint at sealing
Trim & door enamel	350–450	8.6–11.0	Goes on thin over smooth wood — a quart does a room
Textured/masonry paint	200–300	4.9–7.4	Surface area of texture is larger than it looks
Exterior siding paint	250–350	6.1–8.6	Rough-sawn and weathered surfaces drink paint

What the label really measures

Coverage claims come from spreading paint on sealed, non-porous test panels and measuring the wet film. Your walls are neither sealed nor smooth: fresh drywall compound wicks water from paint, orange-peel texture hides 10–20% more surface area than a tape measure reports, and any porous substrate swallows the first pass. That’s why professionals bid materials at ~350 sq ft/gal and treat 400 as what a second coat might achieve over its own first coat.

The factors that move your number

• Surface porosity. New drywall, patched compound and bare wood are the big three coverage killers — the reason primer exists is that it’s cheaper to seal with a $25 gallon of PVA than a $55 gallon of finish paint.
• Texture. Popcorn ceilings and knockdown walls can halve the label figure. Use the textured/masonry preset (~300 sq ft/gal, ~8 m²/L) rather than trusting the wall rate.
• Color change. Painting navy over white needs a third coat or a tinted primer more often than it needs more area math — the per-coat number doesn’t change, the coats do.
• Application. Rollers lay down a predictable ~3–4 mil film. Brushing trim wastes little. Spraying puts more paint on the wall per gallon but loses some to the air.
• Sheen. Flatter paints cover slightly less per gallon than satin at equal coats — the sheen guide has the trade-offs.

Plan at 350, buy at the plan

For a typical room — measured properly, openings deducted — the math is net sq ft × coats ÷ 350, rounded up to cans. If your finish coat beats 350 (many do on a primed wall), the surplus is touch-up stock, not waste. Undershoot the rate and you’re back at the store mid-wall with a half-dry roller — the single most common outcome the overbuy list warns about, in reverse: underbuying is just overbuying delayed.

The calculator ships presets for each of these — pick the one that matches your paint and surface, or type the exact figure off your can.

Frequently asked questions

Is 400 sq ft per gallon realistic?

As a best case, yes — smooth primed drywall, a quality paint, a similar color underneath. As a planning number, no: most real rooms land at 325–375 sq ft/gal per coat. Budget at 350 and treat anything better as a bonus; buying to the label claim is how second trips to the store happen.

Why does primer cover less than paint?

Because it's designed to be absorbed. Primer's job is to seal a porous or stained surface, so a chunk of every gallon goes into the wall rather than sitting on top — hence ~200–300 sq ft/gal vs paint's ~350–400. New drywall especially drinks it. That's not waste; that's the product working.

Does paint go further with a sprayer?

Volume on the wall, yes — sprayed coats are thinner and more even, so a gallon can stretch 10–20% further in coverage terms. But sprayers also lose paint to overspray and back-rolling requirements. For a single room the roller wins on total paint purchased; for whole exteriors the sprayer usually wins.

Do darker colors need more paint?

Not more paint, more coats — deep reds, navy and charcoal often need three passes over a light base (or a tinted gray primer underneath, which is cheaper than a third gallon of finish paint). Coverage per coat is unchanged; coats to opacity is what grows.

How do US and UK coverage numbers compare?

UK cans quote ~10–12 m²/L for emulsion, which sounds bigger than the US ~350–400 sq ft/gal — and is. Converted exactly, 400 sq ft/gal equals ~9.8 m²/L, so UK label claims run a bit more optimistic per unit. Plan at ~10 m²/L for standard emulsion and ~6–7 for primer or undercoat, same discount-for-reality logic.