Coverage is a film-thickness promise measured in mils. Texture and porosity cut it down, primer restores it, and coat count follows the colour change rather than the room.
Numbers to plan with
What 350 square feet per gallon actually measures
A gallon is 231 cubic inches. Spread at a wet film of 1 mil, one thousandth of an inch, that volume covers about 1,604 sq ft. Divide 1,604 by the wet film thickness in mils and you have the coverage figure printed on the can. The familiar 350 to 400 sq ft range for interior latex corresponds to roughly 4 to 4.6 wet mils, which dries to about 1.5 to 1.8 mils because the coating is 35 to 45 percent solids by volume.
This matters because coverage is a thickness promise, not a colour promise. Roll too thin to stretch a gallon and the film loses hide, scrub resistance, and sheen uniformity. Roll too heavy and the film sags, blocks in humid weather, and takes days longer to cure. The label number assumes the manufacturer's film thickness on a smooth sealed surface, so treat it as a target to hit rather than a guarantee that arrives in the can.
Check three numbers on the product data sheet before you estimate: volume solids, recommended wet and dry film thickness, and stated coverage. A coating at 45 percent solids reaches the same hide at a thinner wet film than one at 33 percent, which is why a premium gallon often finishes more wall than a builder-grade gallon at a similar shelf price.
Texture and porosity multiply the area you have to cover
Paint does not know that a textured wall has the same flat dimensions as a smooth one. Orange-peel texture adds roughly 10 to 15 percent to the surface a roller must wet. A heavy knockdown or skip-trowel finish adds 20 to 30 percent. Painted brick, split-face block, and stucco can add 40 to 60 percent, because the profile is deep and the surface pulls liquid sideways into the pores instead of letting it sit on top.
Porosity is the second multiplier, and it shows up entirely in the first coat. New drywall, fresh compound, and patched plaster absorb binder at different rates, so the first finish coat disappears in patches and reads as a dull map under raking light. A primer seals that variation and brings the second coat back toward label coverage. Skipping primer to save one gallon usually costs two, because the finish coat then goes on heavier to hide the flashing.
Coverage to expect on common interior surfaces, single coat
| Surface | Multiplier vs smooth | Plan on |
|---|---|---|
| Smooth primed drywall | 1.00 | 350-400 sq ft/gal |
| Light orange-peel texture | 1.10-1.15 | 300-350 sq ft/gal |
| Heavy knockdown or skip trowel | 1.20-1.30 | 270-320 sq ft/gal |
| Bare drywall, first finish coat | 1.15-1.25 | 280-340 sq ft/gal |
| Painted brick or concrete block | 1.40-1.60 | 220-280 sq ft/gal |
| Bare wood siding or trim | 1.25-1.50 | 230-320 sq ft/gal |
Primer earns its gallon in six specific situations
Paint-and-primer products combine a binder package with extra pigment, and they perform on clean, previously painted walls in sound condition. They are not a substitute for primer on bare or damaged substrates. Use a dedicated primer when the surface needs sealing, bonding, or stain blocking, and save the finish paint for colour, sheen, and washability.
- Bare drywall and fresh joint compound: a drywall primer equalizes absorption. Without it, compound reads as a dull stripe under any sheen above flat.
- Bare or weathered wood: a wood primer blocks tannin bleed from cedar and redwood, which otherwise turns white paint pink within one season.
- Water stains, smoke, crayon, or ink: a shellac or oil-based stain blocker. Water-based finish paint re-wets the stain and lifts it back through.
- Glossy trim, melamine, or previously enameled cabinets: degloss, then a bonding primer. Adhesion failure shows up as peeling sheets months later, not as a thin spot you can see now.
- A dramatic colour change from deep to light: a tinted primer at about 50 percent of the new colour saves one finish coat on a 400 sq ft room.
- Chalky or powdery masonry: a masonry conditioner or alkali-resistant primer, because free lime attacks latex and causes saponification and peeling.
Coats follow the colour change, and the primer follows the coat count
Count coats by the light value of the old colour against the new one, not by the room type. A white over white refresh in the same sheen can be one coat on a clean wall. Two coats is the working default for any colour change. Three coats, or a tinted primer plus two, is standard when moving from deep to light because the old pigment keeps influencing the new colour from underneath.
Sheen changes add a wrinkle that catches homeowners. Flat to eggshell at the same colour usually takes two coats, because the new sheen reveals every roller lap, patch, and sanding mark. Semi-gloss to flat hides those defects better but demands better prep, since flat paint telegraphs dents that gloss concealed. Sand repairs to 120 or 150 grit, spot prime them, and the finish will read even under a window where light rakes across the wall.
Exterior work runs higher. Bare siding usually needs one primer plus two topcoats, and a dark to light change on a south-facing wall can need three, because ultraviolet exposure accelerates chalking and fade. Horizontal surfaces such as rails, fascia, and window sills take 20 to 30 percent more product per square foot than vertical siding, since the film sits thicker and drips instead of running off.
Typical coat counts for interior repaints
| Change | Finish coats | Primer |
|---|---|---|
| White to white, same sheen | 1-2 | None |
| Light to light or mid-tone | 2 | None |
| Mid-tone to deep | 2 | None unless patched |
| Deep to mid-tone | 2 | 1 coat |
| Deep to white or pale | 3 | 1 tinted, or 2 finish over grey |
| Bare drywall | 2 | 1 drywall primer |
| Red or yellow over white | 3 | 1 grey or tinted |
Sheen decides where each gallon goes
Sheen controls washability, hide, and how much surface imperfection you see. Flat hides drywall flaws and touches up invisibly, but it scuffs and does not scrub. Higher sheen scrubs clean and reflects light, which also highlights every ridge of mud and every lap mark. Match sheen to the room's cleaning reality rather than to a photograph of a finished room.
Interior sheen by room
| Sheen | Best rooms | Trade-off |
|---|---|---|
| Flat or matte | Ceilings, low-traffic bedrooms, formal walls | Poor scrubbing; marks burnish |
| Eggshell | Living rooms, dining rooms, hallways | Shows lap marks under raking light |
| Satin | Kitchens, family rooms, kids rooms | Reveals wall flaws more than matte |
| Semi-gloss | Trim, doors, kitchens, bathrooms | High reflectivity; needs a smooth substrate |
| Gloss | Doors, cabinets, exterior trim | Every imperfection is visible |
Recoat windows, temperature, and the failures caused by rushing
Typical interior latex is recoatable in 2 to 4 hours at 70 F and 50 percent relative humidity. Alkyd and oil-based enamels need 8 to 24 hours, and cool or damp air stretches both numbers. Read the data sheet instead of the can's headline. Painting below 50 F, or above 85 F with low humidity, changes how the film coalesces and can leave a surface that never fully hardens no matter how long you wait.
Four failures trace back to timing. Lap marks appear when a wet edge dries before you get back to it, which is why you cut in one wall at a time and keep the roller moving in the same direction. Blocking happens when doors and windows close against paint that has not cured, so rehang hardware after 24 hours. Surfactant leaching leaves a glossy brown film on bathroom ceilings when moisture hits partially cured paint. Flashing appears over unprimed patches, where absorption differs and the sheen reads unevenly under a light.
Price the coat before you skip it. In 2026 a gallon of quality interior latex runs about $32 to $85 depending on tier and region, and a small room takes 2 to 3. Hiring out a repaint of that room at $2.50 to $5.50 per sq ft of wall runs $900 to $2,000. The arithmetic is not close, which is why the second coat is the cheapest insurance in the project.
Audit the order against the wall before you buy
Run one cross-check before checkout. Take total paintable wall area, divide by the coverage on the can, round up per coat, and compare that count against the gallons in your cart. Then compare against a sanity figure any painter carries in their head: one gallon covers roughly one coat of a 10 x 12 ft room's walls. A 12 x 15 ft room at two coats should land at 2 to 3 gallons, not 6.
Check the physical count too. Measure height in two corners, because a room that reads 8 ft at one end and 8 ft 4 in at the other adds about 2 percent to wall area over a 54 ft perimeter. Confirm the ceiling is included if it is being painted, confirm closets are counted, and confirm trim sits on its own line with its own product. Three minutes of arithmetic prevents the two standard outcomes: a second trip to the store mid-job and four gallons of unreturnable custom tint aging in the garage.
Questions that come up on this job
Why did my gallon cover less than the can said?
The label assumes a smooth, sealed surface and a specific film thickness. Texture, bare drywall, patched areas, and a heavy roller cover all increase consumption. Measure the area the first gallon actually finished, divide the remaining area by that number, and reorder from the real figure.
Is a tinted primer worth it on a deep-to-light repaint?
Yes on saturated colours. A primer tinted to about half the new colour hides the old pigment in one pass and saves a finish coat, which is usually $50 to $90 in material plus several hours of labour on a 400 sq ft room.
Can I put a second coat on after two hours?
Check the data sheet first. Many interior latex products allow recoating in 2 to 4 hours at 70 F, but cool, humid, or poorly ventilated rooms stretch that window. If the first coat feels cool and slightly tacky to a light touch, it is not ready.
Does higher sheen mean better coverage?
No. Sheen describes the surface after curing, not the solids content. A semi-gloss and a flat in the same product line often share coverage, but the semi-gloss reveals substrate flaws that flat hides, so it usually needs better prep rather than more paint.
Sources & further reading
References behind this guide
These sources support the safety, measurement, or industry conventions used above. Product instructions, approved plans, and local requirements control your project.