Do I need to prime before painting?

Prime bare drywall, new plaster, bare wood, patched repairs, water or smoke stains, and any glossy surface you have scuffed. Prime also when going from a dark colour to a much lighter one, because primer blocks the old colour more cheaply than extra coats of finish paint. Repainting a similar colour over a sound painted wall does not need primer, and a paint-and-primer product is enough.

How much does a gallon of paint cover?

A gallon of interior wall paint covers about 350 to 400 square feet on smooth, previously painted drywall. This calculator uses 350 square feet per gallon, which is the figure most manufacturers print for a single coat. Porous or textured surfaces drink more, so expect 250 to 300 square feet on new drywall or heavy texture, and as little as 150 on rough masonry.

Does the calculator include the ceiling?

No, the estimate covers wall area only. To add a ceiling, multiply the room length by its width and add that square footage separately, then divide by 350 per coat. Ceilings are usually painted with a dedicated flat ceiling paint rather than the wall colour.

Paint Calculator

Q: Should I buy gallons or a 5-gallon bucket?

A 5-gallon bucket holds the same paint as five separate gallons but usually costs less per gallon, so it is worth it once you need four or more. Buckets also avoid slight colour differences between separately mixed cans, which matters on large continuous walls. Individual gallons are easier to carry, store and return unopened, and are the better choice for one or two rooms.

Measure your walls, subtract the doors and windows, get the gallons.

Wall lengths (feet)

Ceiling height

Coats

Doors (−20 sq ft each)

Windows (−15 sq ft each)

— gallons

Wall area to paint

—

Total coverage needed

—

Coverage per gallon

350 sq ft

Buy an extra quart for touch-ups. Cans mixed later rarely match exactly, and you will want paint on hand for scuffs and nail holes.

How to Use the Paint Calculator

Measure each wall along the floor and enter the four lengths in feet. Leave a wall at 0 if the room has fewer than four.
Enter the ceiling height — 8 feet in most homes, 9 or 10 in newer builds.
Set the number of doors and windows. Each door subtracts 20 sq ft and each window 15 sq ft of wall you do not have to paint.
Choose your coats — two is standard. The gallons figure rounds up, because you cannot buy 2.1 cans.

How It's Calculated

Wall area is the perimeter multiplied by the ceiling height, minus the openings you are not painting.

Wall area = (wall₁ + wall₂ + wall₃ + wall₄) × height − (doors × 20) − (windows × 15)
Gallons = ⌈ wall area × coats ÷ 350 ⌉

Worked example: walls of 12, 14, 12 and 14 feet give a 52-foot perimeter. At 8 feet high that is 416 sq ft. Subtract one door (20) and two windows (30) for 366 sq ft of wall. Two coats is 732 sq ft of coverage, and 732 ÷ 350 = 2.09, which rounds up to 3 gallons.

The deductions are standard sizes: an interior door is roughly 20 sq ft (3 ft × 6 ft 8 in) and an average window about 15 sq ft. Large picture windows or double doors are bigger, so add an extra opening or two to compensate.

Rounding up is deliberate. Running out mid-wall means a second trip and a batch that may not match, so the last partial gallon is always bought whole.

Paint Coverage Reference

Square feet covered by one gallon, one coat. Manufacturer figures vary; these are the common working numbers.

Surface	Sq ft per gallon	Notes
Smooth painted drywall	350–400	The standard case, and what this calculator assumes.
New or unprimed drywall	250–300	Porous. Prime first or the first coat disappears.
Light texture (orange peel)	300–350	More surface area than it looks.
Heavy texture / knockdown	250–300	Back-roll to work paint into the valleys.
Bare wood	200–300	Depends heavily on species and grain.
Smooth exterior siding	300–350	Two coats standard on exteriors.
Rough stucco, brick, block	150–200	Masonry drinks paint. Budget nearly double.
Primer, all surfaces	200–300	Thinner build than finish paint.

Can sizes

Size	Covers (1 coat)	Best for
Quart	~100 sq ft	Touch-ups, trim, accent panels, samples.
Gallon	~350 sq ft	One average room, one coat.
5-gallon bucket	~1,750 sq ft	Whole floors, open-plan spaces, exteriors.

Frequently Asked Questions

Should I buy gallons or a 5-gallon bucket?

A bucket holds the same paint as five gallons but usually costs less per gallon, so it pays off once you need four or more. Buckets also sidestep slight colour differences between separately mixed cans, which shows up on large continuous walls. For one or two rooms, individual gallons are easier to carry, store and return unopened.

Do I need to prime first?

Prime bare drywall, new plaster, bare wood, patched repairs, water or smoke stains, and any glossy surface after scuffing it. Prime as well when going from a dark colour to a much lighter one — primer blocks the old colour far cheaper than extra coats of finish paint. Repainting a similar colour over a sound painted wall does not need it, and a paint-and-primer product is fine.

How much does a gallon of paint actually cover?

About 350–400 sq ft on smooth, previously painted drywall for a single coat. This calculator uses 350, the figure most manufacturers print. Porous or textured surfaces drink more — expect 250–300 on new drywall or heavy texture, and as little as 150 on rough masonry.

Does this include the ceiling?

No — wall area only. For a ceiling, multiply the room's length by its width and divide that by 350 per coat, separately. Ceilings are usually done in a dedicated flat ceiling paint rather than the wall colour anyway.

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