How Much Paint Do I Need? Calculate Gallons, Coverage & Cost

Every paint project starts with the same anxious moment at the hardware store: standing in the paint aisle trying to figure out exactly how many gallons to buy. Order too few and you are back tomorrow hoping the store can match your custom color again. Order too many and you have leftover paint sitting in the garage for a decade.

Paint math is actually simple once you know the two facts that drive every calculation: a gallon of good interior paint covers about 350 to 400 square feet in one coat, and you usually need two coats. The rest is just measuring carefully and subtracting the doors and windows. This guide walks through the formula, gives a room-size chart you can look up directly, covers walls, ceilings, trim, doors and exteriors, and ends with the cost ranges for both DIY and professional jobs.

The Quick Answer

Three numbers run all of paint math, and if you only remember these you can ballpark any project:

• 1 gallon ≈ 350–400 sq ft (smooth, primed interior walls, one coat).
• 1 quart ≈ 100 sq ft — the right size for trim, doors and small rooms.
• 2 coats is the standard, not a luxury. Plan for it.

The most common mistake by a wide margin is multiplying for one coat and then realizing halfway through that two were needed. Built into every estimate below: two coats is the default unless you have a specific reason for one.

How to Calculate Paint Needed

The step-by-step:

1. Measure the perimeter of the room. Add the lengths of all four walls. A 12 by 14 foot room has a perimeter of 12 + 14 + 12 + 14 = 52 ft.
2. Multiply by the ceiling height to get the gross wall area. Most homes are 8 ft. For a 9 or 10 ft ceiling, use that instead.
3. Subtract openings. Standard rule: 20 sq ft per door, 15 sq ft per window. Use exact figures if your doors or windows are unusual.
4. Multiply by the number of coats (usually 2).
5. Divide by 400 (square feet per gallon).
6. Round up to the next whole gallon, or to a gallon plus a quart for a tighter fit.

Paint Coverage Per Gallon

The 400-sq-ft-per-gallon rule is a useful default, but the real coverage varies more than people think. Coverage depends on the surface, the paint, and how heavy a hand you have with the roller.

The big surprise on this chart is usually stucco and rough siding — where coverage can drop to 100 sq ft per gallon. A house with stucco walls needs roughly twice the paint of the same-sized house with smooth siding. Always check the back of the can: every manufacturer publishes the coverage rate for that specific product, and premium "one-coat" paints sometimes genuinely deliver on the claim.

How Much Paint for a Room

For everyday rooms with 8-ft ceilings, one door and two average windows, this table gives the gallon counts at a glance. Add an extra quart for a high-ceiling (9 or 10 ft) version of the same room.

A practical note on the lower rows: a 12×12 bedroom technically only needs 1.67 gallons for two coats, but you cannot buy 1.67 gallons. Buying 2 full gallons leaves you about a third of a gallon left over, which is exactly what you want for touch-ups in the years after — paint stored sealed and indoors keeps for several years easily.

How Many Coats of Paint Do I Need?

The short answer: two for walls, one or two for ceilings, two for trim. The longer answer depends on what is underneath.

• Repainting the same color over a wall in good shape: one coat is often enough.
• Any color change: two coats, minimum. Bright or deep colors (reds, navies, dark greens) frequently want three.
• Light over dark, or dark over light: always two coats, sometimes with a tinted primer underneath.
• New drywall: primer (or PVA sealer) plus two coats of paint. The primer is non-negotiable on bare drywall — paint alone soaks in unevenly and looks blotchy.
• Patched walls: spot-prime the patches, then two full coats.
• Ceilings: typically one coat of flat ceiling paint if the existing finish is in good shape, two if there are stains or you are switching to a brighter white.
• Trim and doors: two coats of semi-gloss, with a light sanding in between for a smooth finish.

The estimating rule is simple: when in doubt, assume two coats. A wall that only needs one feels like a bonus; a wall that ran short of the second coat is a disaster.

Walls vs Ceilings vs Trim vs Doors

An interior paint job is usually four different products, all bought separately:

• Walls get the bulk of the paint — the room-size chart above handles this. Use eggshell, satin or matte/flat depending on traffic.
• Ceiling needs flat ceiling paint (low sheen hides imperfections best). Calculate it separately: length × width of the room, divided by 400. A 12×12 ceiling is 144 sq ft — a single quart at 100 sq ft is short for two coats; budget a half-gallon (two quarts) for a 12×12 ceiling and a full gallon for anything bigger.
• Trim, baseboards and door frames use semi-gloss for durability and washability. For an average bedroom, plan on one quart of trim paint for two coats. A whole house typically needs 1–2 gallons of trim paint total.
• Doors take about a quart per door for two coats, semi-gloss. A six-panel door needs a brush and a small foam roller and a steady hand.

Exterior Paint: How Much for a House?

Exterior estimating uses the same formula — perimeter times wall height, minus doors and windows — but the surface varies far more than indoors, and you almost always want two coats outside.

Rough ranges by home size:

• 1,500 sq ft single-story home: roughly 1,200–1,500 sq ft of siding. Plan on 5–8 gallons for two coats on smooth siding, more on stucco or rough surfaces.
• 2,000 sq ft two-story home: roughly 2,000–2,400 sq ft of siding. Plan on 8–12 gallons for two coats.
• 2,500–3,000 sq ft two-story home: 2,500–3,500 sq ft of siding. Plan on 10–16 gallons.

Add 1–3 gallons for exterior trim, soffits and fascia in a contrasting color. Garage doors and porch ceilings are often counted separately. Stucco, brick and rough cedar can easily double the gallons needed because of how much paint they absorb — if your exterior is anything other than smooth siding, measure carefully and budget extra.

How Much Does Painting Cost?

Paint cost has two big components: the paint itself, and — if you are not doing it yourself — the labor. The paint cost is the same whether you DIY or hire out; labor is what swings the bill.

Paint, by quality tier:

• Budget paint: $20–$30 per gallon. Works for closets, basements, and rentals you do not want to invest in.
• Mid-range: $30–$50 per gallon. The sweet spot for most projects — good coverage, acceptable durability.
• Premium: $50–$80+ per gallon. Better one-coat coverage, washability and longevity — worth it on high-traffic walls and shows on the wall.
• Exterior paint: $30–$70+ per gallon. Always more than the equivalent interior.

Interior labor (professional): $2–$5 per square foot of wall area, depending on prep work, ceiling height, the painter's market, and how much repair the walls need. An average bedroom (around 12×12) typically runs $400–$800 professionally painted. A whole-house interior repaint usually comes in at $2,000–$6,000.

Exterior labor (professional): $1.50–$4 per square foot of wall, plus surcharges for tall, multi-story or heavily-prepped jobs. A typical 1,500 sq ft home runs $3,000–$7,000 professionally; a 2,500 sq ft two-story home is more like $5,000–$12,000.

DIY costs are basically just paint and supplies. An average bedroom DIY paint job runs $80–$200 all-in (paint, rollers, tray, painter's tape, drop cloth, brushes). A whole-house DIY interior repaint is usually $400–$1,500 in materials. The trade-off is your time — figure on a long weekend per room if you are not experienced.

Common Estimating Mistakes

The errors that send you back to the paint store mid-project:

• Forgetting to multiply for the second coat. The single most common reason people run short. Always assume two coats unless you have a clear reason otherwise.
• Not subtracting doors and windows. Skipping the subtraction overstates the wall area and overstates the gallons — less costly than underbuying, but you end up with leftover paint you did not need.
• Using the floor-area square footage instead of wall area. Your home's listed square footage is floor space — wall space is usually three to four times larger. Calculate wall area separately.
• Skipping primer on new drywall. Bare drywall absorbs paint at very different rates from the rest of the wall. Without primer (or a self-priming paint clearly rated for new drywall), the finished wall looks blotchy under sidelight.
• Buying by the room instead of by the project. If you are painting three bedrooms in the same color, buy one larger order. A 5-gallon bucket is usually cheaper per gallon than five single gallons and ensures color consistency.
• Not budgeting extra for textured or porous surfaces. Stucco, brick and unprimed walls drink paint. Cut your coverage assumption in half on rough surfaces.

Key Takeaways

• Wall area = perimeter × ceiling height, minus 20 sq ft per door and 15 sq ft per window. Multiply by coats, divide by 400, round up.
• One gallon covers about 350–400 sq ft on smooth primed walls. Less on textured, porous or rough surfaces.
• Two coats is the default for walls and trim. One coat may be enough only for same-color refresh on existing paint in good shape.
• A standard 12×12 bedroom needs about 2 gallons of wall paint for two coats, plus a quart of trim and a half-gallon of ceiling.
• Exterior estimates: roughly 5–8 gallons for a 1,500 sq ft single-story, 10–16 gallons for a 2,500+ sq ft two-story — more for stucco or rough siding.
• Mid-range paint at $30–$50 per gallon is the sweet spot for most projects; premium paint earns its price on high-traffic walls.
• DIY a bedroom for $80–$200, or pay $400–$800 for a pro. Whole-house interior is $400–$1,500 DIY or $2,000–$6,000 professional.