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How Much Paint for a 20×20 Room with a 9 ft Ceiling?

A 20 × 20 room with a 9 foot ceiling needs this much paint per coat, with doors and windows already deducted. Here is the answer, plus a calculator you can adjust if your measurements are slightly different.

Answer: 4 gallons

How it changes with coats

Same area, different coats:

Coats	1	2	3
Paint needed	2 gallons	4 gallons	6 gallons
Openings deducted	51 ft²	51 ft²	51 ft²

What a 20×20 room with a 9 ft ceiling actually involves

At 20×20 ft this is the size of a typical bedroom, home office or dining room. Stripping out the openings leaves about 669 ft² of wall to cover, and at 2 coats that is 1,338 ft² of paint. It is a comfortable one-person weekend job, and the only real decision is whether to roll or spray.

Four walls give 720 ft² of raw surface. After 1 door and 2 windows — deducted at 21 ft² and 15 ft² each — you are painting 669 ft². At 2 coats and 350 ft² per gallon, that rounds up to 4 gallons. Everything below comes from those figures.

Where the square footage goes

Surface	Area	Notes
Two 20 ft walls	360 ft²	20 ft × 9 ft each
Two 20 ft walls	360 ft²	20 ft × 9 ft each
Walls, gross	720 ft²	perimeter of 80 ft × 9 ft ceiling
Doors deducted	21 ft²	1 × 21 ft²
Windows deducted	30 ft²	2 × 15 ft²
Paintable wall	669 ft²	what the 4 gallons actually covers

Because the room is square at 20 ft × 20 ft, all four walls are the same width and each takes about the same rolling time. Break the job into four identical passes and keep the wet edge moving around the room in one direction.

At 9 ft the top of the wall sits at the awkward height where a stepladder is a stretch but scaffolding is overkill; an extension pole and a stable platform are the practical answer.

Working at a 9 ft ceiling

At 9 ft the ceiling line is out of reach from the floor but too low to justify scaffolding. The practical setup is a 6 ft platform stepladder plus an extension pole on the roller. Budget about 10 repositioning moves around the 80 ft perimeter — more than an 8 ft room, and it is the top few inches where a rushed cut-in shows.

One coat, two coats, three coats

Most repaints need two coats, but the quantity changes sharply with each pass, so it is worth seeing the three side by side:

Coats	Paint required	Typical use
1	2 gallons	same colour, already primed, or a touch-up
2	4 gallons	the standard: full colour change and durability
3	6 gallons	dark over light, bare drywall, or a deep accent colour

This page is worked through at 2 coats, which is why the answer box says 4 gallons. If you are painting a dark wall back to a light colour, treat two coats as the minimum and buy for three — running out mid-wall is worse than a spare can.

Ceiling, trim and primer — the extras

The calculator covers walls only. If you are doing the whole room, these are the add-ons:

Item	Quantity	Notes
Ceiling	400 ft² — about 2 gallons per coat	20 ft × 20 ft flat area, priced as a separate surface
Baseboard and trim	77 lin ft — about 1 quart	trim paint covers roughly 100 linear ft per quart
Primer	2 gallons	one coat over 669 ft²; needed over bare drywall or a dark colour

Primer is usually cheaper per gallon than colour and does two jobs: it seals bare drywall so the topcoat does not get sucked in, and it lets a colour change cover properly in two coats instead of three. Skimping on primer is the most common reason a two-coat job turns into a four-coat job.

Buying the paint

Option	Quantity	Estimated cost
Single gallons	4	$152
5 gallon pail	not worth it below 5 gal	—

At 4 gallons you are below the level where a 5 gallon pail pays off, so buy single cans. Buy them all in one trip and check the batch codes match: two cans from different batches can be slightly different tints, and a wall repainted from a mismatched can shows the seam forever.

Costs assume $38 per gallon and $165 per 5 gallon pail of mid-range interior paint ($30 per gallon of primer, $16 per quart of trim enamel). Brand, sheen and finish move these numbers a lot, so treat them as a planning figure rather than a quote.

Common questions

How much paint for a 20×20 room?

4 gallons of wall paint. That covers 1,338 ft² of surface at 2 coats, after deducting 1 door and 2 windows from the 720 ft² of wall. A 1-coat refresh would take 2 gallons and a 3-coat job 6.

Does that include the ceiling and trim?

No. The 4 gallons is for the four walls. The ceiling adds 400 ft², about 2 gallons per coat, and the baseboard and trim come to roughly 77 linear ft, about 1 quart of enamel. Add them to the order if you are doing the whole room.

How much primer do I need for this room?

About 2 gallons for one coat over the 669 ft² of paintable wall. Primer is only needed over bare drywall, fresh patching or a big colour change — over a similar existing colour you can skip it and go straight to two coats of paint.

How long between coats for a room this size?

Water-based interior paint is usually recoatable in two to four hours at normal room temperature. In a room this size, plan to finish a full pass before starting the next — that naturally spaces the coats and keeps the finish even. Cold, humid weather stretches the wait; check the can rather than guessing.