How much paint do I need for a room?

The arithmetic, and the one input everyone gets wrong

Paint quantity is a perimeter calculation wearing a floor-area costume. The wall is a strip whose length is the distance around the room and whose height is the ceiling. Nothing about the floor enters it.

So the first number is not 144 — that is the floor — but 384 sq ft of wall, from 2 × (12 + 12) × 8. Everything after that is subtraction, multiplication and one rounding.

The subtraction is the step people skip, and it is worth doing properly. A standard interior door is 21 sq ft; a standard window is 15. One of each opening plus a second window comes to 51 sq ft, which leaves 333 sq ft to actually paint. Two coats of that is 666 sq ft of film. Divide by the 350 sq ft a gallon claims and you land on 1.90 gallons.

Then comes the only rounding that has a price: paint is sold in whole cans, so 1.90 becomes 2. There is no half-gallon at the till, and a quart tinted on a different day may not match the gallons you already bought.

Why floor area is the wrong input — and the "× 2.5" shortcut that is only sometimes right

There is a shortcut in circulation that says wall area is about two and a half times floor area. For a 12 × 14 ft room with 8 ft ceilings it is almost exactly right: 416 ÷ 168 = 2.48. That is presumably where the number came from, and it is why it fails everywhere else.

The ratio is not a constant. For a square room it is exactly 4 × ceiling height ÷ the side length, and a rectangle is never lower for the same floor area. Fix the height at 8 ft and shrink the room, and the ratio climbs; grow the room and it collapses.

Swipe the table → for the ratio column

Gross wall area ÷ floor area, no openings deducted. The only thing that changes down this table is the room size and the ceiling height — and the ratio moves by more than a factor of two.

An 8 × 10 ft bathroom has 3.6 times as much wall as floor. A 20 × 20 ft open-plan room has 1.6 times. Apply × 2.5 to both and you come up 88 sq ft short in the bathroom — about a third of a can — and 360 sq ft over in the big room, which is more than a whole extra gallon. The shortcut does not err on the safe side; it is simply wrong, and which way it errs depends on the shape of the room.

The same trap sits in ceiling height, and it is the single most expensive inch in the job. Going from 8 ft to 9 ft adds 48 sq ft of wall to this room. That is enough to push 1.90 gallons to 2.18, which means a third can.

The 1.90 problem: you are one wall away from a whole extra gallon

Look again at that unrounded figure. 666 ÷ 350 = 1.90. Two gallons covers 700 sq ft of film, so you are 34 sq ft of film — 17 sq ft of actual wall — short of a third can. One extra window's worth of wall is almost exactly that. Almost any real-world deviation closes the gap.

Swipe the table → — the last column is the one that costs money

Same 12×12 room, 8 ft ceiling, two coats unless stated. Gallons are rounded up to whole cans, which is how paint is sold — there is no half-gallon option at the till.

Seven of those eight rows cost a whole can, and not one of them is a mistake in the arithmetic. Paint the ceiling, and the ceiling is 144 sq ft that was never in the wall figure. Discover the walls are textured, and the same gallon now covers 300 sq ft instead of 350. Decide to spray because the room is empty and it will be faster, and 15–30% of the paint goes into the air and the lines rather than onto the wall.

This is the honest answer to "why does the calculator always give me one more can than I expected". It is not padding the estimate. It is that the typical room lands at 1.9, and the typical room also has one of those seven conditions.

Deducting the openings is worth exactly one can

This is the least intuitive number in the article. Measure this room's walls whole — no deductions — and you get 384 × 2 = 768 sq ft of film, or 2.19 gallons. Three cans. Deduct one door and two windows and you get 1.90 gallons. Two cans.

Fifty-one square feet of unpainted opening, worth $42. The reason is not that 51 sq ft is large; it is that it straddles the rounding boundary. Had the room been bigger, the same 51 sq ft would have been absorbed by slack elsewhere and the can count would not move at all.

Two practical notes. First, measure your own openings rather than assuming — a double door is two doors, a bay window is a lot more than 15 sq ft, and a room with four windows has 60 sq ft of glass you are not painting. Second, the calculator's 21 and 15 are a 3 ft 0 in × 7 ft 0 in door and a 3 ft × 5 ft window; if yours differ, adjust the count of openings rather than pretending the areas match.

Coverage is the variable you control least

Every figure above assumes 350 sq ft per gallon, which is the middle of what manufacturers publish. The label is a theoretical spreading rate measured on a sealed test panel at a recommended film thickness. Almost no wall is that panel.

Swipe the table → for the notes

Coverage per coat, on a surface in that condition. Manufacturer figures are theoretical spreading rates on a sealed test panel; the field numbers are what estimators plan with.

Look at what the table does to this one room. At 400 sq ft per gallon the answer is still two cans — 1.67 gallons. At 300 it is three. So the whole spread from a good can to a mediocre one, on a small room, is a 50% difference in paint spend, and the only way to know which you are in is the can you are holding.

This is why the calculator has a coverage field instead of hiding the assumption. Type your product's number in and the answer changes immediately; leave it at 350 and you are getting the planning estimate, which is what 350 is for.

Two coats is not twice the paint

The calculator multiplies area by coats, which is the correct way to plan a purchase — but it is slightly pessimistic, and knowing why saves you from thinking you got shorted.

A second coat goes onto paint that has already sealed the wall. It draws roughly 10–20% less than the first coat, because the surface is no longer porous and you are not fighting absorption. So a job planned at 1.90 gallons frequently finishes with a third of a can still in it. That surplus is not an error in the estimate; it is the estimate being deliberately conservative, and it becomes your touch-up stock.

Two thin coats also hide better than one thick one. A thick coat looks covered while wet and then sags, runs, and takes days to cure hard enough to sand. The instinct to save a can by loading one coat heavier is how a wall ends up looking worse than it did before.

Primer is a separate purchase with its own arithmetic

None of the numbers above include primer, and on three surfaces you cannot skip it: bare drywall, patched or stained walls, and any dramatic colour change.

Primer covers 200–300 sq ft per gallon, because it is designed to be drunk by a porous surface. That means the same 333 sq ft of wall takes 1.3 to 1.7 gallons of primer — call it two cans, or one gallon plus a quart if the patches are local. Budget it as a separate line, not as a coat of paint.

On sound, previously painted walls in a similar colour, a paint-and-primer product is usually enough and a separate primer coat is money you do not need to spend. On a deep red going to off-white, primer is the cheaper of the two ways to get there: it costs less per gallon than the finish paint and it does the hiding work in one pass instead of three.

What the calculator cannot see

It has your room and your cans. It does not have these, and any one of them can move the answer:

1. Trim, doors, skirting and the door frames. A bedroom typically needs about a quart of enamel for the trim, which is a separate product with a different sheen. It is not in the wall figure and it is not optional if the trim is getting painted.
2. Patches and repairs. Fresh joint compound is as absorbent as bare drywall. A wall with a dozen patched nail holes and one drywall repair will drink noticeably more on the first coat than a uniform wall.
3. How much furniture you are moving. Not a paint quantity, but a labour one — and it is why an empty room and a full room cost different amounts for the same gallons.
4. Whether you are spraying. The calculator has no "application method" field. Spraying loses 15–30% to overspray and line priming; rolling loses 5–10% to the tray. If you are spraying, add that to the film area yourself — the table above shows what a 20% uplift does.
5. The colour you are covering. Pale over pale is two coats. Deep over pale is three, and a saturated red, yellow or orange may need a tinted primer as well. The calculator will happily tell you two coats for a job that needs four.

None of that argues against doing the arithmetic. It argues for doing the arithmetic and then buying the next can up, because a second trip to the store with a tinted colour is a half-day, and the leftover gallon is the cheapest insurance on the invoice.

What it costs, and where the money actually goes

For the 12 × 12 bedroom, two gallons of mid-range interior latex at $40–60 a gallon is $80–120 in paint. Consumables — a 9 in roller and sleeve, tray, a 2.5 in brush for cutting in, tape, drop cloths — run $25–40 if you own none of it.

Hiring it out changes the shape of the bill completely. A bedroom repaint typically quotes at $400–900 depending on market and how much patching and trim work is included, so the paint is roughly a fifth to a quarter of the price. The rest is preparation, cutting in, two trips up a ladder, and the fact that a professional will spend longer taping than you expect.

Which is the practical argument for getting the quantity right rather than generous: on a $600 job, saving one can of paint is $42 — 7%. The thing that moves the invoice is the preparation, not the gallon count.

Frequently asked questions

Where these numbers come from

1. Paint calculator on this site — The arithmetic here is the calculator’s: 2 × (length + width) × height, minus 21 sq ft per door and 15 sq ft per window, plus the floor if the ceiling is included, times the number of coats, divided by coverage, rounded up to whole gallons.
2. Manufacturer coverage figures — Sherwin-Williams publishes a typical 350–400 sq ft per gallon and 100 sq ft per quart. BEHR quotes 250–400 sq ft for two named interior products, which is the honest width of the range. Primer states 200–300 sq ft.
3. Opening sizes — The 21 sq ft and 15 sq ft used above are a 3 ft 0 in × 7 ft 0 in interior door and a 3 ft × 5 ft window. Measure yours if the openings are unusual — a double door is worth two of them.
4. Trade practice, not published standard — The texture and spray adjustments, and the observation that a second coat over a sealed surface draws less paint than the first, are what painters do rather than anything written in a standard. They are the reason a careful estimate usually finishes with paint to spare.

Last reviewed 5 November 2026. Coverage rates change between products and between batches. If your can disagrees with this page, the can wins — type its figure into the calculator.