Calculator

Paint Calculator

A gallon of quality interior wall paint covers roughly 350-400 square feet in a single coat on a smooth, previously-painted wall — that's the figure printed on most paint-can labels, and it's the number this calculator starts from. The real math isn't the area of the room, though; it's the area of the walls minus doors and windows, multiplied by however many coats your color change actually needs.

This calculator takes your room's length, width, and wall height, subtracts standard door and window openings, and rounds up to whole gallons per coat so you buy enough on the first trip. For the actual step-by-step measuring method, see how to measure a room for paint; for ceilings specifically, use the separate ceiling paint calculator since flat ceiling paint has a different coverage rate than wall paint.

Calculate your quantity

Room length (ft)

Room width (ft)

Wall height (ft)

Doors

Windows

Coats

Coverage rate (sq ft)

sq ft per gallon (check your paint can label)

Paint needed

2 gallons

Net wall area: 366 sq ft
Coats: 2 coat(s)
Exact gallons: 1.95 gallons

Based on 375 sq ft/gallon coverage.

Shopping summary

• 2 gallon(s) of wall paint

This is an estimate — confirm structural work with a professional.

How this calculation works

Wall area = room perimeter x wall height, minus roughly 20 sq ft per standard door and 15 sq ft per standard window. Divide the net wall area by your paint's labeled coverage rate (350-400 sq ft/gallon for most latex wall paint; some premium and eco-formula paints run lower, closer to 250-300 sq ft/gallon, so check your specific can) to get gallons needed for one coat.

Then multiply by coat count. Two coats is the safe default for any color change, and mandatory when you're going from a dark wall to a light one or vice versa — a single coat of white over red or navy will look patchy no matter how good the paint is. If you're repainting the same or a very similar color, one coat can be enough, but budget a touch-up gallon regardless.

Primer changes the math: unprimed drywall or a big color swing (dark-to-light) usually needs a primer coat first, which is priced and bought separately from your finish coat.
Textured walls (orange peel, knockdown) eat 10-15% more paint than smooth drywall because of the extra surface area in the texture.
Always round up to the next full gallon — paint stores don't sell partial gallons, and running out mid-wall means a dry-time delay while you go back for more.

The formula

gallons = ceil( ((perimeter × height − doors × 20 − windows × 15) / coverage) × coats )

perimeter × height: The gross area of every wall in the room, in square feet — this calculator sums all four walls from room length and width rather than pricing one wall at a time, so a long narrow room and a square room of the same perimeter price identically.
doors × 20, windows × 15: Flat per-opening deductions in square feet, not measured openings — a single doorway is treated as 20 sq ft and a single window as 15 sq ft regardless of its actual size, which is why an oversized picture window or an undersized closet door both use the same subtraction.
coverage: Square feet one gallon covers for ONE coat, taken from the field above rather than fixed in the formula, because a can's printed rate ranges from roughly 250 to 400 depending on formula and finish.
ceil(...): The final rounding happens once, after coats are multiplied in — not once per coat — because paint is bought in whole gallons for the whole job, not one purchase per coat.

Where these numbers come from

20 sq ft per door, 15 sq ft per window: Trade convention used across most quick paint estimators for a standard interior door (roughly 3 × 7 ft) and a standard window (roughly 3 × 5 ft). An oversized opening — a sliding door, a bay window — is undercounted by this flat figure, which is why the deduction is a rule of thumb and not a measured input.
375 sq ft/gallon default coverage: Manufacturer-published mid-range figure for latex wall paint in a single coat on a smooth, previously painted surface. The field is editable because premium and eco-formula lines commonly print 250–300 sq ft/gallon on the can, and this calculator reads whatever number is entered rather than assuming the mid-range figure applies.
rounding after the coat multiplication, not before: Derived on this page from how the function is written: coats are applied to the exact gallons-per-coat figure and only the final total is rounded up. Rounding each coat separately would over-order on any job needing more than one coat, since two half-gallon roundups cost a full gallon more than one roundup on the doubled total.

Worked examples

A 17 by 13 foot living room, two coats for a dark-to-light change

Inputs
Room length	17 ft
Room width	13 ft
Wall height	9 ft
Doors	2
Windows	4
Coats	2
Coverage rate	400 sq ft

Result
Paint needed	3 gallons
Net wall area	440 sq ft
Coats	2 coat(s)
Exact gallons	2.2 gallons

Two doors and four windows take a real bite out of this room's 540 sq ft of gross wall — the net area lands at 440, about 19% less than the raw perimeter times height would suggest. At 400 sq ft/gallon coverage for two coats, that's 2.2 gallons exactly, which rounds up to 3 — a full extra can beyond the exact figure, because a paint store has no half-gallon-of-the-third-can option.

That third gallon is not wasted: it is the touch-up reserve every custom-tinted job needs, since re-mixing the exact same tint months later rarely matches. On a room this size with this many openings, buying to the exact 2.2 gallons and hoping the color holds across a fourth trip to the store is the riskier plan.

A 9 by 7 foot laundry nook, one coat of low-VOC paint

Inputs
Room length	9 ft
Room width	7 ft
Wall height	8 ft
Doors	1
Windows	1
Coats	1
Coverage rate	300 sq ft

Result
Paint needed	1 gallons
Net wall area	221 sq ft
Coats	1 coat(s)
Exact gallons	0.74 gallons

This room's net wall area is 221 sq ft — roughly half of the living room above — but at a single coat and a lower 300 sq ft/gallon coverage rate typical of low-VOC formulas, the exact need is 0.74 gallons. It still rounds up to a full gallon, the same purchase size a much bigger single-coat room would need.

That is the real lesson of a small room: below roughly one gallon's worth of paint, the room's size stops driving the purchase and the gallon floor does. A nook half this size would still need one gallon, not a quart, unless the store sells quarts of that specific tint.

Common mistakes

Assuming 'one gallon per room' regardless of room size — a 10x10 bedroom and a 16x20 great room need very different amounts; see how much paint does a room really take for why that rule of thumb fails.
Forgetting to subtract door and window area, which overstates gallons needed on rooms with a lot of glass or multiple doorways.
Buying exactly the calculated amount with zero cushion — a small pour-off for touch-ups later (especially with custom-tinted colors that are hard to rematch exactly) is worth the extra few dollars.
Skipping primer on new drywall to save a trip, then needing three finish coats instead of two to get even coverage.
Not checking the can's actual coverage figure before calculating — budget and premium lines from the same brand can have meaningfully different coverage rates, so a number pulled from memory of a past project can be off for this specific product.
Painting over glossy trim or a slick prior finish without scuff-sanding or a bonding primer first, which can cause peeling regardless of how accurately the gallon count was calculated.

Shopping summary

Buy in whole gallons (or quarts only for very small rooms/closets), one can per coat calculated above, plus a primer gallon if you're covering new drywall or making a major color change. If your room's walls are a single custom-mixed color, buy it all in one batch — different mixing batches of the same color name can shade slightly differently.

Worked example: a 12x14 bedroom with an 8-foot ceiling has a perimeter of 52 feet, so gross wall area is 52 x 8 = 416 sq ft. Subtract one door (20 sq ft) and two windows (15 sq ft each, 30 sq ft total) for a net area of 366 sq ft. At 375 sq ft/gallon coverage for two coats, that's (366/375) x 2 ≈ 1.95 gallons — round up to two gallons, which leaves a small margin for touch-ups without buying a third can.

FAQ

Does paint sheen change coverage?

Not the coverage rate itself, but flat and matte finishes tend to hide texture and prior color better in one coat than eggshell, satin, or semi-gloss, which reflect light and can show streaking if you skimp on coats.

How much extra should I buy for touch-ups?

A pint to a quart held back from your last gallon is usually enough for nail-hole patches and scuffs over the next year or two; label and store it airtight since paint separates and skins over time.

Do I need to paint the closets and trim separately?

Closets count as extra wall area in your total; trim and baseboards are a different product (usually semi-gloss enamel) and aren't included in this wall-paint estimate — budget them separately.

Does an accent wall change the gallon math?

Yes — calculate the accent wall's area separately at its own coat count (often just one or two coats of a bolder color) and subtract it from the main-color wall area, since painting it as part of the main-color total would over-order the main color and under-order the accent color.

Is eco-formula or low-VOC paint's lower coverage rate worth planning around?

It's worth checking the specific can — some low-VOC and eco-formula lines run closer to 250-300 sq ft/gallon rather than the 350-400 range, meaning the same room can need noticeably more gallons than a standard-formula estimate would suggest.

Where to go next

The projects this number is a step of, the guides that explain the method behind it, and the rest of its trade group.