Paint Calculator

What Is Paint Coverage?

Coverage is the area one gallon covers in one coat, printed on the back of the can as a figure between 350 and 400 square feet for most interior wall paint. It is a laboratory number measured on a smooth, sealed, primed surface at the film thickness the maker intends, which makes it a promise about film thickness rather than a guarantee about your wall.

Where 350 to 400 Square Feet per Gallon Comes From

A US gallon is 231 cubic inches, which NIST's SI guide lists as 3.785 412 liters. Spread that gallon one mil thick — a thousandth of an inch — and it covers 231,000 square inches, or 1,604 square feet. That constant is the hard ceiling on any coverage claim, and it belongs to a paint that is 100 percent solid with nothing to evaporate.

1 gallon = 231 in³ = 3.785 412 L
spread 1 mil thick:  231 ÷ 0.001 = 231,000 in² = 1,604 ft²

coverage (ft²/gal) = 1,604 × volume solids ÷ dry film thickness (mils)

Real paint is mostly liquid that leaves. Work the label backwards and it describes a film: 400 square feet per gallon is a wet coat 4.0 mils thick, and if that dries to the 1.5 mils a finish coat is usually specified at, the paint has to be about 37 percent solids by volume. A 350 rating implies roughly 33 percent at the same dry thickness. Volume solids is a measured property with its own ASTM test method, D2697, so the two settings on this calculator's coverage control are not marketing language — they bracket the range interior wall paints are labeled in.

Which end to pick is a judgment about the wall, not about the can. Smooth, sealed, previously painted drywall earns the 400 setting. Texture, plaster, sand finish, fresh joint compound and anything porous take the 350, because the same volume of paint has more surface — and more thirst — to deal with.

What the Calculator Counts, and What It Leaves Out

The result covers wall area, and ceiling area only when Surfaces is switched to Walls + ceiling. Trim, baseboards, crown molding, window casings, door slabs and the insides of closets sit outside the number entirely. In practice that is the right split, because those surfaces are normally a different product in a different sheen, bought by the quart rather than the gallon.

Primer sits outside it too. Primer carries its own coverage rating and deserves its own trip through the arithmetic: run the room once with Coats set to 1 to size the primer, then again at your real coat count for the finish paint. On bare drywall that first sealing coat is the one that disappears fastest, and neither coverage setting here is a promise about how much it will drink.

Why a Door Is 21 Square Feet and a Window Is 15

Both allowances are plain rectangles at the trade's standard sizes. A 3-foot by 7-foot door opening is exactly 21 square feet; a 3-foot by 5-foot window is exactly 15. The calculator multiplies your counts by those figures and takes the total off the wall area.

Your doors are probably smaller. A 30 by 80 inch interior slab is 16.7 square feet, so the tool over-deducts about 4.3 square feet for each one — which at two coats and 400 square feet per gallon is 0.02 gallons, under three fluid ounces. The allowance is deliberately generous because you do not roll the casing around the opening either.

Windows are where a count can genuinely mislead, because the field counts units rather than area. A 6 by 5 foot picture window is 30 square feet and should be entered as two windows; a small awning window over a bath is arguably none. When the glazing is unusual, measure the area you will not be painting and divide it by 15.

Measuring a room that is not a simple rectangle? Break the floor into rectangles first with the Square Footage Calculator.

How Do You Calculate Paint for a Room?

Four steps in a fixed order: perimeter to wall area, subtract the openings, add the ceiling if it is included, then multiply by coats and divide by coverage. The order matters in exactly one place — the ceiling is added after the door and window allowances come out, so an opening can never eat into ceiling area.

The Paint Formula, Written Out

This is what the code runs, in order:

area = 2 × (length + width) × height
       − doors × 21 ft²
       − windows × 15 ft²

Surfaces = Walls + ceiling:  area = area + length × width

gallons = area × coats ÷ coverage

buy line:  whole gallons = floor(gallons)
           quarts       = ceil(remainder × 4)
           4 quarts print as one more gallon

liters = gallons × 3.785

Two guards sit around that arithmetic. Any of length, width or height that is blank, zero, negative or not a number stops the calculation before anything is multiplied. And if the door and window allowances come to more than the wall area — a walk-in closet entered with three doors, say — the tool refuses rather than returning a negative area.

The liter line converts at 3.785 liters per gallon. NIST's exact factor is 3.785 412, so the printed liter figure occasionally lands a tenth low: 1.07 gallons prints as 4.0 liters where the exact conversion is 4.05. It is a display rounding, not an error in the gallons.

Step by Step

• Measure length and width along the floor at the widest points, in feet, and take wall height from the top of the baseboard to the ceiling line. Decimals are fine: type 8.5 for an 8-foot-6 wall.
• Count the doors. The box arrives holding 1 and reads digits only, so type 2 rather than the word two.
• Count the windows as units of roughly 15 square feet each. The box arrives holding 2.
• Choose the coats. Two is the default and the honest answer for a color change, bare board, a sheen change or any wall that has been patched; one coat is for refreshing a sound surface in the same color.
• Choose the coverage. 400 for smooth, sealed, previously painted drywall; 350 for texture, plaster, porous or thirsty surfaces.
• Set Surfaces. Walls only is the default; Walls + ceiling adds the floor footprint to the area.
• Press Calculate. The three measurement boxes start empty, so nothing runs when the page loads and nothing recalculates while you type — the panel updates when the button is pressed. Reset returns it to the empty dash.

Doors, windows, coats, coverage and surfaces all arrive pre-filled, so the only work between an empty page and an answer is three measurements and one press of the button.

Worked Example: A 12 × 12 Bedroom with 8-Foot Walls

Length 12, width 12, height 8, one door, two windows, two coats, 400 square feet per gallon, Walls only. Press Calculate and the panel reads:

• 1.67 gallons
• Paintable area: 333 sq ft (walls) − 1 door (21 sq ft each) − 2 windows (15 sq ft each)
• 2 coats at 400 sq ft per gallon
• Buy: 1 gallon + 3 quarts (6.3 liters)

Behind those four lines: the perimeter is 2 × (12 + 12) = 48 feet, and 48 feet of wall 8 feet high is 384 square feet. One door at 21 and two windows at 15 take off 51 of it, leaving 333. Two coats means 666 square feet of paint actually applied, and 666 divided by 400 is 1.665 gallons, which the readout rounds to 1.67.

Read the second line carefully, because it is easy to double-count. The 333 is already net — the door and window clauses that follow it name what has been taken out, not what is still to come off.

Adding the Ceiling

Switch Surfaces to Walls + ceiling and the same bedroom reports 2.38 gallons over 477 square feet. The extra 144 square feet is the 12 × 12 ceiling, and two coats of it at 400 square feet per gallon is 0.72 gallons.

Subtract the two figures on screen, though, and you get 0.71. Both are right: the panel rounds 1.665 and 2.385 separately before either reaches you, and a difference taken between two rounded readouts inherits both roundings. Take differences from the underlying arithmetic instead — here, from the 144 square feet of ceiling.

Whether to switch it on is a paint question rather than an arithmetic one. Ceilings normally take flat white while walls take an eggshell or satin color, so the two areas usually belong to two different cans. When that is the case, leave Surfaces on Walls only for the color and size the ceiling from the add-on chart below.

Paint Chart: Gallons by Room Size

Every figure in the first three tables below came out of this calculator rather than a rule of thumb, so a row and a run of the tool agree to the last digit. The fourth isolates the ceiling, which no setting on the form does on its own, so its gallons are worked as area × coats ÷ coverage — the tool's own division applied to the footprint alone.

Rooms at Eight-Foot Walls

One door, two windows, two coats, 400 square feet per gallon, walls only:

Rooms of noticeably different size can share a buy line — the 10 × 12 and the 12 × 12 both come out at one gallon plus three quarts — because that line rounds up to the next whole quart. The gallon column is where the real difference lives, 1.50 against 1.67, and it is the column to price.

Taller Walls Move the Number Faster Than Floor Area

The same 12 × 12 room with one door and two windows, two coats at 400, with only the wall height changed:

Two extra feet of height on that room adds 96 square feet of wall; two extra feet of width adds 32. Height multiplies the entire perimeter, which is why a stairwell or a room with a raised ceiling consumes paint out of all proportion to its footprint — and why the height box is the one worth measuring rather than assuming.

The Same Room at Every Setting

A 12 × 12 room with 8-foot walls, one door and two windows, run at each combination of the three controls:

One unchanged room spans 0.83 to 2.73 gallons across that grid, better than three to one, decided by three controls that take a second each. Coats is the biggest lever because it doubles the number outright; coverage moves it by about an eighth; the ceiling adds whatever the footprint happens to be.

Ceiling Add-On by Room Size

Ceiling area is simply length × width, so the extra paint is the same division at your coats and coverage. Two coats:

Two things push a ceiling toward the 350 column. It is often flat paint over sprayed texture, whose true surface area is larger than its footprint, and it is rolled overhead, where the film tends to go on thinner than on a wall you can lean into.

How to Read Your Result

A successful run returns four lines, always the same four, in this order:

• The headline — exact gallons to two decimals. This is the figure to price and the figure to compare against a contractor's estimate; it is not what you can buy.
• Paintable area, with the surfaces it covers in brackets and the allowances that were applied. The square footage shown is already net of them.
• The settings line, repeating the coats and coverage the answer was built on, so a screenshot still explains itself a week later.
• The buy line — whole gallons and quarts, with the liter equivalent for anyone shopping outside the US.

The Buy Line, and the Paint You Have Left

That last line never rounds down. It keeps the whole gallons, rounds the remainder up to the next whole quart, and prints four quarts as one more gallon instead.

For the 12 × 12 bedroom that means carrying home 1.75 gallons against a need of 1.665 — 10.9 fluid ounces of margin, about a third of a quart. Treat it as a reserve rather than waste. Paint left in the tray, in the roller cover and on the brush never reaches a wall, and whatever survives the day becomes the touch-up can.

Two habits make that reserve worth having. Have every can of one color mixed in the same visit and box them — pour them together into one bucket and stir — because tint varies slightly between batches and a boxed batch cannot band across a wall. Then label the leftover with the room and the date: the color name on the lid will not tell you the sheen or which wall it went on.

Turning Gallons Into a Cost

There is no price field on this calculator, deliberately. Paint prices move, and a figure baked into a web page would be wrong within a season. Price it against your own store, and price the buy line rather than the headline — nobody sells 1.67 gallons.

So multiply your price by 1.75 gallons, not by 1.665. Before buying the fourth quart of anything, check what it costs against a gallon of the same product: four quarts and a gallon hold the same paint, and the shelf rarely prices them the same way. Trim and doors are a separate line in a separate sheen, and they are not inside this number at all.

One coat of an expensive paint can beat two coats of a cheap one, and this tool prices both sides of that argument: the same bedroom needs 0.83 gallons at one coat and 1.67 at two. If the premium paint costs less than twice the budget one per gallon, and genuinely covers your wall in a single pass, it wins on materials before you count the day of labor you did not spend.

When the Panel Shows an Amber Notice

Two conditions stop the calculation, each with its own message under a Check your inputs heading:

• “Enter the room's length, width, and height in feet” — one of the three measurements is blank, zero, negative or not a number. Letters typed into a measurement box land here too.
• “Openings exceed the wall area — check the inputs” — the door and window allowances add up to more than the walls. In practice that is a typo in the doors or windows box rather than a real room.

Neither is a browser validation message. The form is submitted without validation and the measurement boxes carry no minimum or maximum, so 500 in all three boxes is accepted and answered — the panel reads 4999.74 gallons, with nothing flagged. Half a foot is caught, but only by the second guard above — at that size the pre-filled door and window allowances swallow the wall. Set both counts to 0 and the same half-foot room answers 0.01 gallons, unremarked.

Limits: When This Calculator Does Not Apply

The arithmetic is exact. What it assumes about your room and about the paint is where an estimate goes wrong, and most of those assumptions are worth knowing before you order:

• Rectangular rooms only. Wall area is built as 2 × (length + width) × height, which describes a box. An L-shaped room, a bay, a chimney breast or an alcove all carry more wall than that.
• A single wall height. Sloped ceilings, stairwells and vaulted rooms have no one number to type.
• No trim, no door slabs, no closet interiors — the figure covers flat wall, plus the ceiling when you ask for it.
• No primer, no texture-filling first coat, and no allowance for a dramatic color change, which often wants a tinted primer underneath or a third coat over the top.
• Coverage is a label figure earned on a smooth sealed test panel. A wall that has been patched, skimmed or left bare drinks the first coat whichever setting you choose.
• Nothing about the work itself: no drying time between coats, and no adjustment for spraying, which puts more paint into the air and onto the drop cloths than a roller does.

Rooms That Are Not Rectangles

There is a workaround, and one trap inside it. Length and width are only ever used to build a perimeter, so any pair of numbers whose sum is half the true wall run produces the correct wall area.

Measure the real distance around the room at the baseboard — say it comes to 56 feet. Half of that is 28, so entering 16 and 12 gives 2 × 28 × height, which is right whatever shape the plan actually is. Doors and windows are counted the same way regardless.

The trap is the ceiling. Walls + ceiling adds length × width, and for a made-up 16 by 12 that is 192 square feet with no relationship to an L-shaped floor. Keep Surfaces on Walls only whenever you are using the perimeter trick, and size the ceiling separately from its real floor area.

The Doors and Windows Boxes Take Digits Only

Both are plain text fields, and the code strips every character that is not a digit before reading them. That has two consequences worth knowing before you trust a result:

• A decimal loses its point. Typing 1.6 does not give one and a half doors — the point is stripped, the calculator reads 16 doors, deducts 336 square feet, and returns a nonsense answer without complaint.
• A word becomes zero. Typing the word two leaves nothing once the digits are extracted, so it counts as no doors at all and nothing on the page says so.

Whole numbers, then. Neither box has a maximum, and the only thing standing between a typo and a wrong answer is the openings-exceed guard, which fires only when the deductions swallow the entire wall.

Homes Built Before 1978

A gallon count says nothing about what is already on the wall. The federal government banned lead-based paint for use in homes in 1978, and the EPA states that approximately three-quarters of US homes built before then still contain some.

Sanding, scraping and heat-stripping those surfaces is what creates the hazard, because the danger is lead dust rather than intact paint. The EPA's Renovation, Repair and Painting Rule requires anyone paid to disturb painted surfaces in pre-1978 homes, childcare facilities and preschools to work for a certified firm with trained renovators. It generally does not reach homeowners painting their own pre-1978 home — but it does apply if you rent out all or part of it, run a childcare center in it, or buy and resell homes for profit.

For your own home the EPA's recommendation is to hire a lead-safe certified contractor, and to follow lead-safe work practices if you do the work yourself. Either way it is a decision that belongs before the paint order, not after the sanding has started.

Painting new board instead? It has to be hung, taped and finished before any of this matters — count the sheets with the Drywall Calculator.