Paint calculator: how many gallons your room actually needs

A paint estimate is two problems wearing one coat. The first is geometry: how much surface is there? That part is exact arithmetic and you can trust it. The second is coverage: how far does a gallon actually go over that surface? That number comes from a manufacturer's test on smooth, primed, mid-tone drywall, and it is the part that goes wrong on real walls.

This calculator handles the geometry properly and then makes the coverage assumption visible instead of hiding it. It measures the walls from the room perimeter, subtracts standard allowances for doors and windows, adds the ceiling if you are painting it, multiplies by the number of coats, and divides by the coverage rate you enter.

The headline figure is rounded up to the nearest quarter gallon, because paint is sold in gallons and quarts. The exact unrounded figure is shown too, so you can see how close the job sits to the next tin.

#The formula

Formula: gallons = ((perimeter x height) - (doors x 21) - (windows x 15) + ceiling area) x coats / coverage, where perimeter = 2 x (length + width) and ceiling area = length x width when the ceiling is included.

Every symbol:

• length, width -- the floor dimensions of the room in feet.
• height -- floor to ceiling, in feet.
• perimeter x height -- the gross wall area, all four walls treated as one continuous band.
• 21 sq ft per door -- a standard 30 in by 80 in interior door slab is 16.7 sq ft; 21 sq ft is the conventional allowance that covers the opening plus the casing you will not roll.
• 15 sq ft per window -- the conventional allowance for a typical double-hung unit and its trim.
• coats -- how many full passes of finish paint go on every wall.
• coverage -- square feet one gallon covers in a single coat, taken from the can.

The gallon figure is then rounded up: ceiling(gallons x 4) / 4. The cost line multiplies that rounded figure by the price you enter, because you buy whole containers, not exact volumes.

#A worked example, done by hand

Take a 14 ft by 12 ft bedroom with an 8 ft ceiling, one door, two windows, two coats on the walls only, 350 sq ft per gallon on the can, and a price of $42 a gallon used purely as an example figure.

1. Perimeter: 2 x (14 + 12) = 52 ft.
2. Gross wall area: 52 x 8 = 416 sq ft.
3. Deductions: (1 x 21) + (2 x 15) = 21 + 30 = 51 sq ft.
4. Paintable wall area: 416 - 51 = 365 sq ft.
5. Ceiling not painted, so nothing is added.
6. Total area across two coats: 365 x 2 = 730 sq ft.
7. Exact gallons: 730 / 350 = 2.086.
8. Rounded up to the nearest quarter: 2.25 gallons.
9. Cost at the assumed $42 a gallon: 2.25 x 42 = $94.50.

In practice that is bought as two gallons plus a quart, or as three gallons if you want the leftover for touch-ups. Add the ceiling and the same room becomes (365 + 168) x 2 = 1,066 sq ft, or 3.05 gallons exact, 3.25 rounded.

#Why perimeter times height beats measuring four walls

Measuring each wall separately and adding the four results gives the same answer when the room is a rectangle, but it invites two errors. You have to write down four numbers instead of two, and you have to remember which wall you already did. On a job with several rooms, that is where transcription mistakes creep in.

The perimeter method also degrades gracefully. If a room has a bay, an alcove or a chimney breast, walk the baseboard with a tape, total the run as one length, and multiply once by the height. The wall band does not care how many corners it has, only how long it is.

The method breaks down in exactly one place: sloped or vaulted ceilings, where "height" is not a single number. For those, split the wall into a rectangle up to the lowest ceiling point and a triangle above it, and add half the base times the rise. The same decompose-into-shapes habit is covered in more depth on the room area and volume calculator.

#Where each input comes from

Length and width. Measure at floor level along the baseboard, not at eye height where furniture and your own posture push the tape. Take each dimension twice, at opposite ends of the room, and use the larger reading. Rooms are rarely square.

Height. Measure from the finished floor to the ceiling, at a corner. Wall hidden behind fitted shelving can be subtracted, but the surplus simply becomes touch-up paint.

Doors and windows. Count the openings, not the leaves. A double door is one opening but roughly twice the area, so enter it as two doors. A patio slider or a picture window is far larger than 15 sq ft; measure it and convert the extra area into equivalent window counts, or simply drop the standard deduction and lower your coverage input to compensate.

Coverage per gallon. This is on the can, usually as a range such as 250 to 400 sq ft. The upper end assumes an ideal surface. Enter the lower end unless you know the wall is smooth, primed and close in color to the new paint.

Price per gallon. Use the shelf price for the specific product and sheen you intend to buy, including any color-mixing charge. Prices vary by region, retailer and finish, so this has to be your own number.

The answer is most sensitive to coverage and coats, in that order. Both scale the whole result. Getting a door count wrong changes the area by 21 sq ft, which is under 3 percent of a typical bedroom; assuming 400 sq ft per gallon when the wall delivers 275 changes it by nearly half.

#What actually reduces coverage

The figure on the can is a best case: one coat, smooth substrate, roller, moderate color, ideal spread rate. Each of these conditions pulls the real number down.

The practical translation: for a repaint over an existing, similar-colored latex wall in good condition, using the can's stated coverage is reasonable. For anything else, drop the coverage input by 15 to 30 percent before you read the answer.

#How many coats the job really needs

Two coats is the working default for a color change and for any wall you want to look uniform in raking light. One coat is defensible only when you are repainting the same color on a sound surface.

Three coats is usually a sign that something upstream was skipped. Going light over a saturated red or navy, or covering nicotine or water stains, is a priming problem. A tinted primer plus two finish coats almost always uses less finish paint than three finish coats, and gives a more even result.

Deep bases -- the ones used for strong colors -- contain less white pigment and hide less per coat by design. If the color card shows the color mixed into a deep or accent base, plan on two finish coats over a primer tinted toward the final color.

#Primer is a separate calculation

Primer is not paint and should not be folded into the gallon total. Run the same area figure through the calculator with coats = 1 and the primer's own coverage rate, which is often higher than the finish paint's because primers are formulated to seal rather than hide.

Primer earns its place on bare drywall or plaster, on patched walls where repairs would otherwise flash, and on severe color changes. Over a sound painted wall going to a similar color, it is optional.

The arithmetic favors priming whenever it removes a finish coat. One gallon of primer that turns a three-coat job into a two-coat job pays for itself on any room bigger than a closet, and finish paint is generally the more expensive of the two.

#Ceilings, trim and closets

Switch the ceiling on and the calculator adds length x width to the wall area. Ceilings are their own job: flat white ceiling paint, more coats over stains, and a coverage rate that drops sharply on textured surfaces.

Trim is not in this model. Baseboards, casings, crown and doors are estimated by linear foot, not by area. A quart of trim enamel roughly covers the baseboard and casings of an average bedroom.

Closets are easy to forget and surprisingly hungry. A 3 ft by 6 ft closet with an 8 ft ceiling adds 2 x (3 + 6) x 8 = 144 sq ft of wall -- nearly 40 percent of the bedroom it opens off. Treat each closet as its own small room and add the results.

#How to read the result and what to buy

The exact gallon figure tells you how the job sits relative to the next container. At 2.09 gallons you are barely into the third gallon and a quart will finish it. At 2.85 the third gallon will be opened and largely used.

Notice how slowly the paint requirement grows with floor area. The 20 x 16 room has 2.7 times the floor of the 10 x 10 but needs only about 1.7 times the wall paint, because wall area scales with perimeter and perimeter scales with the sum of the sides, not their product.

Buying one container more than the strict minimum is cheap insurance. Paint is mixed in batches, and a can tinted weeks later can differ slightly. Boxing -- pouring all your cans into one bucket and stirring -- removes that risk entirely, but only if you bought the cans together. If you are weighing the cost of leftover paint against a return trip, the same logic appears on the cost per use calculator.

#What this model leaves out

• Room shape. It assumes a rectangle. Bays, alcoves, angled walls and stairwells need to be measured and added by hand.
• Sloped ceilings. No allowance for gables or vaults.
• Surface condition. No adjustment for texture, porosity or the old color; you express all of that through the coverage input.
• Waste. Nothing is added for what stays in the tray, the roller, the brush and the can. That is easily a tenth of a quart per session.
• Trim, doors and radiators. Estimated separately, by length.
• Primer and sealer. A separate run of the same arithmetic.
• Sundries. Tape, caulk, filler, roller covers and drop cloths are often a third of the spend on a small room.
• Labor. If you are comparing doing it yourself against paying someone, put a value on your own hours first -- the real hourly wage calculator is one way to set that number.

#Common mistakes

Using the top of the coverage range. The can says 250 to 400. People enter 400. Real walls rarely deliver it.

Forgetting the second coat. The single most common cause of running out mid-job. The area doubles, not the room.

Deducting every opening. Cutting in around a window frame uses paint even though the glass does not. The 15 and 21 sq ft allowances are already generous; deducting a further margin for the trim double-counts.

Measuring the ceiling height at the door. Floors slope and ceilings sag. Measure at a corner, and if the readings differ by more than a couple of inches, use the larger.

Buying paint before measuring the closets. They are wall area too, and they are often the difference between two gallons and three.

For the floor, volume and flooring side of the same room, the room area calculator runs the companion arithmetic, and the full set of tools is listed on the calculators index.