Paint Calculator

The chain, written out

gallons = (perimeter × height − openings) × coats ÷ coverage, rounded up to whole cans. The worked room is a 12 × 8 ft bedroom, so its perimeter is 2 × (12 + 8) = 40 ft. 40 × 8 = 320 sq ft gross, minus 20 sq ft of openings leaves 300 sq ft. Two coats make the demand 600 sq ft; at 350 sq ft/gal that is 1.71 gallons, so you buy 2 and finish with about 0.29 gallons for touch-ups. Engine-computed at build time, same code as the form.

The one figure in that chain that is not yours is the coverage: 350 sq ft per gallon by default, inside the 250–400 range the form quotes under the field. Where that default and that range come from, what kind of fact each is, and how far the gallon count moves across the range is the subject of the guide Coverage figures and where they come from, which lists every constant the estimators divide by with its source, as-of date, and status. Read it when the coverage figure is the question. The rest of this page explains what this calculator does with the numbers you give it, stage by stage, and how to read what comes back.

What the calculator computes, stage by stage

Every estimator on this site follows the five-stage pipeline documented on the methodology page. The paint tool runs its own geometry stage and then the same shared purchasing layer as every other estimator, with two particulars: its geometry stage carries a coats multiplier that no other tool has, and its waste stage is deliberately set to nothing.

1. Geometry. Perimeter times height is the gross wall area — every wall you listed, treated as one rectangle of that total length. The openings you enter are subtracted from it once to give the paintable area. Then the coats multiply: the quantity the engine hands to the next stage is paintable × coats, the square feet of surface the paint has to cover, the figure the result box shows before the arrow on its coats line. For the bedroom that is 300 × 2 = 600 sq ft. Doors and windows are not modelled as shapes; they are an area you supply, and because they are removed before the multiplication, an opening is removed from every coat, not just the first. In metric mode the metres and square metres you type are converted first — metres ÷ 0.3048 and square metres ÷ 0.09290304, both exact definitions — so the arithmetic downstream is identical in both systems.
2. Coverage. The demand is divided by how many square feet one gallon covers in one coat. The default divisor, 350 sq ft per gallon, is an adjustable default recorded in the site's materials data inside the range the two cited manufacturers publish; type the figure from your own label and the engine divides by that instead. The quotient is the gallons needed — 1.71 for the bedroom — carried at full precision and never rounded here. This is the net quantity, in the methodology page's terms: what the walls need with nothing added.
3. Waste. The shared purchasing layer applies net × (1 + waste ÷ 100) at this stage, and the paint engine passes it a waste percentage of zero. The stage runs and changes nothing, which is why the form has no waste field. The reasoning is in the engine itself: paint is bought in whole cans, so the round-up in the next stage already leaves a margin in the last can, and touch-ups come from that leftover rather than from a percentage added on top of it. If you want an explicit margin, the inputs are yours to pad.
4. Package rounding. The engine's package unit is one gallon, so the package count is ceil(gallons needed ÷ 1): 1.71 rounds up to 2. This is the only rounding in the chain and it always goes up. The leftover is the count minus the need — 0.29 gallons here — reported as information, not as a suggestion to buy less. When the division lands on a whole number within floating-point noise, the engine treats it as an exact fit rather than adding a phantom can; the exact-fit example below shows that case.
5. User-priced cost. If you type a price per gallon, the cost is that price times the rounded can count — never the fractional need — because the rounded count is what you pay for. The site holds no paint prices and suggests none; leave the field blank and the line does not appear.

Why each input matters

Wall perimeter is the total length of the walls you intend to paint, in decimal feet. For a whole room it is 2 × (length + width); for a subset of walls it is the sum of just those lengths, and for a single wall it is that wall's length. The engine never sees the room's shape, only the product of perimeter and height, so any pair with the right product gives the right gross area — which also means walls of two different heights can be totalled as gross square feet first and entered as the perimeter with a height of 1. A tape reading with inches in it is converted to decimal feet before entry: the stairwell example below measures 16 ft 6 in and enters 16.5. The feet & inches calculator does that conversion for any reading.

Wall height is in feet, not inches, and it multiplies everything. It is the input to check first when a gallon count looks wrong, because the two fields beside it are also lengths — the misreading section below shows what a height typed as 96 does to the bedroom.

Doors and windows is a single area in square feet, subtracted once from the gross wall area. Leave it blank and the engine subtracts 0, which pads the estimate by the area of the openings; the FAQ below gives the rough per-opening convention the living-room example uses. The engine refuses an openings area that equals or exceeds the gross wall area, since that describes a wall with nothing left to paint — usually a sign that the perimeter or the height is in the wrong unit.

Coats is a whole number from 1 to 6, and it is a multiplier on the paintable area. A second coat is not a small addition; it is the entire wall again, and the gallon count moves accordingly. The form's default of 2 matches the FAQ's note that most color changes need two.

Coverage is the divisor, in square feet per gallon for one coat, and it is the one field whose starting value comes from a source rather than from you. The default of 350 sits inside the 250–400 range the cited manufacturers publish, with porous or textured surfaces near the bottom and smooth repaints near the top, as the materials data notes. The field accepts anything from 50 to 2,000, a validation bound held in the engine so an unusual label figure can still be typed; that bound is an input guard, not a coverage claim. Because coverage divides, a lower figure raises the gallon count in proportion: the living-room example runs the same walls at the default and at the bottom of the range.

Price per gallon touches only the last line. It never feeds back into the quantity, so you can add or change it after the fact without disturbing the can count.

The units toggle changes what the fields expect and how the result is presented, not what is computed. In metric mode the perimeter and height fields take metres and the openings field square metres; each is converted with the exact factors above before the engine runs, and the result leads in litres — converted from the engine's gallons at 3.785411784 L per US gallon, exact — with the gallon count beside it. The coverage field stays in square feet per gallon in both modes, because it is a per-can figure read off the label rather than a measurement of the room.

The figures this page relies on

• Default coverage, 350 sq ft per gallon — an adjustable default recorded in the site's materials data, chosen to sit inside the ranges published by Behr (interior product pages) and Sherwin-Williams (paint calculator FAQ), with the verification date recorded beside it. It is a starting point, never a fact about your paint; the label figure wins.
• Published range, 250–400 sq ft per gallon — the manufacturer-published range from the same two sources, shown under the coverage field so the figure you type can be checked against it.
• Engine limits: 50–2,000 sq ft per gallon and 6 coats — validation bounds held in the paint engine. They decide what the form will accept, not what paint covers.
• Waste, none — the engine passes zero to the shared waste stage; there is no convention to adjust on this tool, and the leftover in the last can is the only margin.
• Package unit, one gallon — the engine's unit, not a claim about can sizes; the count is the gallons needed rounded up to a whole number.
• Metres per foot, 0.3048; square metres per square foot, 0.09290304; litres per US gallon, 3.785411784 — exact definitions, used only to convert metric entries into the engine's feet and square feet and to present the result back in square metres and litres.
• Inches per foot, 12 — a definition; the engine does not use it, but the misreading below does.

Worked examples

Every figure in this table was produced at build time by the same engine that runs the form above. The inputs are chosen to show different behaviours of the chain: the coats multiplier, the coverage divisor, openings that are a large share of the wall, a tall wall, an exact fit, a priced run, and a room entered in metres and converted exactly before the engine ran, which is what the metric toggle does.

The bedroom at one coat. The form's own room, 40 ft of perimeter at 8 ft with 20 sq ft of openings, has 300 sq ft to paint whatever the coat count. At 1 coat the demand is 300 sq ft, 0.86 gallons, and the purchase is 1 gallon with 0.14 spare. At 2 coats the demand doubles to 600 sq ft and the purchase to 2. Nothing else moved; the multiplier did all of it.

The living room, at two coverages. An 18 × 13 ft room has 62 ft of perimeter; at 9 ft that is 558 sq ft gross. Its 2 doors and 3 windows, taken at the FAQ's rough per-opening convention, total 85 sq ft, leaving 473 sq ft paintable and 946 sq ft of demand over 2 coats. At the default 350 sq ft/gal that is 2.7 gallons, 3 to buy with 0.3 spare. Set the coverage to the bottom of the published range, 250, for a textured wall and the same 946 sq ft of demand becomes 3.78 gallons and 4 to buy. The walls did not change; the divisor did, and the label on your can is what should decide it.

The accent wall. One wall 14 ft long and 9 ft high with nothing to subtract is 126 sq ft. A bold colour at 3 coats makes the demand 378 sq ft and the need 1.08 gallons — a little over one — so the purchase is 2 gallons with 0.92 spare, the largest leftover in the table — a comparison the build of this page checks. A need that just crosses a whole number buys a whole extra can; that is the round-up rule, not a mistake in the measurement.

The hallway. 36 ft of wall at 8 ft is 288 sq ft gross, but 4 doors at the FAQ's rough per-door convention — 80 sq ft in all — take 27.8% of it away, leaving 208 sq ft. A single refresher coat in the same colour needs 0.59 gallons and buys 1. Leaving the openings field blank here would have added the full 80 sq ft back into the demand — in a space that is mostly doorways, the subtraction is worth making.

The stairwell, measured in feet and inches. The tape says 16 ft 6 in, so the height is entered as 16.5 ft. With 30 ft of perimeter that is 495 sq ft gross, 471 sq ft after 24 sq ft of openings, and 942 sq ft of demand over 2 coats: 2.69 gallons, 3 to buy, 0.31 spare. Height is the multiplier that turns a short perimeter into a large order; a tall wall and a long one are the same thing to the engine.

The exact fit. 50 ft at 8 ft minus 50 sq ft is 350 sq ft, and 2 coats make the demand 700 sq ft — exactly 2 gallons at 350 sq ft/gal. The purchase is 2 gallons and the leftover is 0: the round-up adds nothing when nothing is owed, and the engine's exact-fit tolerance keeps binary arithmetic from turning a whole number into a phantom extra can. The build of this page checks that the leftover is zero.

The priced run. 48 ft at 8 ft minus 36 sq ft is 348 sq ft, 696 sq ft over 2 coats, and 1.99 gallons — a hair under two, so the purchase is 2 gallons with only 0.01 spare. With $10.00 typed into the price field as a stand-in for whatever your own store quoted, the cost line reads $20.00 — 2 × $10.00, the can count times the price. That price is an arbitrary placeholder chosen only to show the arithmetic, not a figure to shop by; the site holds no paint prices, and the cost is computed on the rounded count because the rounded count is what you buy.

The metric room. In metric mode the form takes 12 m of perimeter, 2.4 m walls, and 1.9 m² of openings and converts them exactly: the perimeter becomes 39.3701 ft, the height 7.874 ft, the openings 20.4514 sq ft. The engine then runs unchanged and the figures are converted back: 28.8 m² gross, 26.9 m² paintable, 53.8 m² of demand over 2 coats at the same 350 sq ft/gal, and 6.26 L needed — 1.65 gallons. The purchase leads as about 7.57 L with the 2 US gallons it stands for beside it, and about 1.31 L left in the last can. The count is identical in either unit system; only the presentation changed.

How to read the result

The headline line is the purchase decision: Buy N gallons. Beneath it, Paintable wall area shows the chain gross − openings = paintable, so a typo in a dimension is visible before it is multiplied; the next line names the coat count and the coverage figure that were applied and shows demand → gallons needed, the unrounded quotient from stage two; and Left in the last can is the count minus that quotient — the margin the round-up created, shown as information rather than as a suggestion to buy less. Read the area line to check your geometry, the coats line to check the multiplier and the divisor, and the headline to shop. Estimated paint cost appears only when a price was entered and multiplies the count, never the need. In metric mode the same lines read in square metres and litres, with the gallon count kept beside the litres because the engine's package unit is the US gallon. The copy button places a one-line summary on the clipboard: paintable area, coats, the count, and (in imperial mode) the coverage figure.

Common misreadings

Coats multiply — they don't add a little. The most common misjudgment here is mental, not typographical: a second coat is not "a bit more paint", it is the entire wall again. The bedroom's demand goes from 300 sq ft at one coat to 600 at two, and the purchase from 1 gallon to 2. Halve your coverage expectations for bold color changes and you will stop making mid-project store runs.

Height in inches. The height field takes feet. Enter the bedroom's 8 ft wall as 96 — the same wall in inches — and the engine sees a wall 96 ft tall: 3,840 sq ft gross instead of 320, 21.83 gallons needed, and a purchase of 22 gallons instead of 2 — 12 times the real wall, because there are 12 inches to the foot. A gallon count that looks wildly high usually means the height field.

Floor area in the perimeter field. The first field wants a length, not an area. The bedroom's floor is 96 sq ft; type that as the perimeter and the engine multiplies it by the height as if it were 96 ft of wall: 768 sq ft gross, 4.27 gallons needed, and 5 gallons to buy instead of 2. If you already know the wall area rather than the perimeter, enter it as the perimeter with a height of 1 — the product is then the area, which is all the engine uses.

Metric figures in the imperial fields. The metric room above is 12 m by 2.4 m with 1.9 m² of openings. Type those same numbers with the toggle still on Imperial and the engine sees 12 ft of wall 2.4 ft high with 1.9 sq ft of openings: 26.9 sq ft paintable, 0.15 gallons needed, and a purchase of 1 gallon against the correct 2. The error is not a simple factor, because every field shrank at once; if the count looks implausibly small for the room, check the toggle.

Subtracting the openings per coat. The openings field is entered once. The engine removes it from the gross area before the coats multiply, so a door is already absent from every coat; doubling the openings figure for a two-coat job removes it twice and understates the demand.

Reading the leftover as an error, or a two-decimal figure as the one used. The spare in the last can is the round-up rule doing its job, not a measurement to correct; the accent wall shows how large it can be when a need just crosses a whole number. And the two-decimal gallons on the result are display copies — the engine rounded up from the unrounded quotient, which is why an exact fit stays exact.

Where the model stops

The engine models one rectangle of wall — a total length at one height — with one area removed from it, painted a whole number of times at one coverage figure, and bought in whole gallons. It assumes the walls you describe are the walls that get painted. It does not model ceilings, trim, or the doors themselves; each of those is its own area or its own product, and the FAQ's advice is to run the tool once per product. It applies no waste allowance and has no field for one. It uses a single coverage figure for all the surface you entered, so a room whose walls differ in texture or porosity is yours to split into separate runs or to cover with one conservative figure from the label. It does not decide how many coats a colour change needs; the coats field is yours. It counts whole gallons only and does not re-pack the count into any other container size. And, as the note under the form says, gallon counts are purchasing estimates only — surface preparation, lead-safe practices in older homes, and code questions call for qualified professionals. The methodology page lists the simplifications every estimator on the site shares.

Same walls, different finishes

Papering instead of painting? The wallpaper calculator works the identical walls the way paper actually hangs — full-height strips cut to pattern repeats, which no area ratio gets right. And if the walls themselves are still going up, start with the drywall calculator. Outside, the same perimeter-times-height area feeds the siding calculator's squares and pieces. A ceiling's own area is a length-times-width job for the square footage calculator, the feet & inches calculator turns tape readings into decimal feet, and the construction unit converter moves any figure between square feet and square metres or gallons and litres. For the coverage figure and its sources, read Coverage figures and where they come from; for the unit toggle, Metric and imperial in the estimators; for the pipeline every tool shares, How material estimates are calculated; or browse all guides.

Frequently asked questions

All figures compute in your browser and are never sent anywhere. Coverage sources and the full pipeline live on the methodology page.