Paint and Stain Calculator

Paint and Stain Calculator

How much paint, primer or stain a job takes, figured the way painters figure it: the real area after doors and windows come out, multiplied by coats, divided by the coverage printed on the can, then rounded up to purchasable cans, gallons and quarts, or litres. Works for a room (with or without its ceiling), a flat surface such as a fence or deck with one side or two, or an area you already know. Primer is calculated separately, because primer covers differently and skipping that distinction is how jobs run short.

Product and coats
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Disclaimer: This calculator gives estimates for general information only. It is not financial, medical or other professional advice. Coverage varies with the product, the substrate and the applicator; the figure printed on your can governs, and this page’s coverage fields exist so you can use it.

What this calculator does

Nobody runs out of paint because the wall was hard to measure; they run out because the estimate skipped a step. This calculator refuses to skip them: gross area, minus doors and windows at sizes you can correct, times the number of coats, divided by the coverage of the actual product on its actual substrate, plus an allowance, then converted into purchasable cans, because paint is sold in gallons and quarts, not in the 1.72 gallons the arithmetic produces. Primer is a separate line with its own coverage, stain on raw wood is not credited with paint’s coverage, and a fence can be counted on both faces. The result is the number you take to the store, with the leftover for touch-ups stated rather than discovered in five years.

How to use it

  1. Room mode wants three measurements and two counts. Length, width and wall height give the perimeter walls; the ceiling is one tick away. Doors default to 21 square feet (a standard 3 by 7 door) and windows to 15, and both sizes are fields, not constants, because a wall of sliding glass is not two windows. Measure your openings if they are unusual; the subtraction is only as honest as its inputs.
  2. Coverage is the number on your can, not a universal constant. The presets are the typical figures: paint on smooth walls at 350 square feet per gallon per coat, and 250 for rough or textured surfaces, for stain drinking into wood, and for primer. Manufacturers print their own coverage on every can, and where it differs, the can wins: type it in. Second coats usually cover slightly better than first coats over bare substrate; the calculator holds coverage constant, which errs on the safe side.
  3. Coats are multiplied, not wished. Two coats is the honest default for a colour change or bare drywall; one coat can work for a same-colour refresh with a quality paint, and the page will calculate it without arguing. What it will not do is pretend one coat at full coverage equals two: each coat consumes its own paint.
  4. Primer is its own purchase. New drywall, bare wood, stains bleeding through, a violent colour change: primer earns its money, and it covers less per gallon than finish paint, which is why it has its own coverage field and its own buy line. Skipping the primer line in an estimate does not skip it in the store.
  5. Flat mode is for fences, decks and single walls. A fence is two surfaces wearing one name, so the sides selector exists; a deck is one surface plus, in honesty, the edges of boards, which the extra allowance absorbs. Stain on a deck is the classic under-estimate, because deck boards drink from the top and the sides at once: rough-wood coverage and the allowance both matter there.
  6. Buy lines round up; leftovers are stated. 1.72 gallons becomes 1 gallon plus 3 quarts, 1.75 gallons bought, 0.03 left. That leftover line is not trivia: it is your touch-up supply, and knowing it before you choose between a quart and a gallon is worth real money across a house.

How it is calculated

Room wall area = 2 x (length + width) x height, plus length x width if the ceiling is included. Openings = doors x door area + windows x window area. Net area = gross – openings. Any finish or primer quantity = net area x coats / coverage per coat x (1 + extra/100). Purchases round up: in gallons, whole gallons plus quarts (1 quart = 0.25 gallon) to cover the need; in litres, up to the next half litre. Coverage conversions: 350 sq ft per gallon is about 8.6 sq metres per litre; 250 is about 6.1. Labour, when a rate is entered, = net area x rate per square unit.

Worked examples

The standard bedroom: 12 by 10 feet, 8-foot walls: 352 square feet of wall. One door (21) and two windows (30) come out, leaving 301 square feet. Two coats of paint at 350 coverage: 1.72 gallons, bought as 1 gallon plus 3 quarts. One coat of primer at 250 goes underneath: 1.20 gallons, bought as 1 gallon plus 1 quart. Every number in that paragraph is the kind of number people guess at the store; none of them needed guessing.

The fence, both sides: 60 feet long, 6 feet high is 360 square feet a side, 720 both faces. Two coats of stain at 250 coverage: 5.76 gallons, bought as 6 gallons. Fences are where one-side thinking goes to die: the neighbour’s view of your fence is also your fence.

The metric room: 45 square metres to coat, two coats at 8.6 square metres per litre: 10.47 litres, bought as 10.5 litres.

Ceiling included: the same 12 by 10 bedroom with its ceiling ticked and no openings: 472 square feet, one coat at 350: 1.35 gallons. Ceilings are routinely forgotten in estimates and never forgotten by the person lying under one with an empty tray.

Reading your result

Compare the buy line, not the need line, against your budget: stores sell the buy line. If the leftover is large, a colour matched in a smaller can later may beat overbuying now, unless the colour is a store mix that varies batch to batch, in which case buying the full purchase in one batch is the professional move and the leftover is insurance, not waste. And when the primer and finish numbers look close, resist merging them into one product line unless you are genuinely using a paint-and-primer product, whose coverage should then be entered as its own figure: the two coatings behave differently on bare substrate, which is the entire reason they are separate lines.

Limitations

  • Coverage is the softest number in paint. Substrate porosity, texture, application method, colour depth and even temperature move real coverage by a fifth or more. The extra allowance is the calculator’s acknowledgement; the can’s printed figure, adjusted for your surface with the rough-surface preset, is the best available estimate, not a promise.
  • Trim, doors and mouldings are not separate lines. Their area is usually inside the openings subtraction (a door comes out of the wall, then gets painted as a door). Painting trim in a contrasting product is a second, smaller job: run the calculator again with the trim area in “I know the area” mode.
  • Sprayer use drinks more. Spraying wastes a meaningful share to overspray and back-rolling differences; sprayers should raise the extra allowance substantially, commonly to 20 to 30%, rather than trust the roller figures.
  • No structural or safety advice lives here. Lead paint in older homes, mould remediation, and exterior work at height are regulated and hazardous jobs in their own right; a quantity calculator is not the tool for them and does not pretend to be.

Frequently asked questions

How much paint do I need for a 12 by 12 room?

With 8-foot walls, the walls are 384 square feet; take out a door and a window (about 36) for roughly 348 net. Two coats at the standard 350 coverage is almost exactly 2 gallons, which is why that room size lives in paint folklore. Add the ceiling and it climbs by 144 square feet and about 0.8 of a gallon over two coats. This page’s room mode does the same arithmetic with your real openings rather than the folklore’s.

Why does the calculator subtract doors and windows if I paint the doors too?

Because the door is not wall, and it is usually not wall paint either. The subtraction keeps the wall figure honest; door and trim paint is a separate, smaller quantity in a different product (typically a trim enamel), best estimated through the known-area mode. Leaving openings in the wall area while also buying trim paint counts the same square feet twice, once in each product.

Does one coat of paint and primer in one really replace primer?

On previously painted surfaces in good condition, often yes, and the calculator handles it: untick the primer and enter the product’s own coverage. On bare drywall, bare wood, patched areas or over stains, the combined products still underperform a dedicated primer at sealing and adhesion, which is what primer is for. The page keeps the two lines separate so the choice is yours, made with both numbers in front of you.

How far does a gallon of stain go on a deck?

Plan on about 250 square feet per gallon per coat on raw or weathered wood, less on very dry, checked boards; the stain preset uses exactly that. Decks also get walked on between coats and weathered unevenly, so the second coat often covers a little better than the first. Two coats on a 720-square-foot two-sided fence came to 5.76 gallons in the worked example; a 200-square-foot deck at the same figures is 1.6 gallons, bought as a gallon plus 3 quarts.

Related calculators

Method: net area after openings, quantity = net area x coats / coverage, purchases rounded up into gallons and quarts or half litres. Typical coverages pre-filled by product class (paint on smooth walls 350 sq ft per gallon, rough surfaces, stain and primer 250) after the figures manufacturers commonly print; every coverage field is editable to the can in hand. Last reviewed: 13 October 2026.