Paint Calculator
Figure out how much paint to buy for a room. Enter your room dimensions (or a wall area you already know), the number of doors and windows, and how many coats you're planning, and get a purchasable amount, not just a raw number.
- Volume Needed (raw)
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- What to Buy
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Assumptions
- Coverage assumes 375 square feet per gallon (about 9.2 square metres per litre), the middle of paint manufacturers' typical published range of 350 to 400 square feet per gallon on a smooth, primed surface. The metric figure is a direct unit conversion of the same manufacturer coverage assumption, not a separately sourced metric coverage rate. Textured, porous, or unprimed surfaces cover noticeably less, so budget extra paint for those conditions.
- Door and window deductions use common estimating figures of 20 square feet per door and 15 square feet per window (about 1.9 and 1.4 square metres). These are widely used approximations for a typical interior door and window, not a professional trade standard: the Painting and Decorating Contractors Association's own surface-area standard actually recommends against deducting any opening smaller than 100 square feet, since the labor to cut in around a small opening usually costs more than the paint it would save. That guidance is written for contractor billing; a fixed per-opening deduction better serves the question this calculator answers, which is how much paint to buy.
- Two coats is the realistic default for most jobs, not one: a single coat rarely gives even coverage, especially over a different existing color. Bare drywall, or a noticeably darker color painted over a lighter one, often needs a separate primer coat in addition to two topcoats, which this calculator does not add automatically.
- If you already know your total wall area, entering it directly overrides the room-dimensions calculation, but room length and width are still used to compute ceiling area if you choose to include the ceiling.
- Metric purchase recommendations round up to common retail tin sizes (1, 2.5, 5, and 10 litre), the standard consumer range rather than trade bulk drums. Imperial recommendations round up to quarts, gallons, and five-gallon buckets. Imperial is the default unit system here, matching this site's other US-sourced calculators.
- This calculates one paint product for the walls (and ceiling, if included). It doesn't separately account for trim or a different ceiling paint formulation, which are typically bought and calculated separately.
Sources
Why coverage rates vary so much
The 350 to 400 square feet per gallon figure on a paint can is measured on a smooth, primed, uniform surface under controlled conditions, which is closer to a best case than an average. Textured walls (like orange peel or knockdown), porous surfaces like bare drywall or masonry, and unprimed surfaces all soak up more paint per square foot, sometimes dropping real coverage well below 300 square feet per gallon. Darker colors and more dramatic color changes also tend to need a heavier coat, or an extra one, to look even.
Doors, windows, and why estimates differ
Consumer paint calculators almost universally subtract a fixed allowance per door and window (commonly around 20 and 15 square feet) to avoid buying paint for space that will not be painted. Professional painting contractors often do the opposite for small openings: the official PDCA measurement standard says openings under 100 square feet should not be deducted at all, because the extra labor to carefully cut in paint around a door or window frame usually costs the contractor more than the few square feet of paint it saves. Both conventions are legitimate, they just answer different questions: this calculator deducts openings because its job is estimating how much paint to buy, not how to price labor.
Worked example
A 12 ft x 10 ft room, 8 ft ceilings, 1 door, 2 windows, 2 coats, ceiling not included.
Gross wall area = 2 x (12 + 10) x 8 = 352 square feet. Deductions = (1 x 20) + (2 x 15) = 50 square feet, leaving 302 square feet net. At 2 coats, that's 604 square feet to paint, or 604 / 375 = 1.61 gallons raw. Rounded up to a purchasable size, that's 2 gallons.
Frequently asked questions
How do I calculate paint needed from square footage alone?
Enter your known total wall area in the optional override field and it replaces the room-dimensions calculation entirely. Room length and width still matter if you also want ceiling paint included, since ceiling area comes from those two numbers regardless of which wall area you use.
Does this account for doors and windows on the wall?
Yes. It subtracts a fixed allowance for each door and window you enter (20 square feet and 15 square feet respectively, or about 1.9 and 1.4 square metres in metric) from the gross wall area before applying coats, so you aren't buying paint for space that will not actually be painted.
Why does this ask for the number of coats?
Coats multiply directly against the area to paint, since each coat uses roughly the same amount of paint as the first. Two coats is the default because a single coat rarely gives fully even coverage, especially over an existing different color; if you're covering bare drywall or a much darker color, budget for a primer coat on top of that.
Can I use this to calculate ceiling paint too?
Switch Include Ceiling to Yes and the ceiling's area (room length times width) gets added to the total before applying coverage and coats. Ceiling paint often has a different formulation and sometimes a different coverage rate than wall paint, but this calculator treats them the same for simplicity, so treat the result as a reasonable estimate rather than a precise ceiling-specific figure.
How accurate is the amount of paint this recommends?
The raw volume figure depends on a coverage assumption (375 square feet per gallon, or about 9.2 square metres per litre) that can vary meaningfully with surface texture and porosity, so treat it as a planning estimate rather than an exact number. The purchase recommendation deliberately rounds up to the next practical size so you're more likely to finish the job without a second trip to the store, rather than buying the bare minimum and running short.
What if my room isn't a simple rectangle?
Measure or estimate the total wall area yourself (adding up each wall, or each section of an irregular room) and enter it in the optional wall area override field. That skips the length-times-width-times-height calculation entirely and uses your own total instead.
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