Paint Calculator — litres or gallons + cost

The paint calculator above turns four simple room measurements into a shopping-list number: how many gallons or litres of paint to buy, and what the job will cost before you ever step foot in the hardware store. Guessing at paint quantity is one of the most common ways homeowners overspend on a repaint — either buying two extra gallons "just in case," or running short mid-coat and having to hunt down a matching batch of tint days later.

At Arb Digital we build tools like this one for the same reason we build websites for our clients: good numbers, presented clearly, save people time and money. This paint calculator is free to use as often as you like, on any room, in any unit system.

What This Paint Calculator Does

Painting a room isn't just length times width times height. A real wall area calculation has to account for the doors and windows you won't be painting, the number of coats your color and surface require, a waste allowance for cutting-in, rollers, and spills, and the actual coverage rate printed on your specific can of paint. This calculator combines all five of those variables into one net figure, then converts that figure into gallons or litres and multiplies it by your price per container to show total project cost.

It also separates out the gross wall area (before subtracting anything) from the net paintable area, so you can see exactly how much surface your door and window openings are saving you from having to paint — useful context if you're deciding whether it's worth swapping a solid door for glass panes, or just curious where the numbers come from.

How to Use the Paint Calculator

1. Pick your units. Choose feet & gallons or metres & litres. The labels and default coverage rate update automatically.
2. Enter room length, width, and wall height. Measure wall to wall, not including baseboards.
3. Enter the number of doors and windows in the room, along with their average size (defaults are typical US sizes, but adjust for oversized or French doors).
4. Set the number of coats. Two coats is standard for a color change; one coat may be enough for a same-color refresh over a good primer.
5. Leave the waste allowance at 10% unless you're an experienced painter with minimal spill — first-timers should bump it to 15%.
6. Check the coverage rate against your actual paint can label, and set your price per gallon or litre.
7. Click Calculate to see gallons/litres needed and total cost.

The Formula Behind the Paint Calculator

The core formula is: Wall area = 2 × (length + width) × height, which gives the gross surface area of all four walls. From that gross figure we subtract (doors × door area) and (windows × window area) to get the net paintable area. That net area is then multiplied by the number of coats and by (1 + waste % ÷ 100) to get the total area you actually need to cover. Dividing that number by your paint's coverage rate (square feet per gallon, or square metres per litre) gives the volume of paint required, and multiplying by price per container gives total cost.

Coverage rate matters more than most people realize — it varies by paint brand, sheen, and the porosity of your wall surface. The EPA and most manufacturers recommend checking the actual label on your can rather than relying on a generic average, since flat paints often cover more surface per gallon than glossy enamels, and a heavily textured or unprimed wall will absorb noticeably more paint than a smooth, primed one.

Why Primer Matters (And When to Skip It)

Primer isn't just an extra step to sell more product — it does a job that topcoat paint isn't designed to do. Primer seals porous drywall and plaster so your finish coat doesn't soak in unevenly, blocks stains, water marks, and smoke residue from bleeding through, and gives glossy or slick surfaces (like old oil-based paint) a surface that new paint can actually grip. Skipping primer on a drastic color change — say, painting a dark red wall a pale cream — often means you'll need three or four coats of the new color to fully hide the old one, which costs far more in paint than a single coat of primer would have.

As a rule of thumb: prime new drywall, prime over any stain or water damage, prime when changing from a dark color to a light one, and prime when switching between sheens (like glossy trim going matte). If you're repainting the same color over a wall that's already in good shape, you can usually skip primer and go straight to two coats of your finish paint — which is exactly what the default settings in this calculator assume.

Coverage Rates by Paint Type

Not all paint spreads the same. Flat and matte interior paints typically cover 350–400 square feet per gallon (roughly 8.5–9.8 m² per litre), while satin and eggshell finishes usually land a little lower, around 300–350 sq ft/gallon, because their binders are formulated for washability rather than maximum spread. Semi-gloss and high-gloss enamels, often used on trim and doors, cover even less — sometimes as little as 250–300 sq ft/gallon — since the resins that create the shine are thicker and less absorbent-friendly. Exterior paints and masonry paints applied over rough stucco or brick can drop to 200 sq ft/gallon or lower because the textured surface simply eats more product per square foot of wall.

If you don't know your paint's exact rate, checking the manufacturer's technical data sheet (usually a quick search of the product name plus "spec sheet") will give you a far more accurate number than guessing, and it's worth doing before you calculate quantities for an expensive premium paint.

Ceilings, Trim, and Rooms with Multiple Colors

This calculator focuses on the four walls of a room, which is the calculation most people need. If you're also painting the ceiling, treat it as a separate area (length × width) and run the numbers again with the ceiling's own coverage rate — ceiling paint is often flatter and covers slightly more surface per gallon than wall paint. Trim, baseboards, and doors are best estimated separately too, since they use small quantities of a different sheen and running them through a wall-sized calculation would drastically overestimate what you need. For a multi-color accent wall, calculate each wall's paint separately using its own length and coat count rather than averaging the whole room together.

Common Mistakes to Avoid

• Ignoring the coverage rate on the can. Using a generic "one gallon covers 400 sq ft" assumption when your actual paint covers 300 sq ft leads to running short mid-job.
• Forgetting waste allowance entirely. Rollers, trays, cutting-in with a brush, and touch-ups all use paint that a pure area calculation won't capture.
• Assuming one coat is enough for a big color change. Dark-to-light or light-to-dark transitions almost always need at least two coats, sometimes three without primer.
• Not subtracting door and window area. In a small room with a lot of openings, this can overstate paint needed by 15–20%.
• Buying paint in odd quantities. Retailers sell in whole gallons, litres, or quarts — round up to the next full container, don't try to buy 3.4 gallons.

Related Free Tools From Arb Digital

Planning a bigger renovation? Pair this paint calculator with our Flooring Calculator to budget new floors, our Tile Calculator for a bathroom or kitchen refresh, and our Concrete Calculator if the project includes a patio, walkway, or foundation work. If you need to convert measurements between systems first, try our Area Converter or Length Converter. Browse everything we offer at the free online tools hub.

Paint Sheen Levels Explained: Matte, Eggshell, Satin, and Gloss

Sheen is how much a dried paint surface reflects light, and it matters for both looks and durability, not just style preference. Flat or matte paint reflects the least light and does the best job of hiding wall imperfections like uneven drywall taping or old patchwork, which makes it a favorite for ceilings and low-traffic adult bedrooms, but it's also the hardest sheen to wipe clean without leaving a duller patch where you scrubbed. Eggshell adds a slight sheen — named for its resemblance to the soft glow of an actual eggshell — and strikes a middle ground of being easy enough to spot-clean while still hiding minor wall flaws reasonably well, making it a common default for living rooms and hallways.

Satin has noticeably more shine than eggshell and holds up to scrubbing and moisture better, which is why it's the standard choice for kitchens, bathrooms, kids' rooms, and other high-traffic or high-humidity spaces where walls get touched, splashed, or bumped regularly. Semi-gloss and full gloss sit at the shiniest end of the scale and are almost always reserved for trim, doors, cabinets, and railings rather than whole walls, since their high shine also highlights every surface imperfection underneath — a benefit on smooth millwork, a liability on a textured or patched wall. As a rule of thumb, sheen tends to climb with how much wear and moisture a surface will see: flat for ceilings, eggshell or satin for walls, and semi-gloss or gloss for trim and cabinetry.

Frequently Asked Questions

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