Paint Calculator
Work out how many gallons of paint a room needs from its perimeter, wall height, and number of coats — with doors and windows subtracted and an optional ceiling so you buy the right amount the first time.
How to Use
- Enter the room perimeter (add up all four wall lengths) and the wall height in feet to set the gross wall area.
- Enter how many doors and windows the room has — each one is deducted at a standard 21 ft² per door and 15 ft² per window.
- Set the number of coats; two coats is normal for good color depth, and bump to three over a strong color change.
- Tick "Include ceiling" if you are painting it too, and adjust coverage (ft²/gal) — 350 is typical, drop to ~250 for porous new drywall.
- Read the gallons needed and check the step-by-step Show Work below to see the math.
Show Work
Notes
About the Paint Calculator
Working on building, materials estimation and site work? The Paint Calculator is a free browser tool that gives you the answer in seconds. Work out how many gallons of paint a room needs from its perimeter, wall height, and number of coats — with doors and windows subtracted and an optional ceiling so you buy the right amount the first time.
How it works
Put each value in its box and read the answer as you go. Because it recalculates live, you can play with the inputs to see how each one moves the result — handy for checking your own working or planning ahead. Everything happens on your device, so it is fast and private.
Want the deeper story? The Knowledge Base explains the ideas behind the tools in more detail.
Frequently Asked Questions
How much area does a gallon of paint cover?
A gallon covers roughly 350 to 400 square feet of smooth, previously-painted wall in one coat. Rough, porous, or brand-new drywall drinks more, so drop the coverage figure to about 250 ft²/gal. The tool defaults to 350 and lets you change it to match your surface and the paint's data sheet.
How many coats do I really need?
Most repaint jobs over a similar color take two coats for an even, durable finish. Going light over dark, covering patches, or painting fresh drywall can need three, while a touch-up in the same color may only need one. Set the coat count to match — the calculator multiplies the gallon estimate by it.
Should I count the ceiling?
Only if you are actually painting it. Leave the box unticked for a walls-only job. When ticked, the tool adds the ceiling area (length × width of the room) to the wall area before figuring gallons, so factor in that ceilings often need a flatter paint than walls.
Why subtract doors and windows?
Openings are wall area you will never paint, so counting them inflates your order. A standard door is about 21 ft² and a window about 15 ft²; entering the counts removes that area so the gallon estimate reflects the surface you are actually rolling. For lots of trim, buy a separate quart of trim paint.
Do I need primer?
Prime bare drywall, patched areas, stains, or big color changes — a coat of primer seals the surface and saves you a coat of more expensive finish paint. A rough rule is one gallon of primer per 400 ft² of new surface. Self-priming paints can skip it on sound, previously-painted walls.
How do I use the Paint Calculator?
Simply type your numbers and read the result, which refreshes the instant you change something. There is nothing to submit and nothing to wait for.
Do I need to install or sign up for anything?
Not at all — it runs in the browser with nothing to install and no account. After it loads once, it even works without an internet connection.
Is my information private?
Yes. Everything happens in your browser. Nothing you type is sent to a server or saved anywhere.
Common Use Cases
Buying paint for one room
Turn room dimensions into a gallon count so you buy enough in one trip without a half-used can left over.
Pricing an interior repaint
Estimate gallons across several rooms to budget a whole-house or rental turnover before picking up a brush.
Painting new drywall
Use the lower coverage figure for porous new board so you order enough primer and finish paint for fresh construction.
Walls plus ceiling
Include the ceiling area to plan a full top-to-bottom refresh and order the right mix of wall and ceiling paint.
Color-change jobs
Bump the coat count to cover a dark-to-light or bold-color change so you do not run short mid-project.
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