Paint Calculator: How Much Paint Do You Need?
Enter your wall measurements below and this calculator will tell you exactly how much paint and primer to buy — no guesswork, no wasted gallons.
gallons of paint needed (rounded up to the nearest quart)
Your Paint Math
Why Getting the Right Amount of Paint Matters
Buying paint is one of those small decisions that quietly shapes how a room turns out — and how much you spend. Too little paint and you end up mid-wall with a dry roller, rushing back to the store for one more can that never quite matches the first batch. Too much and you are left with half-empty gallons that sit in the garage for years before being thrown away. A little measuring up front takes ten minutes and can easily save you thirty dollars or more on a single room.
The most common mistake beginners make is estimating paint by the room instead of by the wall. A 12-by-12 foot bedroom sounds small, but its four walls add up to far more surface than you would guess — roughly 430 square feet once you subtract the door and windows. That single number, your paintable square footage, is the foundation of every good estimate.
The second mistake is skipping the second coat. One coat of paint almost never looks even, especially if you are painting over a darker color or a wall that has patched drywall, spackle repairs, or primer spots. Two thin coats will always look smoother and more professional than one thick coat, and they resist scuffs and marks far better over time. That is why this calculator defaults to two coats: it reflects how professionals actually paint.
Primer is the part people are most tempted to skip, and the part they most often regret skipping. If your walls are bare drywall, a dramatic color change is planned, or you have stains and patched areas, one coat of primer makes the topcoat adhere better and cover more evenly. In many cases primer means the difference between needing two finish coats and three. The estimate above includes a separate primer figure so you can plan both purchases in one trip.
Finally, remember that the gallon figures are rounded up to the nearest quart. Paint is sold in fixed sizes, so a room that technically needs 2.1 gallons still requires you to buy 2.25 gallons (two gallons plus a quart). Buying slightly more than the bare minimum is also smart insurance — a quart of leftover paint in the exact color is what lets you touch up dings and nail holes a year from now without repainting the whole wall.
How to Use This Calculator
- Select your number of walls. Choose 4 for a full room, or 1 if you are painting a single accent wall. Input fields appear for each wall.
- Measure each wall. Measure the length and the height in feet and enter both. For walls with sloped ceilings, use the average height.
- Count your doors and windows. Enter the counts and the calculator subtracts 21 sq ft per door and 15 sq ft per window. If your openings are unusual, edit the total subtractable area directly.
- Choose your number of coats. Two coats is the professional standard; choose one only if you are refreshing the same color on walls in good condition.
- Click Calculate. You will see your total wall area, paintable area, gallons of paint needed, and a separate primer estimate — with every step of the math shown.
Frequently Asked Questions
How much does one gallon of paint cover?
A standard gallon covers about 350 square feet with one coat on a smooth, previously painted surface. Rough or porous surfaces like bare drywall or textured walls absorb more paint, so real-world coverage can drop to 300 square feet or less. This calculator uses the standard 350 sq ft figure, and rounding up to the nearest quart gives you a comfortable buffer for textured walls.
Should I subtract the doors and windows or just paint over their area?
Always subtract them. A typical bedroom door and two windows remove around 50 square feet of paintable surface — enough to change your purchase by half a gallon in a smaller room. Skipping this step is the most common reason people overbuy.
Do I really need two coats?
In most cases, yes. Two coats even out roller marks, hide the old color completely, and give you the true color shown on the paint chip. The exception is repainting with the same or a very similar color on walls in good shape, where one careful coat can suffice.
How do I calculate paint for a ceiling?
Ceilings are simple: multiply the room’s length by its width to get the square footage, then divide by 350 for gallons. Most ceilings take one coat of flat white ceiling paint. Run this calculator with the room’s length and width as a single “wall” to get the figure quickly.
What should I do with leftover paint?
Keep it. Pour a small amount into a clearly labeled jar (include the color name, code, and date), seal the original can tightly, and store it upright in a cool, dry place away from freezing temperatures. Properly stored latex paint stays usable for up to 10 years and is perfect for touch-ups.
Ready to Paint?
Now that you know exactly how much paint to buy, you can plan the fun part: choosing colors, finishes, and accent walls. Explore the paint guides and room makeover ideas on the HomeDecor blog for help picking the perfect shade for every space in your home.