There’s a moment every painter knows well — standing in the paint aisle, staring at the shelf, quietly guessing. One gallon? Two? Better grab three just in case? It’s one of those deceptively simple questions that turns out to be surprisingly easy to get wrong.
Buy too little and you’re making a second trip mid-project, hoping the store can match your batch. Buy too much and you’ve got a half-used gallon taking up shelf space in your garage for the next five years.
The good news: it’s a simple math problem once you have the right inputs. Use the calculator below to get an accurate number for your specific room — then use the cost estimator to know exactly what to budget before you walk into the store.
How to Use the Paint Coverage Calculator
Enter each wall’s length and height, then subtract any doors and windows. The calculator handles the math from there — including adjusting for multiple coats if you’re doing two.
A few things to have handy before you start:
- The length and height of each wall (in feet)
- How many standard doors and windows the room has
- Whether you’re planning one coat or two
Once you hit calculate, you’ll get your gallon number plus a full area breakdown so you can see exactly how the result was reached.
Paint Coverage Calculator
Enter your wall dimensions to find out how many gallons you need.
Walls
Standard door = 20 sq ft. Standard window = 15 sq ft. Coverage rate varies by paint brand and surface texture.
How to Use the Paint Cost Estimator
If you used the paint calculator above, your gallon number has already been filled in automatically — you just need to select your paint quality and hit calculate.
If you’re starting here directly, enter the number of gallons you need, then choose a quality tier:
- Budget (~$30–$40/gal) — store brands and basic interior paints. Fine for low-traffic areas or rental properties.
- Mid-Range (~$50–$65/gal) — the sweet spot for most homeowners. Good coverage, durable finish, widely available from brands like Behr and Sherwin-Williams.
- Premium (~$70–$90/gal) — top-tier paints with better pigment, coverage, and washability. Worth it for high-traffic rooms or if you want to get away with one coat.
Toggle on Include primer cost if you’re also buying primer — it’ll add a separate estimate based on the same number of gallons.
The result shows a cost range rather than a single number because paint prices vary by retailer, region, and current promotions. Use it as a realistic budget target, not an exact figure.
Paint Cost Estimator
Enter the number of gallons you need, select your paint quality, and get an estimated cost.
Paint Quality
Paint prices vary by retailer and region. Estimates are based on typical US retail pricing for interior wall paint.
Understanding Paint Coverage Rates
The calculator defaults to 350 square feet per gallon — which is the standard coverage rate you’ll see on most interior paint cans. But coverage rate isn’t a fixed number, and it’s worth understanding what affects it.
Surface texture matters most. A smooth, previously painted wall will drink up far less paint than bare drywall, a heavily textured surface, or brick. On rough or porous surfaces, your actual coverage might land closer to 250–300 sq ft per gallon, meaning you’ll need more than the calculator suggests.
Paint quality plays a role too. Better quality paints tend to have higher pigment concentration, which means better coverage per coat. A premium paint at $70 a gallon might genuinely cover more than a budget option at $30 — and require fewer coats to get a solid result.
If you’re painting over a dramatically different color — say, covering a deep navy with a soft white — drop the coverage rate in the calculator to around 300 and add a second coat. You’ll get a more realistic number.
Smart Tips for Buying Paint
Once you have your gallon number, here’s how to translate that into what to actually put in your cart.
Always round up, never down. If the calculator says 1.6 gallons, buy 2. Running out of paint with one wall left is genuinely one of the most frustrating things that can happen on a painting day.
Buy an extra quart for touch-ups. Even after the project is done, you’ll want a small amount on hand for scuffs, chips, and future touch-ups. A quart is usually enough and costs a fraction of a full gallon. Ask the store to mix a quart in the same formula when you place your order.
Keep your batch consistent. If you need 3 gallons, buy all 3 from the same batch if possible — or ask the store to box them together. Color can vary slightly between batches, and on large walls that difference can be visible.
Don’t skip primer if the situation calls for it. Primer isn’t just for bare drywall. If you’re making a dramatic color change, painting over a stain, or working on a surface that’s never been painted, a coat of primer will save you paint and get you a better result. The cost estimator above includes an option to factor in primer if you need it.
Check the return policy. Most major paint retailers will accept unopened cans for return or exchange. If you’re unsure whether to buy 2 or 3 gallons, buy 3 — you can always return the unopened one.