Paint Calculator
A gallon of paint covers roughly 350 square feet, but no room is a perfect rectangle of bare wall. This calculator works the way a professional estimator does: it measures your walls, subtracts the doors and windows, adds the ceiling if you want it painted too, applies your coats, coverage rate and a waste buffer, then tells you exactly how many gallons to buy. It also prices primer, counts quarts for touch-ups, and totals the project cost, with a multiple-rooms mode that prices a whole floor at once and rounds up once instead of once per room.
Optional. Leave blank for DIY quantity only.
Optional. US pros typically charge $2 to $6 per sq ft.
Paint to buy
Buy 0 gallons
Quick Answer
A 12 by 12 ft bedroom with 8 ft ceilings, 1 door and 2 windows needs 3 gallons for two coats of eggshell paint with a 10% buffer: 334 sq ft of wall, 735 sq ft total, 2.26 gallons exact. The calculator above is pre-filled with this exact room, so click Calculate to see every line of the math.
Key takeaways
- Buy the rounded-up gallons, not the exact decimal. Stores sell whole gallons, and the leftover is your touch-up supply.
- Subtract 20 sq ft per door and 15 sq ft per window. Skipping this is the most common reason people overbuy.
- Match the coverage rate to your finish: flat stretches furthest, semi-gloss the least. The can label always wins over any preset.
- Primer is a separate purchase with its own coverage, usually around 300 sq ft per gallon for one coat.
- Quick Answer
- How to use the paint calculator
- How the gallons are worked out
- Worked example: a 12 by 12 ft bedroom
- Why the finish changes the coverage
- Primer: when you need it and what it costs
- How the labor estimate works
- Painting several rooms at once
- What competing calculators miss
- Frequently asked questions
How to use the paint calculator
Stay in Single room mode for one space, or switch to Multiple rooms when you are pricing a floor or a whole house. Results stay hidden until you click Calculate.
- Room shape: pick Rectangular room and enter length, width and ceiling height, or choose “I know my paintable wall area” for L-shaped rooms and open plans you have already measured.
- Doors and windows: count them; the tool subtracts 20 sq ft per door and 15 sq ft per window automatically.
- Ceiling: tick the box only if the ceiling gets painted too. It adds length times width to the total.
- Coats and finish: two coats of eggshell is the pre-filled standard. The finish sets the coverage rate: flat 375, eggshell or satin 325, semi-gloss 275, or your own figure from the can.
- Waste buffer: 10% covers roller absorption, cutting in and touch-ups. Leave the price blank for a quantity-only answer, or enter it to see the paint cost.
- Primer and labor: tick the primer box on bare drywall, stains, or dark-to-light color changes; enter a per-square-foot rate to price professional labor.
How the gallons are worked out
The math follows the same steps a professional estimator uses: measure the walls, subtract what you do not paint, multiply by coats and a buffer, then divide by coverage and round up, because you cannot buy a fraction of a gallon.
Total area = (wall area + ceiling area) × coats × (1 + waste buffer)
Gallons to buy = round up (total area ÷ coverage per gallon)
Two honest details about this tool’s behavior. First, the waste buffer is capped at 50% and negative prices are treated as zero, so stray keystrokes cannot produce a nonsense estimate. Second, in single-room mode the primer and labor lines are priced from the wall area only; if you ticked the ceiling, its paint is counted in the gallons but not in the primer or labor lines, because ceilings usually get a separate flat paint bought apart from the walls. In multiple-rooms mode the primer and labor lines use the combined wall-plus-ceiling area instead. This is how the tool has always behaved, and it is stated here so the numbers never surprise you. Contractors use the same wall-first logic: a standard door is about 20 sq ft and a standard window about 15, and industry coverage figures cluster around 350 to 400 sq ft per gallon per coat, as Blue Door Painters’ coverage guide explains from the contractor side.
Worked example: a 12 by 12 ft bedroom
The pre-filled run is the classic case: 12 by 12 ft, 8 ft ceilings, 1 door, 2 windows, 2 coats of eggshell at 325 sq ft per gallon, 10% buffer. The tool returns 334 sq ft of wall, 735 sq ft total, 2.26 gallons exact, and tells you to buy 3 gallons, plus 10 quarts if you prefer touch-up tins.
| Room (8 ft ceilings, 2 coats eggshell, 1 door, 2 windows, 10% buffer) | Wall area | Exact gallons | Buy |
|---|---|---|---|
| Small bedroom, 10 × 10 ft | 270 sq ft | 1.83 | 2 |
| Bedroom, 12 × 12 ft (worked example) | 334 sq ft | 2.26 | 3 |
| Living room, 14 × 16 ft | 430 sq ft | 2.91 | 3 |
| Large living room, 20 × 20 ft | 590 sq ft | 3.99 | 4 |
| Open plan, 20 × 24 ft | 654 sq ft | 4.43 | 5 |
Every row above was produced by the calculator with the same settings, so you can spot-check any of them. Tick the ceiling box on the 12 by 12 bedroom and the picture changes: 144 sq ft of ceiling joins the total, giving 1,052 sq ft, 3.24 gallons exact, and 4 gallons to buy.
Why the finish changes the coverage
Coverage is not one number. Thicker, higher-sheen films build at different rates, so a gallon of flat paint stretches further than a gallon of semi-gloss on the same wall. That is why this calculator presets a coverage per finish instead of using a single 350 for everything.
The bars are square feet per gallon, per coat, on a smooth wall. Texture, unprimed drywall and brick all drink more paint, so treat any preset as a starting point and type the figure from your own can when you have it. This Old House’s painting guide makes the same point in its quantity steps: measure the walls, double for two coats, divide by the coverage, and let the can override the rule of thumb.
Primer: when you need it and what it costs
You need primer on bare drywall, over stains, and when changing from a dark color to a light one. You can usually skip it on previously painted walls in good condition. One coat is enough under two finish coats, and primer covers less per gallon than finish paint, which is why the calculator defaults its primer coverage to 300 sq ft per gallon and keeps it editable.
One prep note for older homes: if the house was built before 1978, test for lead paint before any sanding or scraping. The EPA’s Renovation, Repair and Painting Rule requires lead-safe certified firms for paid work that disturbs paint in pre-1978 homes, so a pro job on an old house must follow those practices.
For the worked-example bedroom, ticking the primer box adds 2 gallons of primer: 334 sq ft of wall at 300 sq ft per gallon, one coat, rounded up. That matches the real-world range; primer guides generally put one-coat coverage at 200 to 300 sq ft per gallon, lower than finish paint because primer is formulated to seal and bond rather than build color. If your project starts with bare drywall, the drywall labor calculator helps you price the hanging and finishing work that comes before the primer goes on.
How the labor estimate works
Leave the labor rate blank and the tool gives you a DIY materials-only estimate. Enter a per-square-foot rate and it prices your wall area at that rate, then shows a project total of paint plus labor. Professional interior painting in the US typically runs $2 to $6 per square foot including materials, with a single room averaging $400 to $1,600, according to HomeAdvisor’s room-painting cost data. The Bureau of Labor Statistics puts median painter pay at $23.75 per hour in 2025, which is why hourly quotes and per-square-foot quotes can look so different for the same room.
At $2.50 per sq ft on the worked-example bedroom, labor comes to $835.00 for 334 sq ft of wall, and with paint at $45 per gallon the project total is $1,015.00. If you are budgeting a bigger renovation, the bathroom remodel cost per square foot calculator prices the rest of the project on the same per-foot basis.
Painting several rooms at once
Switch to the Multiple rooms tab to price a floor or a whole house in one run. Add a room for each space with its own dimensions, doors, windows and ceiling option; the tool lists every room in a table, totals the paintable area, and rounds up once for the combined total. That single round-up is the quiet saving: rounding up per room would buy you an extra partial gallon for nearly every space.
Primer and labor apply to the combined area in this mode, including any ceilings you ticked. Two example rooms, a 16 by 14 ft living room with 9 ft ceilings and its ceiling ticked, and a 12 by 12 ft bedroom with 8 ft ceilings and its ceiling unticked, 1 door and 2 windows each, come to 1,048 sq ft combined; at 2 coats of eggshell with a 10% buffer that is 7.09 gallons exact, so you buy 8, with 4 gallons of primer and $2,620.00 of labor at $2.50 per sq ft. Paste those numbers into the Multiple rooms tab and you will get the same result. If the project includes new ceilings, the false ceiling area calculator measures drop-ceiling grids, and the floor replacement cost calculator prices the flooring side of the same renovation.
What competing calculators miss
Most paint calculators on the first page are single-room tools with one coverage number, and several leave you to finish the math yourself:
- CalculatorLib: clean single-room math showing exact gallons, rounded gallons, paintable area and total across coats, but the tool stops there: no ceiling option, no finish-based coverage, no primer, no labor, and no way to price more than one room.
- ConstructTools: adds paint quality, primer, labor and supplies inputs, yet still prices one room at a time and never shows the step-by-step numbers behind the gallon count.
- CalcMatePro: a genuinely good worked example and a metric version for litres, but the tool itself returns only gallons: no quarts line, no primer estimate, no cost.
- OverCalculator: refreshingly honest about its editable presets, with a trim toggle and a visible formula, but the trim addition is a fixed area and it prices one room at a time.
- MyPaintCalculator: the strongest FAQ content of the group, answering gallons, ceiling and exterior questions, but the tool is single-room only with no room-by-room pricing.
- The gap this page fills: finish-based coverage presets, a quarts line for touch-ups, a visible step-by-step breakdown, primer and labor priced from the same run, and a multiple-rooms mode with a per-room table that rounds up once for the whole project.
Frequently asked questions
About 350 square feet per coat for typical interior paint on a smooth, primed wall, with most cans printing a 350 to 400 range. Flat and matte finishes stretch furthest at around 375, eggshell and satin sit near 325, and semi-gloss drops to about 275 because the thicker film builds differently. Always use your own can’s figure when you have it, since textured walls, bare drywall and brick all cover less.
Three gallons for the standard case: two coats of eggshell with a 10% buffer, one door and two windows. That room has 334 sq ft of wall, 735 sq ft across both coats and the buffer, and needs 2.26 gallons exact, which rounds up to 3. Add a fourth gallon if you are painting the ceiling too.
Yes. A standard door removes about 20 sq ft and an average window about 15 sq ft, and the calculator subtracts both automatically. Skipping this step is the most common reason people overbuy, especially in rooms with several windows.
Two coats is the standard for full, even coverage and durability. One coat only works for a same-color refresh on already-painted walls in good condition. New drywall needs a primer coat plus two finish coats, and a dark-to-light color change usually needs primer plus two coats as well.
You need primer on bare drywall, over stains and water marks, and when changing from a dark color to a light one. Previously painted walls in good condition usually need none. One coat is enough under two finish coats, and primer covers about 300 sq ft per gallon, less than finish paint, so budget it as a separate purchase.
In single-room mode the primer and labor lines are priced from the wall area only, because ceilings usually get a separate flat paint bought apart from the walls. The ceiling’s paint is still counted in the gallon total. In multiple-rooms mode the primer and labor lines use the combined wall-plus-ceiling area instead. Both behaviors are the tool’s long-standing logic, stated here so the numbers never surprise you.
Professional interior painting typically costs $2 to $6 per square foot including materials, with a single room averaging $400 to $1,600. Enter any rate in that range in the labor field and the calculator prices your wall area and shows a project total of paint plus labor. Leave the field blank for a DIY materials-only estimate.
Yes. The Multiple rooms tab takes up to 12 rooms, each with its own dimensions, doors, windows and ceiling option, lists them in a table, and rounds up once for the combined total. Buying once for the whole project costs less than rounding up room by room, since each separate round-up wastes most of a gallon.
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References
- HomeAdvisor. How Much Does It Cost to Paint a Room? Accessed October 23, 2026.
- U.S. Bureau of Labor Statistics, Occupational Outlook Handbook. Painters, Construction and Maintenance. Accessed October 23, 2026.
- U.S. Environmental Protection Agency. Renovation, Repair and Painting Program: Contractors. Accessed October 23, 2026.
- This Old House. 24 Tips to Help You Paint Trim and Walls Like a Pro. Accessed October 23, 2026.
- Blue Door Painters. How Much Paint Should I Buy? Accessed October 23, 2026.