Paint calculator: how much paint do I need?

Walls only, or walls and ceiling?

That's the first question, and the calculator won't show how much to buy until you answer it. The ceiling can add a lot.

Take a 20 by 15 ft living room with 9 ft walls, two doors and three windows, on two coats at 350 sq ft per gallon. Walls only, you paint 552 sq ft and buy 3 gal 1 qt. Add the ceiling and it's another 300 sq ft. You now paint 852 sq ft, need 4.869 gal and buy 1 x 5 gal. The ceiling on its own is 1.714 gal.

Ceiling paint is usually its own product, so you may want two counts. Run the walls as walls only. Then switch to By known wall area, set doors, windows and any other openings to 0, and type the ceiling's 300 sq ft at two coats: it needs 1.714 gal and you buy 1 gal 3 qt, with 0.036 gal left over. Leave them in and the calculator takes them off that 300 sq ft too, so you buy 1 gal 2 qt and run short.

In walls and ceiling mode, the ceiling gets both coats, and the doors and windows don't come off it. The result repeats your choice back, for example "Walls and ceiling. 2 coats at 350 sq ft/gal."

How much paint do I need for a bedroom?

Take a 14 by 16 ft bedroom with 8 ft walls, one door and two windows. Walls only, two coats, eggshell at the starting 350 sq ft per gallon.

• Walls: 480 sq ft.
• Openings: one 21 sq ft door and two 12 sq ft windows, 45 sq ft, or 9.4% of the wall.
• Paint: 435 sq ft.
• Need: 2.486 gal. Buy 2 gal 2 qt, with 0.014 gal left over.

That leftover is under 2 fluid ounces (0.014 x 128 = 1.8), so this is a room where a waste allowance earns its place. See the waste section below.

How do I check the answer by hand?

The formula in words: add the room's length and width, double it, and multiply by the wall height. Take off the doors, windows and other openings. If you're doing the ceiling, add length times width. Multiply by coats, divide by the coverage rate, then add the waste percentage. Round up to the next quart.

Here's the living room above, worked line by line:

1. Wall area: 2 x (20 + 15) x 9 = 630 sq ft.
2. Openings: 2 x 21 + 3 x 12 = 78 sq ft. That's 12.4% of the wall.
3. Walls to paint: 630 - 78 = 552 sq ft.
4. Ceiling: 20 x 15 = 300 sq ft. Total: 852 sq ft.
5. Two coats: 852 x 2 = 1,704 sq ft of coverage.
6. At 350 sq ft per gallon: 1,704 / 350 = 4.869 gal.
7. Round up to the quart: 5 gal. Fill with 5 gallon cans first: 1 x 5 gal, 0.131 gal left over.

If your sum matches the page, the page is doing what it says.

What coverage rate should you use?

The one on your can. Coverage is how many square feet a gallon covers, and makers print it.

Benjamin Moore lists Regal Select Eggshell at 400 to 450 sq ft per gallon, and Regal Select Flat at the same 400 to 450. Lowe's paint calculator says paint usually covers 350 to 400. We start eggshell at 350, the low end of Lowe's range, so you're less likely to run short.

Here's what the rate does to the bedroom:

Rate (sq ft/gal)	Need	Buy	Left over
350 (our start)	2.486 gal	2 gal 2 qt	0.014 gal
400 (BM low end)	2.175 gal	2 gal 1 qt	0.075 gal
450 (BM high end)	1.933 gal	2 gal	0.067 gal

So the label can save you two quarts. Type its low figure, not its high one, unless you're rolling over a wall the same color.

Flat starts at 300 here. That gap between finishes is our setting, not a maker figure: Benjamin Moore lists the same range for Regal Select Flat and Eggshell. If you change the finish after typing your own rate, the rate resets to that finish's starting figure and the page tells you.

How much extra should you buy, and how does it round to cans?

The waste box starts at 0%, because rounding up to the quart already leaves a little over. Add some when the bedroom's leftover is as thin as 0.014 gal, when the wall is textured or patched, or when you want a can for touch-ups.

The page applies waste to the exact figure first, then rounds. The order matters:

1. Bedroom need: 2.486 gal.
2. Plus 10%: 2.486 x 1.1 = 2.734 gal.
3. Round up to the next quart: 2.75 gal.
4. Fill 5 gallon cans first, then gallons, then quarts: 2 gal 3 qt, 0.016 gal left over.

On the living room with the ceiling, 10% takes 4.869 gal to 5.355 gal, and you buy 1 x 5 gal + 2 qt.

The page only reaches for a 5 gallon can when you need 5 gallons or more. At 400 sq ft per gallon the living room needs 4.260 gal, and the page prints 4 gal 2 qt, not a 5 gallon can. Whether one big can is cheaper than four gallons and two quarts is a price check at the store, and the page doesn't price paint.

Do you need primer, and how much?

Tick primer and it gets its own line at one coat and its own rate. It doesn't count toward the paint total. Any waste you set applies to the primer line too.

On the bedroom's 435 sq ft:

• At our starting 200 sq ft per gallon (the low end of Lowe's 200 to 300): 2.175 gal. Buy 2 gal 1 qt.
• At 400, the low end of what Benjamin Moore lists for Fresh Start High-Hiding All Purpose Primer: 1.088 gal. Buy 1 gal 1 qt.

That's a gallon apart on one room, so the primer can's label matters as much as the paint's. Whether you need primer at all, and how many coats of paint, is covered in how many coats of paint do you need.

How are doors and windows handled?

Doors default to 21 sq ft and windows to 12 sq ft, and the sizes are in the labels so you know what's being taken off. Change either if yours differ, and add anything else, like a wide pass-through, under other openings. The result shows the total taken off as an area and as a share of the wall: 78 sq ft, 12.4%, on the living room.

If your openings add up to more than the wall, the wall area goes to zero with a warning, never a negative number. A ceiling, if you included one, is still counted in full.

Already know your wall area? Switch to By known wall area and type it in. You can't include a ceiling in that mode, because there's no room size to work it out from, and the page tells you so.

Already have paint?

Switch to I have paint and enter what you've got, in gallons or liters, as one total. Set your coats, finish and waste, and you'll see how much wall it will cover. Take 2.5 gallons at 350 sq ft per gallon, three coats, with fifteen percent set aside for waste: that covers 253.6 sq ft of wall. That's painted surface, not floor area. The sum is shown too, so you can check it.

What this doesn't do

It doesn't price paint, compare cans or choose a brand, color or finish. It doesn't count trim, doors, cabinets, floors or exterior siding; each has its own surface area this page doesn't measure. It treats every wall as the same height, so a vaulted or sloped ceiling room needs its walls entered as a known area. It works primer at one coat only.

Before you buy

Read two things off the product you're buying. First, its coverage rate: type the low end into the page and rerun. Second, the sizes it's sold in. When we read Benjamin Moore's Regal Select Eggshell page, its size picker offered 1 gallon and 1 quart, so a "1 x 5 gal" answer may need to become five gallons for that line. Then buy every can of one color at once, from the same batch if you can.

Sources (read 24 Oct 2026): Benjamin Moore, Regal Select Interior Paint Eggshell product page (N549): coverage 400 to 450 sq ft/gal, 1 or 2 coats, sizes 1 gallon and 1 quart. Benjamin Moore, Regal Select Interior Paint Flat product page (N547): coverage 400 to 450 sq ft/gal. Benjamin Moore, Fresh Start High-Hiding All Purpose Primer product page (0046): coverage 400 to 450 sq ft/gal. Lowe's, Paint Calculator page: paint 350 to 400 sq ft/gal, primer 200 to 300 sq ft/gal. Our starting rates (eggshell, satin, semi-gloss, gloss 350; flat 300; primer 200), the 21 and 12 sq ft opening sizes and the 1 quart, 1 gallon and 5 gallon can sizes are our settings.

Questions

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Whether you need primer at all, and how many coats of paint, is covered in how many coats of paint do you need.