What this calculator does
Find out how much paint to buy for a room, how many cans that is and what it will cost. Buying paint is a balancing act. You don't want to run out halfway through the second coat, or stack leftover cans in the garage. The amount you need depends on four things. First, how much wall you'll actually cover. Second, how many coats you'll apply. Third, how far a gallon or liter spreads on your walls. Fourth, how much ends up in rollers, trays and touch-ups.
The calculator measures your walls from the room's length and width, or from a total wall length you enter. It takes away the doors and windows, multiplies by the number of coats, and divides by the coverage rate (how far the paint spreads). Then it adds a little extra for waste and rounds up to whole cans of the size you plan to buy. With 1-gallon cans, it also shows whether a quart or two would cover what's left over. It can work out the ceiling separately, and it multiplies the cans by your price. It works in US or metric units.
Where it's used
Why this math matters outside the classroom:
- Bedroom walls. A 12 × 12 ft room with 8 ft walls has 384 ft² of wall, so two coats at 350 ft² per gallon need about 2.2 gallons.
- Doors and windows. Painters subtract 21 ft² for a 3 × 7 ft door and 15 ft² for a 3 × 5 ft window before working out gallons.
- Ceiling paint. A 12 × 12 ft ceiling is 144 ft², so one coat needs well under a gallon at 350 ft² per gallon.
- Whole-house repaint. Homeowners add up the walls of every room to compare buying single gallons with 5-gallon buckets.
- Rounding up. Students divide total area by coverage per gallon and round up, since paint is sold in whole cans.
Grade level and standards: Grades 6–7; Common Core State Standards for Mathematics 7.G.B.6. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.
How to use it
- Choose US or metric units. Then enter either the room's length and width, or the total length of the walls you'll paint.
- Enter the wall height, the number of doors and windows, and their usual sizes.
- Set the number of coats, the coverage figure from your paint can, the can size and a waste allowance.
- Tick the ceiling box if you're painting it too. Enter the price per can, or clear it to skip the cost.
- Read how many cans to buy, the gallons-and-quarts option, the cost, the exact amount of paint and the area you'll paint.
What the inputs mean
- Walls to paint
- Use total wall length for an accent wall, a hallway, or a room where you're skipping some walls.
- Total wall length
- Add up the widths of every wall you will paint.
- Door width
- A common interior door is about 3 ft × 7 ft.
- Number of coats
- Two coats is the usual advice, especially when you're changing color. Primer is a separate product with its own coverage, so it doesn't count as a coat here.
- Coverage per gallon
- How far the paint spreads, printed on the can. About 350–400 ft² per gallon is typical for indoor wall paint on smooth, primed drywall. Rough, porous or unprimed walls soak up more.
- Coverage per liter
- How far the paint spreads, printed on the tin. About 8.5–10 m² per liter is typical for indoor emulsion (water-based wall paint) on smooth, sealed walls.
- Can size
- 1 gallon is the usual can; a quart is 0.25 gal.
- Waste allowance
- Covers paint left in the tray and roller, plus a little for touch-ups. 10% is a common allowance.
- Also paint the ceiling (counted separately)
- The ceiling is the room's length × width. It uses the same coats, coverage and can size. It gets its own can count, because ceiling paint is usually a different product.
- Price per can (optional)
- The $40 starting value is a stand-in for a gallon of mid-range indoor paint, not a quote. Use your store's price.
Formula and method
wall length = 2 × (room length + room width) (or the total you enter) gross wall area = wall length × wall height openings = doors × door width × door height + windows × window width × window height paint = (gross area − openings) × coats ÷ coverage × (1 + waste % ÷ 100) cans = paint ÷ can size, rounded up to a whole number quarts for the remainder = (paint − whole gallons) × 4, rounded up ceiling paint = length × width × coats ÷ coverage × (1 + waste % ÷ 100) cost = (wall cans + ceiling cans) × price per can
Wall length is the distance all the way around the room (its perimeter). For a rectangular room, that's twice the length plus twice the width. Multiply it by the wall height to get the wall area. You won't paint the doors and windows, so their area comes off.
Each coat covers the whole paintable area again. The coverage rate (area per gallon or liter) turns that area into an amount of paint. Waste % is the extra you add for trays, rollers and touch-ups. Stores sell whole cans, so the final number is rounded up. The calculator ignores tiny rounding leftovers, so a job that needs exactly 3 gallons isn't pushed up to 4.
Worked example
A 14 ft × 12 ft bedroom with 8 ft walls has one 3 ft × 7 ft door and two 3 ft × 4 ft windows. You plan two coats of paint that covers 350 ft² per gallon. You add 10% for waste and buy 1-gallon cans.
- Wall length: 2 × (14 + 12) = 52 ft.
- Total wall area: 52 × 8 = 416 ft².
- Doors and windows: 1 × 3 × 7 + 2 × 3 × 4 = 21 + 24 = 45 ft².
- Area you'll paint: 416 − 45 = 371 ft².
- Paint: 371 × 2 ÷ 350 × 1.1 = 2.332 gallons.
- Cans: 2.332 ÷ 1 rounded up = 3. Or buy 2 gallons, plus 0.332 × 4 = 1.33 quarts, which rounds up to 2 quarts.
- Cost at $40 a can: 3 × $40 = $120.
- If you paint the 14 × 12 ceiling too: 168 × 2 ÷ 350 × 1.1 = 1.056 gallons. That's 2 more cans, and $200 in total.
Result: You need about 2.33 gallons for the walls. Buy 3 one-gallon cans (or 2 gallons and 2 quarts) for about $120.
Understanding the result
The number of cans is what to buy. The paint figure is how much you expect to use. The difference is leftover paint for touch-ups later.
The gallons-and-quarts line appears when you choose 1-gallon cans. It only suggests quarts when you need one or two, because three quarts usually cost more than a gallon. Check the shelf prices before you decide.
Assumptions and limitations
- Walls are flat, rectangular and all the same height.
- All doors are one size and all windows are one size. If yours vary, use average sizes.
- Every coat uses the same amount of paint. In real life, the first coat over bare or dark walls can use more.
- Trim, baseboards and the doors themselves aren't included. Estimate them separately if you're painting them.
- The ceiling option assumes a flat ceiling the same size as the floor. Vaulted or sloped ceilings need their own measurement.
- Textured walls, fresh plaster and unprimed drywall can soak up far more paint than the label suggests.
- The cost covers paint only, at the price you enter. Primer, rollers, tape and tax are extra.
Frequently asked questions
How much does one gallon of paint cover?
Most indoor wall paints say roughly 350–400 square feet per gallon for one coat on a smooth, primed wall. The number on your can is the best guide. Rough walls will get less.
Should I subtract doors and windows?
Yes, if they take up a fair share of the wall. A standard door is about 20 square feet, and an average window is around 12–15. In a room with several of them, that can add up to most of a gallon.
Do I really need two coats?
One coat can be enough if you're repainting the same color over paint in good shape. Two coats give more even color and last longer. You'll usually need two for a noticeable color change.
How many gallons of paint do I need for a 12 × 12 room?
With 8 ft walls, the walls total 384 ft². Take off one door and two windows (about 45 ft²). Two coats at 350 ft² per gallon need 339 × 2 ÷ 350 = 1.94 gallons, or 2.13 with 10% extra. So buy 2 gallons and a quart, or 3 gallons to be safe.
How much paint do I need for a ceiling?
A flat ceiling is the room's length times its width. A 14 × 12 ft ceiling is 168 ft². Two coats at 350 ft² per gallon, with 10% extra, is about 1.06 gallons. That means 2 one-gallon cans, or 1 gallon and 1 quart.
Can I use this for a single accent wall?
Yes. Choose 'Total length of walls' and enter the width of that wall. Then set the door and window counts to whatever is on that wall, which may be zero.
How this calculator is checked
Method: Rectangle area and perimeter; paint volume = area × coats ÷ manufacturer coverage rate. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.
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