Paint Calculator
This paint calculator estimates the gallons of paint for a room, a run of wall or an area you already know. It subtracts doors and windows, multiplies by the number of coats and divides by how far a gallon spreads. Pick the spread rate of a paint or type the coverage from your can, add primer or the ceiling if you need them, and switch to liters if you prefer.
Quick answer: gallons = wall area × coats ÷ coverage per gallon. A 14 × 12 ft room with 372 sq ft of wall needs 2.13 gallons for two coats at 350 sq ft per gallon, so buy 3.
Paint needed depends on the area, the coats and how far a gallon spreads. Wall area is the distance around the room times the wall height, minus doors and windows. Multiply by the number of coats, divide by the coverage per gallon on the can, and round up to whole gallons. A quart is a quarter of a gallon.
Example: a 14 × 12 ft room with 8 ft walls, one 20 sq ft door and two 12 sq ft windows has 2 × (14 + 12) × 8 − 20 − 24 = 372 sq ft of wall. Two coats at 350 sq ft per gallon is 372 × 2 ÷ 350 = 2.13 gallons, so buy 3 gallons, or 2 gallons and 1 quart.
Method
How this calculator works
The area math is exact. What moves the answer is the spread rate and the number of coats, so the calculator lets you pick or type both.
Wall area and gallons
Wall area is the distance around the room times the wall height. For a room that distance is 2 × (length + width). Each door and each window is multiplied by its area and subtracted, and the wall area never goes below zero. The paint needed is that area times the coats, divided by the spread rate: the square feet one gallon covers in one coat.
Wall area = 2 × (Length + Width) × Height − (Doors × Door area + Windows × Window area)Gallons = Wall area × Coats ÷ Spread rate (sq ft per gallon)Spread rates in the list
Sherwin-Williams states 350 to 400 sq ft per gallon for SuperPaint Interior Latex, and Benjamin Moore states 400 to 450 for Regal Select. Behr gives 250 to 400 for Premium Plus, depending on texture, porosity and application method, but that range comes from a search summary because its page could not be opened, so it is marked lower confidence. The calculator uses the low end of each range, so the estimate errs on the high side. Typing the coverage from your own can overrides all three.
Spread rate used = low end of the stated rangeOpenings, ceiling and coats
The 20 sq ft door (3 ft × 6 ft 8 in) and the 12 sq ft window (3 ft × 4 ft) are nominal sizes you can edit. Two coats is the default and you can choose 1 to 4. The ceiling is an option, off by default, because the headline answer is wall paint. When it is on, the ceiling is worked out separately with the same spread rate and coats and shown as its own paint. Trim, baseboards and doors are not counted.
Ceiling paint = Length × Width × Coats ÷ Spread ratePrimer, rounding and liters
Primer is one coat over the walls, and over the ceiling if you include it, at 400 sq ft per gallon, the figure on the Sherwin-Williams primer data sheet, unless you enter your own. Purchases round up to whole gallons. When the amount is a quarter gallon or less over a whole number of gallons, the result also offers the quart option, because 4 quarts make 1 gallon. In the metric view, 1 US gallon is 3.785411784 liters exactly, and liters are rounded up to whole liters.
Primer gallons = (Wall area + Ceiling area) ÷ Primer spread rateLiters = Gallons × 3.785411784Stated coverage and what the calculator uses
| Paint | Stated (sq ft per gal) | Calculator uses | Basis |
|---|---|---|---|
| Sherwin-Williams SuperPaint Interior Latex | 350–400 | 350 | Product data sheet 101.02A, at 4 mils wet |
| Benjamin Moore Regal Select Interior | 400–450 | 400 | Product page |
| Behr Premium Plus Interior | 250–400 | 250 | Search summary only, lower confidence |
| Sherwin-Williams Premium Wall & Wood primer | 400 | 400 | Product data sheet 108.38, at 4 mils wet |
What each field does
| Field | What to enter |
|---|---|
| What are you painting? | Room (length, width and height), Wall length (the total length of all the wall you will paint, any shape, plus the height) or Total area (an area you already know, with nothing subtracted). |
| Room length, Room width, Total wall length, Wall height | Feet and inches in the US view or meters and centimeters in the metric view. |
| Paint the ceiling too? | Room only. No by default. Yes adds the ceiling (length × width) as its own paint, with the same coats and spread rate. |
| Area to paint | Total area only. Doors and windows are not subtracted from it. |
| Doors and Area of one door | How many doors, and the size of each. The default is 20 sq ft, which is 3 ft × 6 ft 8 in. Edit it for yours. |
| Windows and Area of one window | How many windows, and the size of each. The default is 12 sq ft, which is 3 ft × 4 ft. |
| Coats | 1 to 4, with 2 as the default. The spread rate is per coat. |
| Paint (spread rate) | Sherwin-Williams SuperPaint (350 sq ft per gallon), Benjamin Moore Regal Select (400), Behr Premium Plus (250, lower confidence) or My own coverage. |
| Your coverage per gallon | Used with My own coverage. Enter the figure on your can, in sq ft per gallon (m² per liter in the metric view). If it is empty, 350 is used and a warning shows. |
| Add primer (1 coat)? | No by default. Yes adds a primer line for one coat over the painted area. |
| Primer coverage per gallon | Leave it empty to use 400 sq ft per gallon, or enter the figure on your primer's label. |
Worked example: a 14 × 12 ft room
Arithmetic checkedThese are the calculator's own example values: 8 ft walls, one door, two windows, two coats and Sherwin-Williams SuperPaint at 350 sq ft per gallon. The ceiling and primer rows show the options and are not part of the 3 gallons.
| Step | Math | Result |
|---|---|---|
| Distance around the room | 2 × (14 + 12) | 52 ft |
| Wall area | 52 × 8 | 416 sq ft |
| Openings | 1 × 20 + 2 × 12 | 44 sq ft |
| Wall area to paint | 416 − 44 | 372 sq ft |
| Paint for two coats | 372 × 2 ÷ 350 | 2.13 gal |
| To buy | rounded up | 3 gal, or 2 gallons + 1 quart |
| Ceiling if included, as its own paint | 14 × 12 × 2 ÷ 350 | 0.96 gal, so 1 gal |
| Primer, 1 coat on the walls | 372 ÷ 400 | 0.93 gal, so 1 gal |
| You would see | 3 gal of paint (2.13 gal needed) | |
Disclaimer
Results are planning estimates based on the dimensions and assumptions you enter. They are not engineering, structural or building-code advice. Product yields, densities and coverage vary by manufacturer, batch and site conditions. Check product labels, your supplier and your local building department before you order materials or start work. Read the full calculation disclaimer.
Accuracy
What affects your estimate
The formula is simple. What changes the answer is the paint you pick, the number of coats and the condition of the wall.
The spread rate changes the gallons
The same 372 sq ft with two coats needs 2.98 gallons at 250 sq ft per gallon, 2.13 at 350, 1.86 at 400 and 1.65 at 450. Rounded up, that is 3, 3, 2 and 2 gallons. Use the figure on the can you will buy, and the low end of its range on a rough wall.
Coats multiply everything
One coat of the example room is 1.06 gallons, two coats are 2.13 and three coats are 3.19. Rounded up, that is 2, 3 and 4 gallons. The spread rate is per coat, and a strong color change often needs more than two.
Surface and application
Spread rates are for smooth, previously painted walls. Texture, porous surfaces and new, unprimed drywall take more paint, and sprayer overspray is not included. Behr states that its range depends on texture, porosity and application method.
Rounding and quarts
A result of 2.13 gallons rounds up to 3. Because 2.13 is only 0.13 over 2 gallons, the result also offers 2 gallons + 1 quart, which is 2.25 gallons. When the remainder is larger, the next whole gallon is the answer. Can sizes are a retail convention, so check which sizes your paint comes in.
Ceiling, trim and doors
The ceiling has its own line when you turn it on. Trim, baseboards, doors and window frames are not in the wall area. To include them, measure their area and use the Total area view for that paint.
Metric view
The 14 × 12 ft example needs 2.1257 gallons, which is 2.1257 × 3.785411784 = 8.05 liters, so it rounds up to 9 L. Coverage is in m² per liter: 350 sq ft per gallon is 8.59 m² per liter. Cans come in several sizes, so ask your supplier.
This calculator estimates how much paint to buy. It quotes no prices and does not choose a product for you. Homes built before 1978 may have lead paint. Test before sanding or scraping and follow EPA lead-safe (RRP) rules. The estimate does not account for it.
Common mistakes
Forgetting the second coat
The spread rate is per coat. One coat of the example room is 1.06 gallons and two coats are 2.13. If you plan two coats, leave Coats at 2.
Using the top of the range
A data sheet can show 400 sq ft per gallon or more, and a rough wall will not reach it. Planning on 350 instead of 400 turns this room from 2 gallons into 3. Read the can, and use the low end for rough or porous walls.
Mixing the ceiling into the wall paint
Ceilings often get a different paint. The calculator keeps ceiling paint on its own line, so you do not buy wall color for a ceiling that needs 1 gallon of something else.
Skipping primer when the label asks for it
New drywall, bare patches and stains are typical places for primer, and the paint label says when it is needed. Primer has its own coverage, so it is a separate line: 0.93 gallon for the example walls.
Forgetting trim, doors and overspray
None of them are in the wall area, and a sprayer loses paint to overspray. If the result lands close to a gallon line, round up or add a quart.
Questions people ask
How do I calculate how much paint I need?
Multiply the distance around the room by the wall height, subtract doors and windows, multiply by the number of coats and divide by the coverage on the can. Round up to whole gallons. The guide on how many square feet a gallon of paint covers works the same steps by hand.
How much paint is for 1 sq ft?
A gallon is 128 fluid ounces. At 350 sq ft per gallon, one coat on 1 sq ft takes 128 ÷ 350 = 0.37 fluid ounce, and two coats take 0.73. At 400 sq ft per gallon it is 0.32 for one coat.
How many sq ft will 1 gallon of paint cover?
Manufacturers state about 350 to 400 sq ft per coat for Sherwin-Williams SuperPaint and 400 to 450 for Benjamin Moore Regal Select, on smooth surfaces. Behr gives 250 to 400, from a summary we could not check on its page. Rough or porous walls take more paint.
Will 1 gallon of paint cover a 10×10 room?
For one coat, yes. With 8 ft walls and nothing subtracted, the walls are 40 × 8 = 320 sq ft, and 320 ÷ 350 = 0.91 gallon. For two coats the area is 640 sq ft and 640 ÷ 350 = 1.83 gallons, so you need 2.
How much paint do I need for a 12×14 room?
With 8 ft walls, one door and two windows, the walls are 372 sq ft. Two coats at 350 sq ft per gallon is 2.13 gallons, so buy 3. At 400 sq ft per gallon it is 1.86 gallons, so 2.
Related calculators and guides
Sources
Where the numbers come from
- Sherwin-Williams, SuperPaint Interior Latex Flat, product data sheet 101.02A coverage of 350 to 400 sq ft per gallon at 4 mils wet, accessed October 16, 2026.
- Sherwin-Williams, Premium Wall & Wood Interior Latex Primer, product data sheet 108.38 primer coverage of 400 sq ft per gallon, accessed October 16, 2026.
- Benjamin Moore, Regal Select Waterborne Interior Paint coverage of 400 to 450 sq ft per gallon, accessed October 16, 2026.
- Behr, Premium Plus Interior coverage of 250 to 400 sq ft per gallon depending on texture, porosity and application method. Lower confidence: the page could not be opened and the range comes from a search summary, accessed October 16, 2026.
- NIST Handbook 44 (2026), Appendix C 4 quarts = 1 gallon = 231 cubic inches, behind the quart and liter conversions, accessed October 16, 2026.