Paint calculator
Paint needed is the area to cover × the number of coats ÷ the coverage per gallon or litre on the can. A 12 × 10 ft room with 8 ft walls has 352 sq ft of wall; two coats of a flat paint the maker rates at 350–400 sq ft/US gal use 2.01 gallons, so you buy 3. The calculator takes off doors and windows, adds the ceiling if you want it, and uses the coverage each manufacturer publishes, low end first.
Paint calculator
v1 · US and metricEnter the room or wall size, the doors and windows to leave out, the paint and the number of coats. The result gives gallons or litres of paint and primer from the coverage the maker publishes.
Result
Enter your measurements and press Calculate.
Formula and assumptions
- A room (four walls):
Wall area = 2 × (length + width) × height − doors − windows (+ length × width for the ceiling) - Some walls:
Wall area = total wall length × height − doors − windows Gallons = area × coats ÷ coverage per gallon, rounded up to whole gallons; metric:Litres = area × coats ÷ coverage per litre, rounded up to the next 0.1 L- Coverage per coat as the makers state it: Sherwin-Williams SuperPaint Interior Latex Flat covers 350-400 sq ft per gallon (8.59-9.82 m2/L) at 4 wet mils. Behr Premium Plus Interior Flat covers 250-400 sq ft per gallon (6.14-9.82 m2/L). Benjamin Moore Regal Select Matte covers 400-450 sq ft per gallon (9.82-11.04 m2/L). Dulux Trade Vinyl Matt covers up to 17 m2 per litre (about 690 sq ft per US gallon).
- Where a range is given the low end is used, so the estimate is not short on porous, textured or new surfaces.
- Sherwin-Williams Premium Wall & Wood Interior Latex Primer covers 400 sq ft per gallon (9.82 m2/L).
- Converting between sq ft per gallon and m² per litre uses 0.024542; One US gallon is 3.785412 litres.
- Doors and windows are taken off at the size you enter. No standard door or window size is assumed.
How much paint do I need to paint a room?
For a 12 × 10 ft room with 8 ft walls, two coats of Sherwin-Williams SuperPaint flat need 2.01 gallons, so the calculator tells you to buy 3. That is the default it opens with. The walls measure 2 × (12 + 10) × 8 = 352 sq ft, and two coats double that to 704 sq ft of painting. At the low end of the maker's coverage, 704 ÷ 350 = 2.01. A shortfall that small tempts people to buy two gallons and stretch them, but the low end of the range is the figure to plan on for porous or new surfaces.
The metric default is a 3.6 × 3 m room with 2.4 m walls, painted with Dulux Trade Vinyl Matt. The walls come to 31.68 m², and two coats at 17 m2/L need 3.73 litres, rounded up to 3.8. Dulux Trade Vinyl Matt is sold in 1, 2.5, 5, 7.5, 10 and 12 L tins. In practice that means a 5 litre tin, or a 2.5 litre tin and some smaller ones.
Three ways to enter the size
Choose A room to enter length, width and height; the calculator works out all four walls and can add the ceiling. Choose Some walls for a feature wall, a hallway or a stairwell, and enter the total length of the walls you are painting. Choose An area I have measured if you already have square feet or square metres, for example from a plan or from the square feet from feet and inches calculator. The measuring walls, ceiling and trim guide shows how to measure awkward rooms, gable ends and sloped ceilings before you start.
How is paint quantity calculated?
Area × coats ÷ coverage, with doors and windows taken off the area first. The steps are:
- Wall area: 2 × (length + width) × height for a room, or total wall length × height for selected walls.
- Ceiling: length × width, added only if you choose to paint it with the same paint.
- Openings: number of doors × door width × door height, and the same for windows, subtracted from the area. No standard door or window size is assumed, so enter the size of one of each.
- Paint: area × coats ÷ coverage per coat. In US mode the result is rounded up to whole gallons; in metric mode to the next 0.1 litre.
- Primer: area ÷ the primer's coverage, one coat, shown separately.
Worked example: walls, ceiling and openings
Take the same 12 × 10 ft room, add the ceiling, and take off one 3 × 6.67 ft door and two 3 × 4 ft windows. Walls 352 + ceiling 120 = 472 sq ft. Less the door, 20.01 sq ft, and the windows, 24 sq ft, leaves 427.99 sq ft. Two coats at the SuperPaint low-end figure: 427.99 × 2 ÷ 350 = 2.45 gallons, so you still buy 3. The openings saved less paint than the ceiling added, which is common in small rooms.
The coverage figure matters more than anything else in the formula. Pick Behr Premium Plus flat instead, and the low end of its range, 250–400 sq ft/US gal, makes the default walls 2.82 gallons. Add a coat of Sherwin-Williams primer and that is another 0.88 gallons. The room did not change; the product did.
Illustration sources
| Job | Paint and coverage used | Area | Paint needed | Buy |
|---|---|---|---|---|
| 12 × 10 ft room, 8 ft walls | SuperPaint flat, low end | 352 sq ft | 2.01 gal | 3 gal |
| Same room, ceiling, 1 door, 2 windows | SuperPaint flat, low end | 427.99 sq ft | 2.45 gal | 3 gal |
| 12 × 10 ft room, with primer | Behr Premium Plus flat, low end | 352 sq ft | 2.82 gal + 0.88 gal primer | 3 gal + 1 gal primer |
| 3.6 × 3 m room, 2.4 m walls | Dulux Trade Vinyl Matt | 31.68 m² | 3.73 L | 3.8 L |
| 60 m² pebbledash wall | Weathershield, textured, low end | 60 m² | 30 L | 30 L |
| 500 sq ft, three coats | Other paint at 300 sq ft per gallon | 500 sq ft | 5 gal | 5 gal |
What coverage does each paint maker state?
These are the figures behind the product list in the calculator. Each one is per coat, as the manufacturer states it. Where a maker gives a range, the calculator uses the low end.
| Product | Type | Sq ft per US gal | m² per litre | Market |
|---|---|---|---|---|
| Sherwin-Williams SuperPaint Interior Latex Flat | interior topcoat | 350–400 | 8.59–9.82 | US |
| Sherwin-Williams Premium Wall & Wood Primer | interior primer | 400 | 9.82 | US |
| Behr Premium Plus Interior Flat | interior paint & primer | 250–400 | 6.14–9.82 | US |
| Benjamin Moore Regal Select Matte | interior topcoat | 400–450 | 9.82–11.04 | US |
| Dulux Trade Vinyl Matt | interior emulsion | up to 693 | up to 17 | UK |
| Dulux Trade Weathershield Smooth Masonry (smooth render) | exterior masonry | up to 652 | up to 16 | UK |
| Dulux Trade Weathershield Smooth Masonry (pebbledash) | exterior masonry | 163–244 | 4–6 | UK |
Why does the calculator use the low end of the coverage range?
Because a range on the can is not a promise of the top figure. Dulux states that coverage varies with how rough and porous the surface is, and the paint makers' ranges reflect that. A common belief is that the coverage on the can holds for any wall. Dulux's own masonry paint shows how far that is from true: Dulux Trade Weathershield Smooth Masonry covers up to 16 m2/L on smooth, low-porosity render. On textured surfaces such as pebbledash the same masonry paint covers only 4-6 m2/L. On pebbledash the same tin covers as little as a quarter of its smooth-render figure.
The spread between brands is wide as well. Behr Premium Plus flat is rated at 250–400 sq ft/US gal, while Benjamin Moore Regal Select Matte is rated at 400–450 sq ft/US gal. UK emulsion claims are stated as maximums and look much higher: Dulux Trade Vinyl Matt covers up to 17 m2 per litre (about 690 sq ft per US gallon). So a direct comparison between a US and a UK can is not like for like. If your paint is not on the list, choose Other paint and type the coverage from the can or the product data sheet; our page on spec sheet coverage figures shows where to find it and what film thickness it assumes.
How many coats of paint do I need?
Two, in most cases, which is the calculator's default. Sherwin-Williams specifies 2 topcoats on drywall, either self-primed or over 1 coat of primer. Dulux Trade's normal finishing system is 2 full coats; 1 may do on sound, similar-colour paintwork.
A lot of people believe one coat of a paint-and-primer product is enough on new drywall. Sherwin-Williams' own system for SuperPaint says otherwise: two coats self-primed, or primer and then two coats. A paint-and-primer label does not change the number of topcoats on bare board; it means the first of those coats can act as the primer.
Bare drywall and fresh joint compound are the clearest case for a separate primer. Choose the primer option and the calculator adds one coat. Sherwin-Williams Premium Wall & Wood Interior Latex Primer covers 400 sq ft per gallon (9.82 m2/L). The primer is shown as a separate line, because it is a different product and you buy it separately.
Can I use the paint calculator for exterior walls?
Yes for UK masonry paint, and with your own coverage figure for anything else. The product list includes Dulux Trade Weathershield on smooth render and on pebbledash, because Dulux publishes both figures. We could not find a US exterior paint data sheet with a stated coverage, so there is no US exterior preset. For US siding or stucco, choose Other paint and enter the coverage printed on the can, using the low end of any range for rough or porous surfaces.
Measure an exterior wall as a long, flat wall: the total length of the walls you are painting and the height to the eaves, entered in Some walls mode. For gable ends, Area of a triangle is base times perpendicular height divided by two. Add them as a measured area, or calculate them separately and combine. Weather limits matter outside. Dulux Weathershield should not be applied below 7°C (45°F) or above 80% relative humidity. For interior paint, Sherwin-Williams SuperPaint should be applied above 50°F (10°C).
What changes between gallons and litres?
In US mode the calculator works in square feet and US gallons and rounds up to whole gallons. In metric mode it works in square metres and litres and rounds up to the next tenth of a litre, because UK and European paint is sold in a range of tin sizes rather than one standard can. Converting coverage between the two uses one factor. Multiply sq ft per US gallon by 0.024542 to get m2 per litre (divide by it for the reverse). One US gallon is 3.785412 litres.
Watch the gallon if you read older UK labels or advice. One UK (imperial) gallon is exactly 4.54609 litres. It is larger than a US gallon, so a coverage figure per imperial gallon is not the same as one per US gallon. The calculator only uses US gallons and litres.
How much extra paint should I buy?
The calculator adds no percentage, because no paint maker we checked publishes a waste allowance. The extra comes from two places instead: using the low end of the coverage range, and rounding up to whole gallons or the next 0.1 litre. In the default US room, that rounding alone leaves almost a full gallon spare.
Leftover paint is a real problem rather than a free reserve. Oregon's collection scheme shows the scale. Oregon's PaintCare program collected 649,346 gallons of leftover paint in 2024, equal to 7.7% of the 8.4 million gallons of new architectural paint sold that year. That is one state's collection scheme, not a national rate, and it counts only paint brought back for recycling or disposal. Buying close to the calculated amount, and keeping a labelled litre or quart for touch-ups, is cheaper than storing half-full cans for years.
The question of what is in the paint matters too. Federal rules cap VOC content at 250 grams per liter (2.1 lb/gal) for flat interior and exterior coatings and 380 g/L (3.2 lb/gal) for nonflat coatings (40 CFR 59 Subpart D, Table 1). Those are federal maximums, so check the product data sheet for the actual figure.
When is the paint result wrong?
The calculator is accurate for flat walls and ceilings at the coverage you give it. It does not handle these:
- Trim, doors and skirting. Woodwork usually takes a different paint, often a gloss or satin. Measure it separately and use Other paint with that product's coverage.
- Ceilings in a different paint. The ceiling option assumes the same paint as the walls. For a separate ceiling paint, run the ceiling as a measured area on its own.
- Heavily textured walls. Textured plaster, popcorn ceilings and pebbledash take far more paint than a smooth wall. Use the low end of the maker's figure for texture, or the Weathershield textured preset as a guide to how far coverage falls.
- Spraying. Coverage figures assume the film thickness on the data sheet. Paint lost as overspray is not included.
- Colour changes. A deep colour over a pale one, or the reverse, can need an extra coat beyond the default two.
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Rollers, brushes and tape for the job
Picked from the manufacturers' published specifications as the basic kit for applying the gallons above. We have not handled or measured them. Links go to Amazon; see our affiliate relationships.
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Wooster Sherlock Frame 9 in (UPC 071497690791)
Sherlock Frame 9"
Takes 9-inch standard-core roller covers. Compatible with Sherlock GT and standard threaded extension poles. That lets you reach ceilings and high walls from the floor.
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Wooster RR642
Pro/Doo-Z 9" x 3/8" Roller Cover
Nap is 3/8 inch (semi-smooth). Fits a standard-core frame. Also sold in 3/16, 1/2 and 3/4 inch naps (RR641, RR643, RR644).
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Purdy 144152325
XL Glide 2-1/2" Trim Brush
For cutting in along ceilings, corners and trim before rolling. The brush is 2-1/2 inches wide. Bristles are a DuPont Tynex nylon and Orel polyester blend.
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FrogTape CF 120
FrogTape Multi-Surface Painter's Tape
For masking trim, sockets and edges. Standard widths are 0.94 in, 1.41 in and 1.88 in (24 mm, 36 mm, 48 mm). Removes cleanly from indoor and outdoor surfaces for up to 21 days.
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Which calculator do I need next?
If the walls are new, the drywall sheets, screws and mud calculator sizes the board and joint compound before any paint goes on. For a papered feature wall alongside painted ones, the wallpaper calculator counts rolls with the pattern repeat included. The walls category page brings the wall calculators together with the coverage and sheet figures they share.
Paint calculator questions
How many gallons of paint does a 12 × 12 bedroom need?
With 8 ft walls, the walls come to 384 sq ft. Two coats at SuperPaint's low-end coverage need 2.19 gallons, so buy 3. Add the ceiling in the same paint and it rises to 3.02 gallons, which means 4. Enter your own room and paint to check.
Can I use the Benjamin Moore figure for other Benjamin Moore paints?
No. The preset is Regal Select Matte only. Benjamin Moore Regal Select Matte covers 400-450 sq ft per gallon (9.82-11.04 m2/L). Other lines and sheens have their own figures, so choose Other paint and enter the coverage from that product's data sheet. Naming a brand here implies no affiliation; it is the published figure.
Does one coat of paint and primer cover new drywall?
Not according to Sherwin-Williams, whose drywall system calls for two topcoats either way: self-primed, or over a coat of primer. Plan on two coats and use the primer option if the board is bare.
Should I deduct doors and windows from the wall area?
For paint, yes, if you know their sizes. The calculator takes off each opening at the size you enter, and makes no assumption about standard sizes. If you skip them, the result is a little high, which some people prefer as a margin.
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Sources (19)
- Asphalt Roofing Manufacturers Association (ARMA)
- Residential Asphalt Roofing Manual, 2014 edition, Ch. 5 area of rectangles and triangles, checked
- Behr
- BEHR PREMIUM PLUS Interior Flat product sheet, checked
- Benjamin Moore
- Regal Select Waterborne Interior Paint - Matte (N548) product page, checked
- Dulux Trade (AkzoNobel)
- Dulux Trade Vinyl Matt product page, checked
- Dulux Trade Weathershield Smooth Masonry Paint datasheet, checked
- Dulux Trade Weathershield Smooth Masonry Paint product page, checked
- National Institute of Standards and Technology
- Handbook 44 (2026), Appendix C: General Tables of Units of Measurement, checked
- NIST Guide to the SI (SP 811), Appendix B.9 Factors for units listed by kind of quantity, checked
- Special Publication 811 (2008), Guide for the Use of the International System of Units (SI), Appendix B.8, checked
- PaintCare Oregon LLC (report to Oregon DEQ)
- Oregon Paint Stewardship Program 2024 Annual Report, checked
- Purdy
- XL Glide, checked
- Sherwin-Williams
- Premium Wall & Wood Interior Latex Primer B28W08111 product data sheet (108.38), checked
- SuperPaint Interior Latex Flat A86 Series product data sheet (101.02A), checked
- Shurtape Technologies (FrogTape)
- CF 120 FrogTape Brand Painter's Tape - Multi-Surface, Technical Data Sheet, checked
- FrogTape Multi-Surface Painter's Tape, checked
- UK Parliament (legislation.gov.uk)
- Weights and Measures Act 1985, Schedule 1, checked
- US EPA / eCFR (Office of the Federal Register)
- 40 CFR Part 59 Subpart D, Table 1: VOC Content Limits for Architectural Coatings, checked
- Wooster
- Pro/Doo-Z Rollers, checked
- Sherlock Frame, checked
Every number on this page is taken from these documents. How we choose and check sources.