Paint Calculator

What it calculates and who it’s for

This calculator estimates interior paint for one rectangular room. It gives the paintable wall area, wall-paint gallons per coat and in total, and a shopping mix of 5-gallon buckets, 1-gallon cans, and quarts for the wall color. If you include the ceiling, it figures ceiling paint as its own buy, and primer gets its own line too.

It is for a homeowner painting bedroom, bathroom, or living-room walls and, if you choose, the ceiling. It uses feet and square feet. It does not price the paint, and it does not estimate the outside of a house.

How to use

1. Enter the room length, width, and wall height in feet. Leave Include ceiling on Yes to get a separate ceiling-paint amount, or set it to No if you are painting walls only. It never changes the wall paint.
2. Count the doors and windows you will not paint with the wall color. The sizes start at 20 square feet per door and 15 square feet per window. Change those in Opening sizes and primer if you measure something else.
3. Leave Coats at 2. Leave Product on Sherwin-Williams SuperPaint Flat at 350 square feet per gallon, or switch to the 400 high end of that same sheet. For any other can, choose Custom and type the coverage rate from that can’s data sheet.
4. Leave Include primer on No unless you are covering a very different color, priming new drywall, or the can says to prime. Read the wall gallons and the can counts, then the ceiling and primer lines. The donut shows the gallons you carry home of each product.

The formula

Gross wall area (sq ft) = 2 × (length in ft + width in ft) × wall height in ft

That is the four walls, with no doors or windows removed yet.

Opening area (sq ft) = number of doors × door size (sq ft each) + number of windows × window size (sq ft each)

Paintable walls (sq ft) = gross wall area − opening area

Ceiling area (sq ft) = length × width when Include ceiling is Yes, and 0 when it is No

Coverage rate (sq ft/gal) is 350 or 400 when you pick Sherwin-Williams SuperPaint Flat, or the number you type for Custom. The SuperPaint sheet lists 350–400 square feet per gallon at 4 mils wet and 1.4 mils dry.

Wall paint per coat (gal) = paintable walls ÷ coverage rate

Wall paint, all coats (gal) = wall paint per coat × number of coats

Ceiling paint (gal) = ceiling area ÷ coverage rate × number of coats

Primer (gal) = (paintable walls + ceiling area) ÷ coverage rate when Include primer is Yes, and 0 when it is No. That is one coat over everything you are painting.

Walls and ceiling are kept apart because a ceiling usually gets its own ceiling paint, often flat white, not the wall color. Sherwin-Williams’ own calculator also figures ceilings separately from walls.

The can mix starts from the exact gallons, not the rounded gallons on screen. It rounds up to the next quart (0.25 gallon). Four quarts make 1 gallon, and 20 quarts make one 5-gallon bucket, so the mix uses the biggest containers that cover that quart total. If 3 quarts are left over, the mix buys one more gallon instead, because 3 quarts hold less paint than a gallon and smaller containers cost more per gallon. If that makes five 1-gallon cans, they become one 5-gallon bucket. Ceiling paint and primer are rounded the same way and shown as the gallons to buy.

There is no extra waste percent on top of the coverage rate. Choosing 350 instead of 400 already buys more paint than the high end of the same sheet, and rounding up to whole containers adds a little more.

Worked example

A 12 ft by 12 ft room with 8 ft walls, the ceiling included, one 20 sq ft door, two 15 sq ft windows, 2 coats, and 350 sq ft per gallon.

Gross walls = 2 × (12 + 12) × 8 = 384 sq ft

Openings = 1 × 20 + 2 × 15 = 50 sq ft

Paintable walls = 384 − 50 = 334 sq ft

Wall paint per coat = 334 ÷ 350 = 0.9543…, shown as 0.95 gal

Wall paint, all coats = 0.9543… × 2 = 1.9086…, shown as 1.91 gal

Round 1.9086… up to the next quart: 2.00 gal. That is two 1-gallon cans. No quarts and no 5-gallon bucket.

Ceiling = 12 × 12 = 144 sq ft. Ceiling paint = 144 ÷ 350 × 2 = 0.8229… gal. Rounded up to the next quart that is 1.00 gal, so the ceiling line says 1 gal: one gallon of ceiling paint.

Primer is 0 because Include primer is No. The donut shows 2 gallons of wall paint and 1 gallon of ceiling paint.

Those are the numbers the calculator shows for the defaults.

How to read your results

Wall paint, all coats is the wall color, before the cans are rounded up. Wall paint per coat is that amount divided by the number of coats. Areas are rounded to the nearest tenth of a square foot for display. Gallons are rounded to the nearest hundredth for display. The can counts use the exact gallons, then round up, so the cans are always at least the gallons on screen.

Buy the buckets, cans, and quarts, not the hundredths. In the worked example the screen says 1.91 gal and the mix is two 1-gallon cans. If you buy more than one can of the same color, pour them together in a bucket before you start so the color matches from wall to wall.

Ceiling paint to buy is already rounded the same way: 0.25 or 0.5 means 1 or 2 quarts, and a whole number means that many gallons. It is a separate product from the wall color. If you really are rolling the wall color onto the ceiling, add the two amounts together.

Primer to buy is a different product too. Do not pour it into the wall-paint total. It is rounded the same way as the ceiling line.

The 350 rate is the low end of SuperPaint’s 350–400 range, so it plans on more paint than the 400 rate. A porous or textured wall can still use more than either number. If the can you are holding lists a different coverage, choose Custom and type that rate.

Assumptions and limits

The room is a rectangle. Alcoves, closets, and sloped ceilings are not in the four-wall formula. Measure those separately or adjust the length and width until the wall area matches what you measured.

Door size starts at 20 sq ft, the face of a 36 in by 80 in door. The 80 in height is 6 ft 8 in, a height JELD-WEN lists for its Birkdale interior doors. The 36 in width is the common 3-0 width paired with that height, and it is labeled here as a starting size, not a measurement of your house. Window size starts at 15 sq ft, the nominal 36 in by 60 in call size. JELD-WEN’s Builders vinyl double-hung book code 3660 has a frame of 35.5 in by 59.5 in, about 14.7 sq ft. Measure the opening and change the size. Trim, baseboards, and the door slab itself are not in the wall total.

Coverage is for Sherwin-Williams SuperPaint Interior Latex Flat on the kind of surface the data sheet describes, at 4 mils wet. The sheet says to use two coats on drywall (self-prime) or one primer coat plus two finish coats. It also says a primer coat helps when the new color is very different from the old one, and it tells you to use the recommended P-shade primer for hide. Sherwin-Williams Premium Wall & Wood Primer, the drywall primer that sheet names, lists 400 sq ft per gallon. Primer here uses the same rate as the finish paint (350 by default), so it never plans on less primer than that sheet’s rate. The ceiling uses the same coverage rate and the same number of coats as the walls. If your ceiling paint lists a different rate, run the room again with Custom. Textured, masonry, and exterior surfaces are outside that cited rate. Benjamin Moore and Behr rates are not built in. Use Custom and the number on your own can.

Porous or rough surfaces drink more paint than a sealed drywall face. The sheet gives the 350–400 range at a set film thickness and does not give a porosity multiplier, so this calculator does not invent one. If the wall is new, patched, or very thirsty, stay on 350 or type a lower custom rate from the can, and plan on the two coats the sheet recommends.

Ventilation is your job. The SuperPaint sheet says to use the product only with adequate ventilation, and to open windows and doors or otherwise bring in fresh air while you apply it and while it dries. Follow the can and the data sheet. This page is not a safety data sheet.

This calculator is not a lead-paint guide. The SuperPaint sheet warns that scraping, sanding, or removing old paint can release lead dust, and it points readers to the National Lead Information Hotline at 1-800-424-LEAD and to epa.gov/lead. EPA’s lead page covers lead-based paint hazards, including homes built before 1978. This tool only estimates gallons. Use EPA’s guidance before you disturb old paint.

Confirm the coverage number with the retailer or the can you buy, and follow the product instructions.

FAQ

How much does a gallon of paint cover?

Sherwin-Williams SuperPaint Interior Latex Flat lists 350–400 square feet per gallon at 4 mils wet and 1.4 mils dry. This calculator defaults to 350, the low end. Another brand can be higher or lower. Choose Custom and use the figure on that can’s data sheet.

How many gallons of paint for a 12 by 12 room?

At the defaults (8 ft walls, one 20 sq ft door, two 15 sq ft windows, 2 coats, 350 sq ft per gallon) the walls need 1.91 gallons, so you buy two 1-gallon cans. The ceiling needs another 0.82 gallon, so you buy one gallon of ceiling paint. The worked example above is that room, step by step.

Should I buy quarts or a gallon?

Quarts make sense for a small leftover. This calculator buys up to 2 quarts on top of your gallons. If 3 quarts are left over, it buys one more gallon instead, because a gallon holds more and smaller containers cost more per gallon. The same rule turns five 1-gallon cans into one 5-gallon bucket.

How much paint do I need for one wall?

This calculator always uses all four walls. One wall, with no door or window, is length × height. A 12 ft wall at 8 ft is 96 sq ft. At 350 sq ft per gallon and 2 coats, that is 192 ÷ 350 = 0.55 gallon. That rounds up to 3 quarts, so buy one gallon. Subtract any opening on that wall before you divide.

How many gallons of paint per square foot?

Divide 1 by the coverage rate. At 350 square feet per gallon, that is about 0.0029 gallon per square foot for one coat. The easier way to shop is the other direction: one gallon covers 350 square feet once.

How much paint do I need to paint a door?

The main results subtract the door from the walls. They do not estimate paint for the door itself. One face of the default door is 20 sq ft. At 350 sq ft per gallon, one coat of that face is about 0.06 gallon. A quart is 0.25 gallon, so a quart is the smallest can and it covers that face more than once. Paint both sides only if you mean to. The door still needs the same ventilation and product rules as the walls.

How much paint for the exterior of a house?

This page does not estimate exterior paint. Siding, stucco, and trim take coverage rates this interior flat-latex sheet does not give. Use the data sheet for an exterior product.

Shopping list for common room sizes

The table uses one set of assumptions so you can compare rooms: 8 ft walls, ceiling on, one 20 sq ft door, two 15 sq ft windows, 2 coats, 350 sq ft per gallon, primer off. The wall gallons column is the amount on screen. The wall cans and ceiling paint columns are rounded up to the next quart, with 3 leftover quarts bought as one more gallon.

Room (ft)	Paintable walls (sq ft)	Wall paint (gal)	Wall cans to buy	Ceiling (sq ft)	Ceiling paint to buy
10 × 10	270	1.54	2 gallons	100	1 gallon
10 × 12	302	1.73	2 gallons	120	1 gallon
12 × 12	334	1.91	2 gallons	144	1 gallon
12 × 14	366	2.09	2 gallons and 1 quart	168	1 gallon
14 × 16	430	2.46	2 gallons and 2 quarts	224	1 gallon and 2 quarts
16 × 20	526	3.01	3 gallons and 1 quart	320	2 gallons

A 5-gallon bucket shows up once the wall paint passes 4 gallons and 2 quarts, for example walls of about 790 sq ft or more at 2 coats and 350 sq ft per gallon. The donut on your own result shows the gallons to buy of wall paint, ceiling paint, and primer for the room you typed. A product you do not need is left off the chart.