Paint Calculator

Plan on about 1 gallon of paint per 350 square feet of wall, per coat, or whatever spread rate your can lists. For a typical 12 x 12 ft bedroom with 8 ft ceilings, one door and two windows, the paintable area is about 333 sq ft, so two coats cover 666 sq ft ÷ 350 = 1.9 gallons: buy 2 gallons, plus roughly 10% extra for touch-ups. This paint calculator does that math for any room: it finds the wall area, subtracts the doors and windows you are not painting, multiplies by the number of coats, and divides by how far a gallon actually goes.

The whole job rests on one number most people get wrong: a gallon of interior paint covers a few hundred square feet in a single coat (the exact figure is on the label), not "a room." A guest bath and an open-plan living room are both "a room," yet one might take a quart and the other three gallons. Once you switch from counting rooms to measuring square feet, the estimate stops being a gamble. If you only need the wall area itself (to compare quotes, order wallpaper, or price out drywall), the Square Footage Calculator gives you the bare area, and this tool layers the paint-specific math (openings, coats, coverage, and a waste buffer) on top.

Paint is often one line in a larger renovation, and each surface has its own tool. Reach for this paint calculator whenever you are covering interior walls; use the Concrete Calculator for slabs and footings, the Roofing Calculator to size shingles and the Roof Pitch Calculator to find your roof's slope and area multiplier, and the Mulch Calculator for landscaping beds.

A worked example

Take a standard bedroom that is 12 ft long and 10 ft wide with 8 ft ceilings, one door and two windows. The walls run a perimeter of 2 × (12 + 10) = 44 feet. Multiply by the 8 ft height and the gross wall area is 352 sq ft. Now subtract the openings: one door at 21 sq ft and two windows at 15 sq ft each is 21 + 30 = 51 sq ft removed, leaving a net paintable area of 301 sq ft. For two coats you are covering 301 × 2 = 602 sq ft. At 350 sq ft per gallon that is 602 ÷ 350 = 1.72 gallons, which rounds up to 2 gallons. Add 10% for waste and touch-ups (about 1.9 gallons) and the 2 gallons still leave a little to spare: a textbook one-room job, and the calculator's default inputs.

How much paint do I need for a room? (by size)

The table below runs the exact formula for the most common room sizes. All rows assume 8 ft ceilings, one door, two windows, and 350 sq ft per gallon. "Buy (2 coats)" is the exact gallon count rounded up to whole cans.

The formula

Every paint estimate comes down to a single chain of arithmetic:

where wall length is the total horizontal run of every wall in feet (the room perimeter, or the sum of the walls you are painting), height is the ceiling height in feet, coats is how many times you will roll the wall, and coverage is how many square feet one gallon covers in a single coat (from the label; 350 by default). Always round the final gallons up to the next whole can, because you cannot buy a fraction of a gallon off the shelf.

How many gallons of paint per square foot?

If you already know your paintable wall area, use this quick-reference chart. Values are computed at 350 sq ft per gallon and rounded up to whole gallons for each coat count.

How to measure your room

You only need a tape measure and a minute. Work through the inputs in order:

1. Wall length: for a simple rectangular room, enter the length and width and the calculator uses the perimeter automatically. For odd shapes, switch to "Wall run" and add up each wall individually.
2. Ceiling height: measure floor to ceiling. Many homes have 8 ft ceilings; others are 9 or 10 ft. Convert inches to feet by dividing by 12 (a 9 ft 6 in wall is 9.5).
3. Doors and windows: count them. Each door removes about 21 sq ft and each window about 15 sq ft of paintable wall.
4. Coats: choose 2 for most jobs, 1 for a refresh in the same color, 3 for big color changes or vivid hues.
5. Coverage (optional): leave it at 350 unless the can says otherwise or your surface is rough and porous.

The results update as you type: read the whole-gallon number at the top, then check the area breakdown and the coats table below it.

Who this calculator is for

• DIY homeowners repainting a bedroom, living room, or hallway who want to buy the right amount once.
• First-time painters who do not yet know that a gallon covers a few hundred square feet, not "a room."
• Renters doing an accent wall or a move-out repaint on a budget.
• Landlords and flippers estimating paint across several similar rooms quickly.
• Anyone pricing a job who needs a defensible gallon count before getting quotes from a painter.

Key terms explained

• Coverage (spread rate): the square footage one gallon paints in a single coat, printed on every can. The default here is 350 sq ft.
• Paintable area: the wall area left after removing doors, windows, and anything else you will not roll.
• Coat: one complete application over the surface. Hide improves with each coat, which is why two is standard.
• Cut-in: brushing the edges and corners a roller cannot reach; it consumes a little extra paint that the 10% buffer covers.
• Waste factor: the extra you buy (about 10%) to absorb uneven coverage, drips, roller loading, and touch-ups.
• Quart: one quarter of a gallon (about 87 sq ft per coat at 350 sq ft per gallon), handy for trim, a door or touch-ups instead of buying a whole gallon.

Three common scenarios

• Single accent wall: a 12 ft wall at 8 ft is 96 sq ft. Two coats at 350 sq ft per gallon is 192 ÷ 350 = 0.55 gallon. That is more than two quarts (0.5 gallon), so buy one gallon and keep the rest for touch-ups.
• Average living room: a 16 × 14 ft room (60 ft perimeter) with 8 ft ceilings (480 sq ft gross), two doors and three windows nets about 393 sq ft. Two coats need 786 ÷ 350 = 2.25 gallons, so buy 3 gallons.
• Open-plan great room: 70 ft of wall at 10 ft ceilings (700 sq ft gross), minus four windows and two doors, nets about 598 sq ft. Two coats is 1,196 ÷ 350 = 3.4 gallons, so buy 4 gallons; the 10% buffer (3.76 gallons) fits inside the rounding.

What changes the result the most

If you adjust the inputs you will notice a few factors dominate the gallon count:

• Number of coats: the biggest lever, since going from one coat to two doubles the paint required.
• Surface texture: rough or new drywall pushes coverage toward the low end of the label range, raising the count noticeably.
• Color change: covering dark with light (or a vivid hue) can demand a third coat or a tinted primer.
• Ceiling height: a 10 ft ceiling adds 25% more wall area than an 8 ft ceiling for the same floor plan.
• Openings: a room full of large windows and doors can shave a meaningful chunk off the paintable area.

Tips for an accurate, waste-free job

• Always buy about 10% extra. Touch-ups, cut-in waste, and a heavier second coat add up; a leftover can in the exact color is worth its shelf space.
• Buy all your gallons at once and have them boxed (mixed together) if you bought several, so the color is identical wall to wall.
• Use the coverage number on the can, not a rule of thumb, especially for premium one-coat paints or budget lines that spread thinner.
• Prime bare or patched surfaces first; it improves hide and can save you a finish coat.
• Estimate trim and ceilings separately: they use different paints and have different areas.

Primer: when you need it and how to estimate it

Primer is a separate product with its own spread rate on the label, often lower than a finish paint's. It is not always required, but it earns its place in four situations: bare drywall or raw wood that would otherwise drink up an expensive finish coat; a big color change (especially dark to light), where a tinted primer can save you a third finish coat; patched, stained, or smoke-damaged walls that need a sealing base to hide blotches; and any glossy surface that needs a "grip" coat so the new paint adheres. To estimate primer, treat it as one full coat: take the same paintable area, divide by the primer's coverage, and round up. In the calculator, you can re-run the numbers by temporarily setting the coverage field to your primer's spread rate and the coats field to 1, which gives you the primer gallons separately from the finish gallons. Many "paint-and-primer-in-one" products let you skip a dedicated primer on previously painted walls in good condition, but they are no substitute for true primer on bare or problem surfaces.

Coverage, sheen, and quality: why the 350 default moves

The 350 sq ft default is a sensible middle estimate, but the real spread rate shifts with three things. Surface matters most: smooth, sealed drywall lands near the top of the range printed on the label, while new drywall, bare wood, textured ceilings, and stucco soak up paint and land near the bottom. Sheen plays a smaller role: flat and matte finishes often spread a little further than glossy ones. Paint quality is the quiet variable: paints with more solids and pigment hide better and may cover in two coats where a thinner paint needs three, which can make the "expensive" gallon cheaper per finished wall. The single rule that beats every rule of thumb: read the coverage number printed on the can you actually buy, and type that into the calculator instead of the default.

How surface texture changes the gallons

Coverage is the biggest hidden variable. This table holds the job fixed (a 12 x 12 ft room, 333 sq ft paintable, two coats, 666 sq ft total) and only changes the spread rate across a typical label range. The rougher the surface, the more paint it drinks.

How much will the paint cost?

Once you know the gallon count, the paint budget is simple arithmetic: multiply the whole gallons by the shelf price of the product you chose, add primer if the walls need it, and add supplies (rollers, brushes, a tray, painter's tape and drop cloths) if you are starting from scratch. For example, at a hypothetical $40 per gallon, the two-gallon bedroom above costs $80 in paint; plug in the price on the shelf to get your own figure. Because the gallon count drives the bill, the same levers that raise the paint also raise the cost: a third coat, a porous surface that cuts coverage, or a jump to a premium line. If you are buying flooring, tile, or other materials for the same project, the Tile Calculator and Square Footage Calculator help you price those line items the same way.

Gallons vs. quarts: buying the right container

Paint is sold in quarts (a quarter gallon), gallons, and five-gallon buckets. The container you choose changes the price per square foot and the convenience. A quart covers about 87 sq ft per coat at 350 sq ft per gallon, so it suits trim, a door, or touch-ups; a 96 sq ft accent wall with two coats needs about 0.55 gallon, which is already more than two quarts. A gallon is the workhorse for one or two rooms. A five-gallon bucket is often priced lower per gallon and gives you a single consistent batch, which is worth it for whole-house jobs, large open-plan rooms, or any time color uniformity across many walls matters. If your estimate lands at, say, 4.2 gallons, compare the bucket with four gallons plus a quart: the bucket keeps every wall in one batch.

Should you paint it yourself or hire a pro?

The gallon count is the same whether you roll the walls or a contractor does, but it anchors the decision. A DIY one-room repaint is mostly the cost of paint and supplies plus a weekend of your time; the calculator's materials estimate is essentially your whole budget. A professional quote, by contrast, is dominated by labor, so painters typically charge by the square foot or by the room. Knowing the paintable area and gallon count from this tool lets you sanity-check a quote: if a painter's paint line item assumes far more gallons than your measured area needs, ask why. Large, high, or heavily textured walls, exterior work, and homes built before 1978 (where paid painters must follow the EPA's lead-safe Renovation, Repair and Painting Rule) are the jobs where hiring out most often pays off; a flat-ceilinged single room in good condition is the classic DIY candidate.

Limitations and assumptions

This is a planning estimate, not a guarantee. Keep these assumptions in mind:

• It estimates wall paint only: ceilings, trim, doors, and frames are separate jobs and products.
• It uses standard opening sizes (21 sq ft per door, 15 sq ft per window); your actual openings may differ.
• It assumes uniform coverage; real surfaces absorb unevenly, which is exactly why the 10% buffer exists.
• It does not account for primer, which is a separate coat with its own coverage; re-run the numbers with a primer coverage value if needed.
• Color, sheen, and brand all affect hide and spread rate; the can's stated coverage always wins over the default.

Related materials and calculators

Painting is usually one step in a larger project. If you have a different question, a sister tool fits better:

• To measure any area before you start, use the Square Footage Calculator.
• To estimate flooring, backsplash, or wall tile, use the Tile Calculator.
• For pours, slabs, and footings, use the Concrete Calculator and Cubic Yard Calculator.
• For landscaping fill, use the Gravel Calculator and Mulch Calculator.

Sources

• U.S. Environmental Protection Agency (EPA): Lead Renovation, Repair and Painting (RRP) Program (certification and lead-safe work practices for paid work that disturbs paint in homes built before 1978).
• National Institute of Standards and Technology (NIST): NIST Handbook 44 (2026), Appendix C, General Tables of Units of Measurement (PDF) (1 gallon = 4 quarts = 231 cubic inches).
• Coverage figures: use the spread rate on your product's label or technical data sheet. The 350 sq ft per gallon default and the 21 and 15 sq ft door and window deductions are this calculator's planning assumptions, not product data.