Paint Calculator

The most common painting mistake is guessing. Buy too little and you make a second trip mid-project, risking a slightly different batch color; buy too much and you have unopened gallons cluttering the garage. This paint calculator turns a few measurements into a reliable gallon count by doing exactly what a pro does: find the wall area, subtract the doors and windows you are not painting, multiply by the number of coats, and divide by how far a gallon actually goes.

The whole job rests on one number most people get wrong: a gallon of interior paint covers roughly 350 square feet in a single coat, 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.

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 and you would buy 2 gallons with a little to spare - a textbook one-room job.

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 (about 350). Always round the final gallons up to the next whole can, because you cannot buy a fraction of a gallon off the shelf.

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 "total wall length" and add up each wall individually.
2. Ceiling height: measure floor to ceiling. Most homes are 8 ft; older or upscale rooms can be 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.

Press Calculate and 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 and want to buy the right amount once.
• First-time painters who do not yet know that a gallon covers roughly 350 sq ft, 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) - handy for small walls and trim 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 over 350 sq ft coverage is 192 ÷ 350 = 0.55 gallons - a single quart, or one gallon if you want extra for the future.
• 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 fits comfortably 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 - going from one coat to two doubles the paint required.
• Surface texture: rough or new drywall drops coverage to 250-300 sq ft per gallon, 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 coverage, usually a little lower than finish paint at around 200 to 300 sq ft per gallon. 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 modern "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 with a flat finish can reach 400 sq ft, while new drywall, bare wood, textured ceilings, and stucco soak up paint and drop to 250 to 300 sq ft. Sheen plays a smaller role: flat and matte finishes spread slightly further than glossy ones, which is partly why ceilings (usually flat) often stretch a can. Paint quality is the quiet variable - premium paints carry more solids and pigment, so they hide better and frequently cover in two coats where a budget line 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 much will the paint cost?

Once you know the gallon count, a rough budget follows quickly. Interior wall paint runs from about $20 to $35 a gallon for builder-grade lines up to $50 to $80+ for premium brands, with primer landing in the $20 to $40 range. So the textbook two-gallon bedroom above costs roughly $40 to $160 in paint, plus a few dollars for primer if the walls need it. Add a one-time spend on supplies - rollers, brushes, a tray, painter's tape, and drop cloths typically total $30 to $60 if you are starting from scratch - and you have a defensible materials budget before you ever call a painter. 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 premium paint each push the total up. 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, about 87 sq ft per coat), gallons, and five-gallon buckets. The container you choose changes the price per square foot and the convenience. A quart is ideal for a single accent wall, trim, or touch-ups - buying a whole gallon for a 96 sq ft accent wall wastes most of the can. A gallon is the workhorse for one or two rooms. A five-gallon bucket is cheaper per gallon and guarantees 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, a five-gallon bucket is usually both cheaper and safer than four gallons plus a quart from a different mix.

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 - paint is often only 15 to 25% of the bill - 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, lead-paint-era homes that need careful prep, and exterior work 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 for pre-1978 homes.
• U.S. General Services Administration (GSA) - painting and surface-preparation technical guidance.
• Coverage and coat figures follow standard manufacturer can labeling (about 350-400 sq ft per gallon, per coat); always defer to the spread rate printed on your specific product.