🎨 Paint Calculator
Find how many gallons of paint to buy and what it will cost — from your room or wall dimensions. Subtracts doors and windows, handles multiple coats. Free, no sign-up.
A paint calculator works out how much paint to buy for a room. It finds the wall area, subtracts the doors and windows, multiplies by the number of coats, then divides by the coverage of one gallon and rounds up — because paint is sold in whole gallons. The core formulas are:
How to calculate paint in 3 steps:
- Measure the wall area — a whole room, a single wall, or a total you already know.
- Set the number of coats, the paint type, and how many doors and windows to subtract.
- Click Calculate Paint to see the gallons to buy, paintable area, and total cost.
For example, a 12 ft × 15 ft room with a 9 ft ceiling has 486 square feet of wall. After one door and two windows, 435 square feet is paintable — at two coats that is 3 gallons, about $105 of paint.
Paint Calculator
Enter your room or wall — see how many gallons of paint to buy and the cost.
Buy the Right Amount of Paint –
Enough to Finish, Not a Shelf of Leftovers
Paint is sold by the gallon, and the math is easy to get wrong — forget a coat, or forget the doors and windows, and the estimate is off by a third. This free calculator turns your room size into an exact gallon count, with coats and openings handled for you.
⚡ Try the Calculator NowHow Much Paint? The Math Most People Get Wrong
Paint looks like the simplest material to estimate, and that is exactly why it trips people up. They measure the floor instead of the walls. They forget the second coat. They paint right over where the windows are. Each mistake quietly shifts the number — and one gallon short, halfway up a wall, is a frustrating place to be.
The real estimate has four steps. Find the wall area — that is the room's perimeter times the ceiling height, not the floor. Subtract the doors and windows, because you are not painting those. Multiply by the number of coats. Then divide by what one gallon covers, and round up, since paint comes in whole gallons.
This Paint Calculator runs every step. Enter a whole room, a single wall, or a wall area you have already measured; set the coats and the openings; and it returns the gallons to buy, the paintable area, the total surface to cover, and the cost.
How the Paint Calculator Works
Measure the walls, set the coats, read the gallon count. Here is what each field does.
Pick the Surface
Whole Room measures all four walls from the room's footprint. Single Wall handles one wall — an accent wall, say. Total Wall Area lets you enter a figure you have already measured.
Enter the Dimensions
For a room, enter the length, width, and ceiling height in feet. The calculator finds the perimeter and multiplies by the height to get the gross wall area.
📏 Measure the real ceiling height, not an assumed 8 ft.Set the Number of Coats
Two coats is the default and the norm for an even, lasting finish. Drop to one when refreshing a similar color; expect two or more over bare drywall or a big color change.
🖌️ Coats multiply the area — and the paint.Choose a Paint Type
Interior latex, primer, ceiling, exterior, or premium. Each loads a typical coverage rating and price per gallon as a starting point you can adjust.
Subtract Doors & Windows
In Advanced mode, enter how many doors and windows the walls contain. The calculator subtracts a standard area for each so you are not paying to paint glass and panels.
Calculate — Read the Gallons
Instantly see the gallons to buy, the paintable area, the total surface across all coats, the exact gallon figure, and the cost.
✅ Gallons = round up(Paintable × Coats ÷ Coverage)The Paint Formula, Explained
Four steps turn a room measurement into a gallon count. The arithmetic is light; the discipline is in not skipping a step.
Step one: the wall area. Walls, not floor. Add the room's length and width, double that for the perimeter, then multiply by the ceiling height. A 12-by-15-foot room has a perimeter of 2 × (12 + 15) = 54 feet; at a 9-foot ceiling that is 54 × 9 = 486 square feet of wall.
Step two: subtract the openings. You do not paint doors and windows. A standard door is about 21 square feet, a typical window about 15. One door and two windows remove 21 + 30 = 51 square feet, leaving 486 − 51 = 435 square feet of paintable wall.
Step three: multiply by the coats. A single coat rarely covers evenly, so two is standard. Two coats over 435 square feet means painting 435 × 2 = 870 square feet of surface in total. This is the step most rushed estimates forget — and forgetting it halves the answer.
Step four: divide by coverage, then round up. Take the total surface and divide by what one gallon covers. At 375 square feet a gallon: 870 ÷ 375 = 2.32 gallons. Paint sells in whole gallons, so you round up to 3 gallons.
Those 3 gallons cover 1,125 square feet — comfortably past the 870 you need. The surplus is deliberate: it is the paint you keep for touch-ups, and it is the reason a job never stalls one roller-width short of the corner.
Coverage, Coats & Price Tables
Typical figures for common paint types. Use them as a starting point, then confirm against the can you actually buy.
| Paint Type | Typical Coverage / Gallon | Typical Price / Gallon |
|---|---|---|
| Interior latex (walls) | ~375 ft² | $30–$45 |
| Ceiling paint | ~350 ft² | $25–$40 |
| Exterior paint | ~350 ft² | $40–$60 |
| Primer | ~300 ft² | $22–$35 |
| Premium / designer | ~400 ft² | $50–$75 |
The core formula in plain words: perimeter times ceiling height gives the wall area; subtract doors and windows; multiply by coats; divide by gallon coverage; round up. A 486 square foot room, two coats, needs 3 gallons.
How Many Coats to Plan For
| Situation | Coats | Why |
|---|---|---|
| Repaint, similar color | 1–2 | The old color helps the new one cover |
| Standard color change | 2 | The reliable default for an even finish |
| Bare drywall | 2 + primer | Raw surface needs sealing first |
| Dark over light, or light over dark | 2–3 | Strong color shifts need extra build |
Paint for Common Room Sizes
| Room (9 ft ceiling) | Wall Area | Gallons (2 coats) |
|---|---|---|
| 10 × 10 ft | 360 ft² | 2 gallons |
| 12 × 12 ft | 432 ft² | 3 gallons |
| 12 × 15 ft | 486 ft² | 3 gallons |
| 15 × 20 ft | 630 ft² | 4 gallons |
| 20 × 25 ft | 810 ft² | 5 gallons |
These assume two coats over gross wall area before subtracting openings. Take out the doors and windows and a room often drops by close to a gallon.
Factors That Change How Much You Need
Two rooms with the same floor plan can need different amounts of paint. These are the factors that move the count.
Paint Types Compared
The five types in this calculator suit different jobs. Here is what each is for.
| Type | Best For | Coverage | Notes |
|---|---|---|---|
| Interior latex | Bedroom, living-room walls | ~375 ft² | The everyday choice; easy to apply and clean up |
| Primer | Bare drywall, patches, stains | ~300 ft² | A base coat, not a finish — seals before the topcoat |
| Ceiling paint | Ceilings | ~350 ft² | Flat, low-spatter, often dries to show missed spots |
| Exterior paint | Siding, outdoor walls | ~350 ft² | Built to handle sun, rain, and temperature swings |
| Premium / designer | Bold colors, fewer coats | ~400 ft² | Costs more per gallon; can save a coat of labor |
How to Plan & Buy Paint: A Phase-by-Phase Roadmap
From an empty room to paint cans on the drop cloth, in four phases.
Measure the room's length, width, and ceiling height. Count the doors and windows. The Whole Room mode turns the footprint and height into a gross wall area for you.
Choose the number of coats for your situation and a paint type. A big color change or bare drywall pushes you toward two coats plus primer; a simple refresh may need less.
Pick a specific product and read its coverage rating and price. Enter those, set the doors and windows, and calculate to get the gallon count and total cost.
Buy all the gallons at once so they are tinted from the same batch. If you need several, ask the store to box them — combine and re-stir — so the color is perfectly uniform.
What a Paint Job Costs
Paint itself is rarely the expensive part. Knowing the full breakdown helps you budget — and decide whether to roll it yourself.
The paint cost is simple: gallons needed, times the price per gallon. A typical bedroom takes 2 to 3 gallons, so the paint alone is often $70 to $150. What surrounds it adds up — primer, brushes and rollers, painter's tape, drop cloths, trays, and sandpaper or filler for prep.
Labor is the real variable. A professional crew prices by the room or the square foot, and on an interior job the labor frequently outweighs the materials several times over. Wall painting is also one of the most approachable DIY jobs, which is why it is a common place to save — if you have the time and patience for the prep.
Example Paint Calculations
Three jobs, three surfaces — each worked through with verified math so you can check your own.
Buying Paint Without Running Short — or Over
A good paint order has enough to finish, in one matching batch, with a little kept back. These habits get you there.
The most common paint mistake is the forgotten second coat. People measure carefully, then divide by gallon coverage as if one pass will do — and end up halfway through the room with an empty can. Two coats is the default for a reason: it is what an even, durable finish actually takes. Build the coats into the math from the start.
The second habit is treating color consistency as part of the order. Tinted paint can vary slightly between mixes, so when a job needs several gallons, buy them together and ask the store to box them — pour them into one large container and re-stir. One uniform batch means no faint color shift where you switched cans mid-wall.
Six Habits for a Clean Paint Order
When This Calculator Is the Wrong Tool
The arithmetic here is exact, but a real paint job has variables a calculator cannot see. Here is where the output needs judgment.
1. It estimates wall paint only. The calculation covers the walls. Ceilings, baseboards, door and window casings, and crown molding are separate jobs — usually a different paint and finish — and are not included in the gallon count.
2. Coverage ratings are ideals. The 350-to-400 square feet per gallon figure assumes a smooth, primed, mid-tone surface. Real coverage drops on textured, porous, or unprimed walls, and when covering a strong color. On a tough surface, the true gallon count can run higher.
3. Door and window sizes are standardized. The calculator subtracts a typical area for each door and window. Oversized picture windows, sliding glass doors, or French doors remove more wall than the standard figures assume — measure those and adjust.
4. It does not plan the prep or the job. Filler for holes, sanding, caulk, the order of cutting in and rolling — none of that is in the gallon count. Good prep is most of a good paint job, and it is its own separate plan.
Where to go instead: To measure an irregular wall before estimating paint, a square footage calculator handles odd shapes. For the floor of the same room — flooring is measured differently — a flooring calculator is the right tool. This calculator's job is the wall paint once the dimensions are known.
Painting Terms You'll See On This Page
Quick reference for the painting terms used throughout this calculator.
- Wall Area
- The total surface of a room's walls — the perimeter multiplied by the ceiling height.
- Paintable Area
- The wall area left after subtracting the doors and windows you are not painting.
- Coverage
- How many square feet one gallon of paint covers in a single coat, printed on the can.
- Coat
- One full application of paint over a surface. Two coats is standard for an even finish.
- Primer
- A base coat that seals a porous or stained surface so the topcoat covers evenly.
- Perimeter
- The total distance around the inside of a room — twice the sum of its length and width.
- Cutting In
- Painting the edges and corners by brush, where a roller cannot reach cleanly.
- Boxing
- Combining several gallons of paint into one container and re-stirring for a uniform color.
- Sheen
- How glossy a paint dries — flat, eggshell, satin, semi-gloss — affecting look and durability.
- Latex Paint
- Water-based paint, the standard for interior walls; easy to apply and clean up.
- Spread Rate
- Another term for coverage — the area a given amount of paint will cover.
- Dry Time
- How long paint needs before it can be recoated or the surface used again.
- Touch-Up
- A small repair to a painted surface, best done with leftover paint from the original job.
- Drop Cloth
- A protective sheet laid over floors and furniture to catch drips and spatter.
Frequently Asked Questions
The most common questions about estimating and buying paint.
How do I calculate how much paint I need?
Find the wall area by multiplying the room's perimeter by the ceiling height. Subtract the area of doors and windows to get the paintable area, then multiply by the number of coats. Divide that by the coverage of one gallon — around 350 to 400 square feet — and round up to whole gallons.
How many square feet does a gallon of paint cover?
A gallon of interior paint typically covers about 350 to 400 square feet in one coat on a smooth, primed surface. Primer covers a little less, around 300 square feet. Rough, porous, or unprimed surfaces absorb more paint, so coverage drops and you should plan for the lower end of the range.
How many coats of paint do I need?
Two coats is the standard for a durable, even finish, and the calculator defaults to two. One coat may be enough when repainting a similar color over a sound surface. Expect to need two or more coats when covering a dark color, painting over bare drywall, or making a big color change.
How much paint do I need for a 12x15 room?
A 12 by 15 foot room with a 9 foot ceiling has about 486 square feet of wall. After subtracting one door and two windows, roughly 435 square feet is paintable. At two coats that is 870 square feet to cover, which needs about 3 gallons of paint.
Should I subtract doors and windows when calculating paint?
Yes, for an accurate estimate. A standard door is about 21 square feet and a typical window about 15 square feet. Subtracting them from the wall area avoids over-buying. For a quick rough estimate you can skip this, since the leftover paint is useful for touch-ups.
How much does it cost to paint a room?
Multiply the gallons you need by the price per gallon. Interior wall paint runs roughly $30 to $50 a gallon, and premium paint more. A typical bedroom needs about 2 to 3 gallons, so the paint alone is often $70 to $150, before primer, supplies, or labor.
Do I need primer before painting?
Primer is worth it on bare drywall, patched areas, stained surfaces, or when making a dramatic color change. It seals the surface so the topcoat covers evenly and may save a coat. Repainting a similar color over a sound, previously painted wall often does not need a separate primer.
How do I measure a room for painting?
Measure the length and width of the room and add them, then double the total for the perimeter. Multiply the perimeter by the ceiling height to get the gross wall area. The Room mode in this calculator does this for you — just enter length, width, and height.
How much paint do I need for one wall?
Multiply the wall's width by its height for its area, subtract any window or door, then multiply by the number of coats. Divide by the gallon coverage. A 15 by 9 foot accent wall with one window needs about 240 square feet covered at two coats — roughly one gallon.
Why does the calculator round up to whole gallons?
Paint is sold in whole gallons and quarts, so an estimate of 2.3 gallons means buying 3. Rounding up guarantees you can finish the job without a second trip. The small surplus is also useful — keep it sealed for touch-ups, since an exact color match later is hard to find.
Does ceiling height affect how much paint I need?
Yes, directly. Wall area is the perimeter multiplied by the ceiling height, so a room with a 9 foot ceiling needs noticeably more paint than the same room with an 8 foot ceiling. Always measure the actual ceiling height rather than assuming a standard.
How accurate is this paint calculator?
The wall area, paintable area, coverage, and gallon count are exact arithmetic from the figures you enter. Accuracy depends on your measurements and on using the real coverage rating of your paint. Surface texture and absorbency affect real-world coverage, so treat the result as a solid planning estimate.
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