Paint Calculator
- Updated Jul 2026
This free paint calculator works out exactly how much paint you need for any room, wall, ceiling or exterior in gallons, quarts and litters. One gallon covers roughly 350–400 square feet in a single coat. Enter your dimensions, subtract doors and windows, and get your paint quantity and cost in seconds.
Table of Contents
Paint Calculator: Exact Gallons in 4 Easy Steps
Enter Your Details
Fill in the fields, then press Calculate.
Fill in your information on the left to see your result.
Results are estimates for planning purposes. Verify quantities for structural or safety-critical work.
How to use this paint calculator
Buying too little means a second trip and a risk of mismatched batches; buying too much wastes money. The paint calculator removes the guesswork in four steps:
- Enter the room or wall dimensions — length, width and height in feet.
- Subtract openings. Enter the number of doors and windows; the paint calculator deducts a standard 21 sq ft per door and 15 sq ft per window.
- Set coats and coverage. Two coats is standard; coverage defaults to 350 sq ft per gallon.
- Add a price per gallon for an instant material cost.
Results come back in gallons and quarts, rounded up to what you’d actually buy, so you’re never left with an unbuyable figure like 1.4 gallons.
Paint Calculation formula: how much paint do I need?
Every estimate the paint calculator makes comes down to one formula:
Gallons = (Wall area − Openings) × Coats ÷ Coverage per gallonWall area is the perimeter of the room multiplied by its height. Subtract the doors and windows, multiply by the number of coats, then divide by the coverage rate printed on your paint can — usually 350 to 400 square feet per gallon. That’s the entire calculation, and it’s the same whether you’re painting one wall or a whole house.
The number to remember: 1 gallon ≈ 350–400 sq ft per coat. A 12×12 room needs about 2 gallons for two coats.
How to measure a room for paint
Accurate measurement is where the paint calculator earns its keep. Here’s what to measure:
- Measure the perimeter. Add the length of every wall you’re painting. A 12 × 12 room is (12 + 12 + 12 + 12) = 48 feet.
- Multiply by ceiling height. 48 × 8 = 384 square feet of wall.
- Subtract openings. One door (21 sq ft) and two windows (30 sq ft) leaves 333 square feet.
- Multiply by coats, divide by coverage. 333 × 2 ÷ 350 = 1.9 gallons — round up to 2.
For irregular rooms, split the space into sections and total them, or find each wall’s area with our square footage calculator first, then drop the total into the paint calculator.
Use the first when your length is in feet, and the second when every dimension is in inches. Both give the same answer. If your measurements are in centimetres, run them through our cm to inches converter first, then use the board foot calculator above.
How much paint do I need for a room? Quick chart
These are the figures people search for most. All assume 8-foot ceilings and two coats on the walls, with openings already deducted.
| Room size | Wall area | Walls (2 coats) | + Ceiling |
|---|---|---|---|
| 8 × 8 (small bedroom) | ~256 sq ft | 1.5 gallons | 2 gallons |
| 10 × 10 | ~320 sq ft | 2 gallons | 2.5 gallons |
| 12 × 12 (bedroom) | ~384 sq ft | 2 gallons | 2.5 gallons |
| 12 × 16 (living room) | ~448 sq ft | 2.5 gallons | 3 gallons |
| 16 × 20 (great room) | ~576 sq ft | 3.5 gallons | 4 gallons |
| Whole house interior (1,500 sq ft home) | ~4,500 sq ft | 13–14 gallons | 16–18 gallons |
So the most-asked question, how much paint for a 12×12 room, comes out at about 2 gallons for two coats, or 2.5 with the ceiling. Going from a light colour to a dark one, or covering bare drywall, pushes every figure up; the sections below show by how much.
Buy roughly 10% extra. Touch-ups, missed spots and future repairs all need matching paint, and getting an exact colour match later is difficult once the batch changes.
Paint coverage per gallon by surface
The single biggest variable in any paint calculator is the surface itself. A rough or porous wall has far more actual surface area than a smooth one, so it drinks more paint for the same square footage.
| Surface | Coverage per gallon | Notes |
|---|---|---|
| Smooth drywall / plaster | 400 sq ft | Best-case interior coverage |
| Textured walls or ceilings | 350 sq ft | Popcorn and knockdown use more |
| Wood siding | 275 sq ft | Exterior, smooth |
| Brick | 200 sq ft | Porous — soaks up paint |
| Stucco | 175 sq ft | Most paint-hungry common surface |
| New / bare drywall | Add 30% | First coat absorbs heavily — prime first |
Enter the figure from your paint can into the paint calculator’s coverage field; the 350 default is a safe middle ground, but checking the label gives the sharpest estimate.
How many coats of paint do you need?
| Situation | Coats | Primer? |
|---|---|---|
| Same colour refresh | 1–2 | No |
| Light to light colour change | 2 | Optional |
| Light to dark | 2–3 | Tinted primer helps |
| Dark to light | 2–3 | Yes — primer saves a coat |
| Bare drywall or wood | 2 + primer | Yes |
| Stains or water marks | 2 + stain-block primer | Yes |
Two coats is the realistic default for almost any job, which is why the paint calculator assumes it. A single coat only works for a same-colour touch-up with premium paint.
Do I need primer?
Primer isn’t paint, and it covers less — about 200 to 300 square feet per gallon. Use the same formula in the paint calculator with that lower coverage rate to size your primer. You need it when:
- Painting bare drywall, wood, or masonry for the first time
- Making a dramatic colour change, especially dark to light
- Covering stains, smoke damage or water marks (use a stain-blocking primer)
- Painting over glossy or slick surfaces
A tinted primer close to your topcoat colour can save an entire finish coat on a big colour change — often cheaper than the extra gallon of premium paint it replaces.
Does spraying use more paint than rolling?
Yes, noticeably. A paint sprayer atomises paint and loses some to overspray and air, so it uses roughly 25–30% more paint than a roller for the same surface. If you’re spraying rather than rolling, increase the paint calculator’s result by about a quarter.
| Method | Paint use | Best for |
|---|---|---|
| Roller | Baseline | Walls, ceilings, most interiors |
| Brush | Baseline | Trim, edges, cutting in |
| Sprayer | +25–30% | Large areas, exteriors, fast coverage |
Yes, noticeably. A paint sprayer atomises paint and loses some to overspray and air, so it uses roughly 25–30% more paint than a roller for the same surface. If you’re spraying rather than rolling, increase the paint calculator’s result by about a quarter.
Ceiling, exterior and trim paint
Ceiling paint
Ceiling area is simply room length × width — a 12 × 12 ceiling is 144 square feet. Ceilings are usually textured, so use the 350 sq ft coverage rate, and most take two coats. That 144 sq ft room needs a bit under a gallon for two coats.
Exterior paint
Exterior walls use the same perimeter × height formula, but coverage drops because siding, brick and stucco are rougher — 175 to 275 square feet per gallon. Exteriors almost always need two coats plus primer on bare areas. For a whole house, our square footage calculator helps total the wall area before you run the paint calculator.
Trim, doors and baseboards
Trim is measured by length, not area. A rough rule is that the trim, doors and baseboards in an average room take about a quarter of the wall paint — so a room needing 2 gallons for walls needs roughly a quart for trim. Doors take about 1/6 gallon each per coat.
Buy trim paint by the quart. You rarely need a full gallon, and trim is often a different sheen (semi-gloss or gloss) from the walls anyway.
Paint finishes and sheen
Sheen doesn’t change how much paint you need, but it changes which paint goes where — and buying the wrong finish means repainting.
| Finish | Durability | Best for |
|---|---|---|
| Flat / matte | Low | Ceilings, low-traffic adult bedrooms, hiding flaws |
| Eggshell | Medium | Living rooms, dining rooms, bedrooms |
| Satin | Medium-high | Hallways, kids' rooms, family areas |
| Semi-gloss | High | Kitchens, bathrooms, trim, doors |
| Gloss | Highest | Trim, cabinets, high-wear surfaces |
Paint can sizes and coverage
| Can size | Covers (1 coat) | Good for |
|---|---|---|
| Quart (0.25 gal) | ~90 sq ft | Trim, touch-ups, accent walls |
| 1 gallon | 350–400 sq ft | A small room, one coat |
| 5 gallons | 1,750–2,000 sq ft | Whole-house or multi-room jobs |
This answers two common searches directly: a quart covers about 90 square feet, and 5 gallons covers up to 2,000 square feet — enough for the walls of a small house in one coat.
How much does it cost to paint a room?
| Item | Typical cost | Notes |
|---|---|---|
| Interior paint | $20–$60 per gallon | Budget to premium |
| Primer | $15–$40 per gallon | Covers less than paint |
| DIY — average room | $50–$150 | Paint and supplies only |
| Professional — per room | $300–$1,000 | Labour included |
| Professional — per sq ft | $2–$6 | Walls; more with trim and ceiling |
| Whole house interior | $2,000–$6,000 | Varies by size and prep |
Enter your paint price into the paint calculator for the material total. If a store is running a promotion, our discount calculator shows what the sale saves across every gallon you need.
Common paint estimating mistakes
- Forgetting the second coat. Most jobs need two — halving your estimate leaves you short.
- Not subtracting openings — or subtracting too much. Deduct doors and large windows, but ignore tiny ones.
- Using 400 sq ft on a textured wall. Match the coverage rate to the actual surface.
- Skipping primer on bare or stained surfaces, then needing three topcoats instead of two.
- Ignoring the spray penalty — add 25–30% if spraying.
- Buying the exact amount with nothing left for touch-ups.
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Frequently asked questions
How much paint do I need for a room?
Measure the perimeter, multiply by ceiling height, subtract doors and windows, then multiply by coats and divide by 350. A typical 12×12 room needs about 2 gallons for two coats.
How much paint do I need for a 12x12 room?
About 2 gallons for two coats on the walls, or 2.5 gallons including the ceiling, assuming 8-foot walls.
How many square feet does a gallon of paint cover?
Roughly 350 to 400 square feet per coat on smooth drywall, dropping to 175–275 on textured, porous or exterior surfaces.
How much does 5 gallons of paint cover?
About 1,750 to 2,000 square feet in one coat — enough for the walls of a small home.
Do I need primer?
Yes for bare drywall, wood or masonry, big colour changes, stains, or glossy surfaces. A same-colour refresh usually doesn’t need it.
How many coats of paint do I need?
Two for most jobs. Dramatic colour changes may need three, while a same-colour touch-up with premium paint can take one.
Does spraying use more paint than rolling?
Yes, about 25 to 30 percent more, because of overspray and atomisation. Add a quarter to your estimate if spraying.
How much paint do I need to paint a house interior?
A 1,500 sq ft home needs roughly 13 to 14 gallons for the walls in two coats, or 16 to 18 including ceilings.
Sources and further reading: Coverage and application guidance on this page aligns with Paint Quality Institute application standards and manufacturer data from brands such as Sherwin-Williams coverage guidance. Always check the coverage rate printed on your specific paint can, as it varies by product and sheen.