Paint Coverage Calculator
How to Use the Paint Coverage Calculator Getting your paint quantity right before going to the hardware store saves two frustrating trips — one when you run out mid-wall, and one returning leftover cans that the store may not accept. This calculator takes your room dimensions and gives you an accurate gallon count with a […]
How to Use the Paint Coverage Calculator
Getting your paint quantity right before going to the hardware store saves two frustrating trips — one when you run out mid-wall, and one returning leftover cans that the store may not accept. This calculator takes your room dimensions and gives you an accurate gallon count with a built-in 10% waste buffer, so you always have a little extra without massively over-buying.
Step 1: Measure Your Room
Measure the length and width of the room in feet. For ceiling height, measure from the floor to the ceiling — most modern homes are 8–9 ft, older homes can be 10 ft or more. Toggle between feet and metres using the unit buttons at the top of the calculator if you prefer metric.
Step 2: Count Doors and Windows
Every door and window is a surface you are not painting. The calculator uses standard deductions — 21 sq ft per door (a standard interior door is approximately 3×7 ft) and 15 sq ft per window (a standard window is approximately 3×5 ft). If your windows or doors are unusually large, count oversized ones as two. These are deducted automatically from the total wall area.
Step 3: Choose What You Are Painting
Select whether you are painting walls only, walls and ceiling together, or ceiling only. The ceiling option uses your room’s floor area (length × width) as the surface to be painted, which is separate from the wall calculation. If you are doing both walls and ceiling in the same colour, selecting “Walls + Ceiling” gives you a single combined gallon count.
Step 4: Set Your Coat Count and Paint Type
Most repaints need 2 coats for good coverage. If you are making a dramatic colour change — going from deep red to white, for example — select 3 coats. For a touch-up over the same colour, 1 coat is often sufficient. The coverage rate changes based on paint quality: economy flat paints cover more area per gallon on smooth surfaces, while premium thick or satin finishes cover slightly less.
Paint Coverage Formula — How the Calculator Works
The formula behind the calculator is the same one professional painters and decorators use:
- Wall area = 2 × (Length + Width) × Height
- Ceiling area = Length × Width (if selected)
- Net area = Paintable area − (Doors × 21 sq ft) − (Windows × 15 sq ft)
- Paint needed (exact) = (Net area × Coats) ÷ Coverage rate (sq ft per gallon)
- Recommended purchase = Exact amount × 1.10 (10% buffer)
For example, a 14×12 ft room with 9 ft ceilings, 2 doors, and 2 windows, using standard paint at 2 coats: Wall area = 2 × (14+12) × 9 = 468 sq ft. Deductions = (2×21) + (2×15) = 72 sq ft. Net area = 396 sq ft. At 350 sq ft/gallon for 2 coats: 396 × 2 ÷ 350 = 2.26 gallons exact. With 10% buffer: 2.49 gallons. Recommendation: buy 3 gallons.
Always buy all the paint you need in one visit. Colours are mixed by machine, and even the same colour code can look slightly different between batches. If you run short and need a second can mixed later, it may not match perfectly.
How Much Does a Gallon of Paint Cover?
The standard industry figure is 350–400 square feet per gallon for a single coat on a smooth, previously painted surface. However, the actual coverage depends on several real-world factors:
Surface Texture
Smooth, sealed surfaces (existing painted drywall) absorb far less paint than rough or porous ones. Bare drywall soaks up paint and may require a primer plus extra topcoats. Rough masonry, brick, or concrete walls can drop coverage to 200–250 sq ft per gallon. Textured ceilings (popcorn or orange peel) also significantly reduce coverage per gallon.
Paint Finish
Flat and matte finishes typically have the highest coverage per gallon because they contain more pigment and less binder. Gloss and semi-gloss finishes are more transparent and often need an extra coat for the same visual opacity. Satin falls in the middle and is the most popular all-purpose finish for interior walls.
Colour Shift
Going from a light colour to a dark one is straightforward — dark colours generally cover well in 2 coats. Going from dark to light is much harder. A deep red or hunter green wall being painted over with white or cream may need 3–4 coats or a tinted primer first. The calculator accounts for this — select 3 coats for dramatic dark-to-light changes.
Choosing the Right Paint Finish for Every Room
Flat / Matte
Flat paint hides wall imperfections beautifully because it absorbs light rather than reflecting it. This makes it ideal for ceilings and adult bedrooms where scrubbability is less of a concern. The downside is that flat paint marks easily and is difficult to wipe clean — avoid it in high-traffic areas. It has the highest coverage per gallon, typically 400+ sq ft.
Eggshell
A slight sheen that is more durable than flat but still low-key enough for most living rooms and dining rooms. Eggshell is far easier to clean than flat — a damp cloth removes most marks — making it a practical choice for family rooms. Coverage is typically 350–380 sq ft per gallon.
Satin
The most versatile interior finish. Satin has a soft, pearl-like sheen that looks sophisticated without being overtly shiny. It is easy to clean and holds up well to moisture, making it excellent for hallways, children’s rooms, and even bathrooms in well-ventilated spaces. Coverage is typically 350 sq ft per gallon.
Semi-Gloss
The standard choice for trim, doors, and window frames because its higher sheen provides extra durability and moisture resistance. Semi-gloss is also commonly used in kitchens and bathrooms where walls need to withstand steam and frequent wiping. It shows surface imperfections more than lower-sheen finishes. Coverage drops to about 300 sq ft per gallon due to its more translucent formula.
Gloss
High-gloss paint is very durable and easy to clean, but it amplifies every bump and imperfection in the wall surface. It is rarely used on large wall areas but works beautifully on cabinetry, furniture, and exterior trim. Coverage is the lowest of all finishes at around 250–300 sq ft per gallon.
Do You Need Primer?
Primer is a separate product from topcoat paint, and knowing when to use it saves both time and money:
Always prime: Bare drywall or plaster, wood surfaces that have never been painted, stained surfaces (water stains, smoke damage, crayon, marker), and any surface where you are making a significant colour change — especially dark to light.
Consider priming: Walls in a room that has not been painted in many years, surfaces with a glossy finish that you want to paint over (the new paint may not adhere without a bonding primer), and walls in high-humidity rooms like bathrooms.
Skip the primer if: You are repainting a wall with a similar colour using a paint-and-primer-in-one product, the walls are in good condition, and you are making a similar-shade colour change.
Paint-and-primer-in-one products have improved dramatically and are now a legitimate single-step option for most standard repaints. However, for problem surfaces, a dedicated primer is still the better investment.
How Much Paint Do I Need for Common Room Sizes?
Here are quick reference figures for common room sizes using standard paint at 2 coats, with 1 door and 2 windows deducted:
- Small bathroom (6×8 ft, 8 ft ceiling): approximately 1.5 gallons — buy 2 gallons
- Bedroom (10×12 ft, 9 ft ceiling): approximately 2.2 gallons — buy 3 gallons
- Master bedroom (13×15 ft, 9 ft ceiling): approximately 2.8 gallons — buy 3 gallons
- Living room (15×18 ft, 9 ft ceiling): approximately 3.5 gallons — buy 4 gallons
- Open plan kitchen/living (20×24 ft, 9 ft ceiling): approximately 5.2 gallons — buy 6 gallons
- Ceiling (12×14 ft room, 1 coat): approximately 0.5 gallons — buy 1 gallon
Use the calculator above for your exact dimensions — these figures are estimates based on standard assumptions.
Tips for a Better Paint Job
Prep Work Is Everything
The difference between a professional-looking paint job and a mediocre one is almost entirely in the preparation. Fill holes and cracks with lightweight spackle, sand smooth, and wipe down walls with a damp cloth to remove dust and grease — especially in kitchens. Paint applied over a dirty or patchy surface will look patchy no matter how many coats you add.
Buy the Right Tools
A good roller makes more difference than the brand of paint. Use a 3/8-inch nap roller for smooth walls, a 1/2-inch nap for lightly textured walls, and a 3/4-inch nap for rough or heavily textured surfaces. Cheap rollers shed fibres and leave a stippled texture. A quality 2.5-inch angled brush for cutting in corners and edges is equally important.
Cut In Before Rolling
Always brush the edges (cutting in) before rolling the main wall area. Cut in along ceilings, corners, trim, and outlets using a good angled brush. Work in sections and roll that section within 10–15 minutes while the cut-in edge is still wet — this wet-edge technique blends the brush and roller work invisibly.
Two Thin Coats, Not One Thick One
A thick coat of paint drips, takes longer to dry, and often looks uneven when it dries. Two thin coats with proper drying time in between (usually 2–4 hours for latex paint, 6–8 hours for oil-based) produce a far more uniform, durable result.
Frequently Asked Questions
How many gallons of paint do I need for a 12×12 room?
A 12×12 ft room with 9 ft ceilings has a wall area of approximately 432 sq ft. After deducting a standard door and two windows (51 sq ft), the net area is about 381 sq ft. At 2 coats with standard paint (350 sq ft/gal): 381 × 2 ÷ 350 = 2.18 gallons. With a 10% buffer: 2.4 gallons. Buy 3 gallons. Use the calculator above with your exact dimensions for a precise result.
How much does a gallon of paint cover?
One gallon of standard interior paint covers approximately 350–400 square feet with a single coat on a smooth, previously painted surface. On rough or textured surfaces, coverage drops to 250–300 sq ft per gallon. Always apply at least 2 coats for a professional finish — so a 400 sq ft room typically needs 2+ gallons of wall paint.
How many coats of paint do I need?
For most standard repaints over a similar or lighter colour, 2 coats gives excellent coverage with a quality paint. If you are going from a dark colour to a significantly lighter one, or painting bare drywall for the first time, use a primer first and then 2 coats of topcoat — or 3 coats of a paint-and-primer-in-one product. A single coat is only appropriate for touch-ups or very similar colour changes.
Should I buy paint in quarts or gallons?
A quart covers approximately 80–100 sq ft with one coat — suitable for very small areas like a powder room (one coat) or a small accent wall. For most rooms, gallons are far more economical per square foot. A gallon typically costs $35–$70 versus $15–$25 for a quart — meaning gallons are 30–40% cheaper per unit of coverage. If the calculator shows you need less than 1 gallon, consider buying 1 gallon plus 1 quart rather than 2 gallons to avoid excess.
How do I calculate paint for a ceiling?
Select “Ceiling only” in the surface dropdown. The calculator uses your room’s floor area (Length × Width) as the ceiling area. Ceilings typically only need 1 coat of ceiling-specific flat white paint, which provides excellent opacity and coverage. If your ceiling is in very poor condition or you are changing colour, use 2 coats. A standard 12×14 ft ceiling needs approximately 0.5 gallons — buy 1 gallon.
How long does it take for paint to dry between coats?
Latex (water-based) paint is typically dry to the touch in 1 hour and ready for a second coat in 2–4 hours. However, in humid or cold conditions, allow 4–6 hours between coats. Oil-based paint requires 6–8 hours between coats and up to 24 hours before the room is back in normal use. Never rush the second coat — applying paint over insufficiently dried paint causes wrinkling, peeling, and an uneven finish.
How much paint do I need for trim and doors?
Trim and door paint is typically bought separately from wall paint since you use a different finish (semi-gloss or gloss for trim vs. eggshell or satin for walls). A quart of trim paint covers all the baseboard, door frames, and window trim in a standard bedroom. For painting doors themselves, allow approximately 1 quart per 2 standard interior doors with 2 coats applied to both sides.