Enter your room's dimensions below to find out how many gallons of paint and primer you need, plus a full cost estimate in Canadian dollars — including 13% Ontario HST. Built using the same coverage and pricing figures we use throughout our painting guides.
Room Details
Your Estimate
How This Calculator Works
We calculate total wall area from your room's perimeter and ceiling height, subtract standard door and window sizes, then add a 10% buffer on top for cutting-in waste, touch-ups, and uneven coverage. That buffered area is divided by 350 sq ft per gallon per coat — a conservative coverage figure pulled from Benjamin Moore and Sherwin-Williams product data sheets — to get gallons of paint needed. Primer, when selected, uses a 400 sq ft per gallon coverage rate for a single coat.
Pricing reflects typical 2026 GTA retail pricing at Home Depot Canada, RONA, and Home Hardware for each paint tier, plus 13% Ontario HST on the materials subtotal. This is a planning estimate, not a quote — always round up when buying paint, since running out mid-coat on a drying wall causes visible lap marks.
💡 GTA Tip
Home Depot Canada, RONA, and Home Hardware all colour-match Benjamin Moore and Sherwin-Williams tints at their paint desks. Bring a sample swatch or fabric and ask for a match rather than settling for a close house-brand equivalent — the pigment load on true colour-matched paint is more consistent and often needs one fewer coat.
Frequently Asked Questions
How many gallons of paint do I need for a 12x12 room?
A 12x12 room with 8-foot ceilings has about 384 sq ft of wall area before deducting doors and windows. With two coats and a 10% buffer for touch-ups, that works out to roughly 2 to 2.5 gallons of paint, depending on how many doors and windows the room has. Use the calculator above with your exact dimensions for a precise number.
How much wall area does one gallon of paint cover?
Most interior latex paints cover 350 to 400 sq ft per gallon per coat, though the exact coverage varies by brand and by how porous the wall surface is. This calculator uses 350 sq ft per gallon per coat, a conservative figure from Benjamin Moore and Sherwin-Williams data sheets, plus a 10% buffer for cutting-in waste and touch-ups.
Do I need primer before painting?
Prime if you're painting new drywall, covering a dark colour with a light one, painting over a stain, glossy surface, or patched repairs. If you're simply refreshing a wall with a similar colour in good condition, you can usually skip primer. Primer coverage runs about 400 sq ft per gallon for one coat.
How much does it cost to paint a room in the GTA?
DIY materials for an average bedroom typically run $150 to $350 CAD depending on paint tier, including paint, primer, and basic supplies like tape, rollers, and drop cloths. Hiring a painting contractor in the GTA for the same room usually runs $400 to $800. Remember that 13% HST applies to materials purchased in Ontario.
Should I paint the ceiling too?
Ceilings usually need less frequent repainting than walls, but if yours is stained, yellowed, or you're changing wall colour dramatically, painting it fresh white ties the room together. Ceilings typically only need one coat unless there's visible staining. Check the ceiling box in the calculator above to include it in your estimate.
Related Painting Guides
Once you know how much paint to buy, these guides cover the rest of the job: