How the paint estimate works
The method is simple. Add up the area of the walls, subtract the doors and windows, multiply by the number of coats, and divide by how many square feet one gallon covers. The calculator assumes 20 square feet for a standard door and 15 for a window, which is close enough for most rooms. A 14 by 12 foot room with 8 foot walls has 416 square feet of wall. Take off a door and two windows, and you have 366. Two coats at 350 square feet per gallon comes to just over 2 gallons.
Gallons, litres and what Canadian stores sell
Paint in Canada is usually sold in 3.78 litre (one gallon) cans, 946 millilitre quarts, and larger 15 to 18 litre pails. Labels often list coverage in square metres per litre, and the numbers differ slightly between brands, so check the can. A typical interior latex covers about 9 square metres per litre, or 350 to 400 square feet per gallon. If your paint says something different, change the coverage field above.
Do you need primer?
New drywall, bare wood, patched areas and a big colour change all call for a coat of primer. Skip it only if you are repainting a similar colour in sound condition. A primer coat is calculated the same way as paint, using the same area. Many people buy a paint and primer in one, but on fresh drywall a dedicated primer-sealer saves topcoat and gives a more even sheen.
Walls, ceilings and trim
Ceilings are usually painted in flat white. If you are painting them, choose Yes above, and the calculator will add the floor area to the total. Trim and doors are measured separately. A rough rule is a quart of trim paint per room with average casing and baseboards. Semi-gloss or satin finishes hold up better to scuffs than flat, which suits trim, kitchens and bathrooms.
When you need extra paint
Several things push you above the estimate. Rough or textured walls soak up more paint, sometimes 20 percent more. Dark colours over light, and bright reds and yellows, often need a third coat. Painting directly over bare drywall without primer consumes much more. If you are covering a dark colour with a light one, tinted primer saves a lot of topcoat. Always buy a little extra, since matching a tint later is hard when batches vary.
Temperature and timing
Most latex paints want the room at 10 degrees Celsius or warmer and low humidity. In a Canadian winter, that means you are limited by how well the house is heated, and cold basements dry slowly. Check the can for the minimum temperature and allow the recoat time listed before applying a second coat.
Painting a whole house
For several rooms, run the calculator per room and add the totals, then round up to full cans per colour. Using one colour across hallways and living areas lets you buy a 15 litre pail, which is cheaper per litre than gallons. If the walls have drywall repairs, our drywall calculator shows how much compound and sheet to buy before painting.
Frequently asked questions
How much paint do I need for a 12 by 12 room?
With 8 foot walls, one door and two windows, about 1.7 gallons for two coats, so plan on 2 gallons. Add more if you are also painting the ceiling.
How many square feet does a gallon of paint cover?
Most interior latex paints cover 350 to 400 square feet per gallon on smooth, primed walls. Check the can for your product.
How many litres of paint do I need per square metre?
About 0.11 litres per square metre per coat, based on 9 square metres per litre.
Do I paint the ceiling with the same paint?
Usually not. Ceilings generally get flat white paint. You can include the ceiling in the calculator to see the extra amount.
How many coats of paint do I need?
Two coats is the standard. Light over light may need one, and strong colours or new drywall may need three including primer.
Should I subtract windows and doors?
Yes, but only large openings. The calculator subtracts 20 sq ft per door and 15 sq ft per window by default.
Related calculators
Estimates only. See our disclaimer and how we calculate.