Running out of paint halfway through a room is frustrating. Buying too much? Wasteful. Whether you're freshening up a bedroom or painting your whole house in Pickering or Whitby, estimating paint quantities correctly is the first step to a smooth project.

Here’s how we at Altona Painting calculate paint needs to keep every job efficient, clean, and on budget — and how you can too.

1. Measure the Walls

Start by measuring the length and height of each wall. Multiply those together to get the square footage.

Example:
A wall that’s 12 feet long and 8 feet tall = 96 sq ft.

Do this for all walls in the room.

2. Subtract Doors and Windows

Paint doesn't go over doors and windows (unless you're painting trim — in which case, that’s a separate calculation).

• A standard door = ~20 sq ft

• A window = ~15 sq ft

Subtract these from your wall total to get net paintable area.

3. Add It Up

Once you’ve calculated the square footage for all walls and subtracted windows and doors, add everything together to find your total surface area.

4. Know Your Coverage Rate

Most paints cover about 350 to 400 square feet per gallon.
Check the label or product specs — some primers and specialty paints have different coverage rates.

5. Do the Math

Divide your total surface area by the coverage rate.
Example:
If you’re painting 800 sq ft with a paint that covers 350 sq ft per gallon:
800 ÷ 350 = 2.29 gallons

Round up — in this case, you’ll need 3 gallons.

6. Factor in Extra Coats

• Going over dark walls with a light colour? You may need two or even three coats

• Using primer and then colour? Factor both separately

• Planning a bold accent wall? It might need more to reach full saturation

You can learn more about when and why to use primer in our Primer 101 blog.

7. Don’t Forget Touch-Ups

Always plan to keep ½ gallon or so of leftover paint for future touch-ups. This is especially helpful in high-traffic areas (like hallways or entryways). Check out our tips on painting high-traffic areas to learn how to keep those walls looking fresh.

8. Whole-Home Projects? Tackle Room by Room

If you're painting your entire home, calculate each room individually, then total it all up.
Keep notes on:

• Paint colour and finish

• Room size

• Number of coats

This makes future repainting or touch-ups much easier to plan.

Want to Skip the Math?

Calculating paint quantities isn’t hard — but doing it right can save time, money, and frustration. At Altona Painting, we handle everything from prep to cleanup so you don’t have to worry about overbuying or running out mid-project.

Contact us for a free quote — we’ll take care of the numbers and the painting.