How the calculator works
It multiplies the distance around the house by the wall height, counting each storey as 9 ft (2.7 m). It adds the triangular gable ends under the roof, subtracts doors, windows and garage doors, then divides by the coverage rate for your siding. Trim, fascia and soffits add about 10%, and 10% is added for waste.
Why siding changes the answer
Smooth vinyl, aluminium and fibre cement take the least paint. Wood clapboard has edges and absorbs more. Rough cedar, shakes and brick have far more surface than their size suggests, and stucco is the thirstiest of all, often needing twice the paint of smooth siding.
When to paint
Most exterior paints need dry weather and temperatures of at least 50°F (10°C), and some formulas allow 35°F (2°C). Avoid painting when dew or rain is expected before the paint dries. In much of Canada that leaves late spring to early fall, so plan and buy paint before the season starts.
Buying paint for a whole house
For large jobs, 5-gallon buckets usually cost less per gallon than single cans, and mixing all the paint together (called boxing) keeps the colour consistent from wall to wall.
Results are estimates. Coverage depends on the product, the tools and the wall; the rate printed on your can wins.