Calculate Your Runoff!

In order to manage stormwater on your property and also to select the best stormwater management technique, you’ll need to know how much rainfall lands on your property and where it flows. Identify all of the impermeable surfaces on your property. This includes roofs draining into gutters and downspouts, and impermeable surfaces at ground-level such as driveways, patios and walkways.

1. Calculate the total impermeable area.

For designing a stormwater management system, add up the areas of all the impermeable surfaces that will contribute water to the system you choose.

A₁ + A₂ + … = A_T

Example: The roof of a house (160 m²⁾ and part of the roof of a garage (10 m² ) both drain through a single downspout.

160 m² + 10 m² = 170 m²

2. Calculate the volume of stormwater runoff.

In Toronto, the average rainfall over a 24 hour period is 10mm or 0.01m. To find the average daily volume of stormwater runoff for a given area, simply mulitply the area by the daily rainfall (make sure your units are consistent).

A_T = Total Impermeable Area (m²)
R_T = Rainfall in Toronto (m)
V = Volume of Stormwater (m³)

A_T x R_T = V

Example: From the downspout mentioned above, 170 m² are drained. Multiply this area by the 0.01 m average daily rainfall in Toronto.

170 m² x 0.1m = 1.7 m³ of stormwater in 24 hours

1.7 m³ = 1700 litres

• You can use this runoff information towards stormwater management systems on your property or apply it towards using a RainBarrel.

3. Choose your stormwater capture target.

The volume of stormwater runoff resulting from the average rainfall over 24 hours makes a good minimum capture target, but you may want to increase the target to prevent overflows during larger rain events.

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