Activity Runoff Footprint

Calculating Your Runoff Footprint

Grade: 5 to 11
Subject: Mathematics, Geography, Social Studies
Time: 90 - 120 mins.

Key Terms

100-Year Storm

Combined Sewer Overflow



Rainfall Intensity


Runoff Footprint


Sewer System

Stormwater Infrastructure


This activity shows you how to calculate how much of your study area is a permeable, semi-permeable or non-permeable surfaces. Knowing this allows you to estimate how much rain is soaking into the ground and how much is running off into sewers, drains and ditches. This is called a Runoff Footprint. As climate change leads to larger, more intense and more frequent storms, it is important to understand the size of your Runoff Footprint. You will be performing percentage calculations, but you could also use our GreenLearning Runoff Calculator.

Step 1: Measuring Permeability Surfaces

Look over your Permeability Worksheet and the definitions of permeable, semi-permeable and non-permeable. Review your Study Area map from your Stormwater Mapping Tour, or your Permeability Map. You will be calculating the surface area of each section of your study area. This can be done by:
- actually measuring the section and calculating the area (see below)
- using Google Maps (see User Guide)
- using the online tool at  https://www.calcmaps.com/map-area/

If you are measuring an area that is not square or rectangular using calcmaps look at Measuring Unusual Shapes in the User Guide.

Calculating Area

Remember that the basic formula for measuring rectangular area is area = length x width.   There are three basic ways that you can get your length and width measurements if you are doing it without a computer:

  • Use a long tape measure 

  • Use rope or string: cut, measured or bought to a known length like 1 meter or 10 meters.

  • Pace out distance by measuring your pace (from front foot toe to the heel of your back foot) based on a regular step. 

Do you measuements in meters. Record your answers in the Surface Areas Worksheet.

Surface Areas Worksheet

A = L x W
Parking Lot        
Running Track        
Playing Field        
Road 1        
Road 2        
Road 3        
Playing Area 1        
Playing Area 2        
Playground Rubber Mats        
Wood Chip Area        
Artifical Turf        
Road 1        
Road 2        
Sidewalk 1        
Sidewalk 2        
Paved Area 1        
Paved Area 2        
Sports Courts        
Semi-Permeable Pavement        
Naturalized Area        
Swales and French Drains        

Download Surface Areas Worksheet

Step 2: Calculate Permeability Percentages

Review the negative impacts of stormwater runoff.  Where the rain falls is important to the impacts of stormwater. Permeable surfaces handle the stormwater. Non-permeable surfaces pass the water on to the stormwater infrastructure to hand. 

Calculations  of percentages of the permeable, semi-permeable and non-permeable surface will show how well your study area will handle rainfall runoff.

Your Permeability Map shows you key information:

  • The different types of stormwater infrastructure, how many and where they are located.

  • The location and permeability of all surfaces.

  • A series of observations (collected before after and during rainfall events) about where issues of flooding, pooling, erosion and pollution exist.

Based on your answers in your Surface Areas Worksheet, complete the Permeability Percentage Worksheet.

Permeability Percentage Worksheet

Non-Permeable total area (in m2)

(total of NP surfaces in Surface Areas Worksheet)


Semi-Permeable (SP) total area  (in m2)

(total of SP surfaces in Surface Areas Worksheet)


Permeable (P) total area  (in m2)

(total of P surfaces in Surface Areas Worksheet)


total area of Study Area (SA)

= NP + SP + P


Percentage Non-permeable

       __NP total area_______    x 100 = _________ %

           SA total area

Percentage Semi-permeable

       __SP total area_______    x 100 = _________ %

           SA total area


Percentage Permeable

       __P total area_______    x 100 = _________ %

           SA total area




  1. What area of your Study Area is total runoff?

  2. What impacts will all this runoff have?

  3. If you think your non-permeable percentage is high, what impact does that have?

  4. If you decrease your non-permeable percent by 10% what change would that cause in the other two percentages?

  5. What do you think your school could do to improve its percentages?


Share your percentages on the permeability of your Study Area on social media (Facebook, Twitter, Instagram) using #stormwater #challenge

Step 3: Calculate Rainfall Amounts

The next step is to calculate the amount of rainfall in your area. The more rain your area receives the more important it is that you have a high percentage of permeable surface. Fill in the chart below using the data sources that are suggested.

Rainfall Chart

Rainfall Event 
Amount of Rain (mm)
Sample Storm 1
Sample Storm 2
Sample Storm 3
1 year storm

100 year storm  
Real Time Storm


Download Rainfall Chart

Historical Rainfall data

You can find the data for the Rainfall Chart at the Government of Canada web site: http://climate.weather.gc.ca/historical_data/search_historic_data_e.html

You can get help using this Government of Canada site in their How To document?


You can also find rainfall data at:


Real Time Storms

You can use radar and weather reports to view, study and track storms online, in real time as they happen. This site will show you mm of rain per hour in real time as you track the storm as it moves across your area.


There are tips on using the Weather Underground site in the User Guide. and suggestions on Weather Considerations.

Step 4: Calculate Runoff Footprint

You are now ready to calculate the Runoff Footprint of your Study Area. This footprint is basically how much rain falls on your Study Area and runs off into sewers, drains and ditches. 

All of the water runs off non-permeable surfaces. The tricky part is calculating the runoff from semi-permeable surfaces.  For this activity, consider that if a surface is semi-permeable, half of the water soaks in and half runs off. Permeable surfaces absorb all the rain that falls on them.

The Runoff Footprint is the total volume of rain that runs off your Study Area. It is calculated by multiplying the amount of rain by the total area of permeable surface and half the area of semi-permeable surface. 

Runoff Footprint Chart

Rainfall Event

Amount of Rain (mm)
from Rainfall Chart


Total Non-Permeable Surface Area (m2 )
from Surface Areas Worksheet


Total Semi-Permeable Surface Area (m2 )
from Surface Areas Worksheet


Runoff Footprint

Volume of Runoff Water

= R*TNP + R*TSP/2

(use Runoff Calculator)

Sample Storm 1

Sample Storm 2

Sample Storm 3

1 Year Storm

100 Year Storm

Today’s Storm

Download Runoff Footprint Chart

Runoff Footprint Calculator

This calculator will calculate your Runoff Footprint. Use it to complete the chart above.




Runoff Footprint


  1. Which number do you feel best represents the runoff footprint of your Study Area?

  2. What surprised you about the amount of water that falls in one rain event?

  3. Take your rainfall amounts and compare it to some everyday things. (carton of milk 1 l, bathtub 80 l, refrigerator, bedroom, house).

  4. What are the three biggest areas of non-permeability in your Study Area?

  5. Why is the amount of water runoff such a big problem?

  6. List any solutions to runoff that you observed in your tour or marked on your map.


Share your Runoff Footprint on social media (Facebook, Twitter, Instagram) using #stormwater #challenge