Rainwater storage tank ideal sizing program. Rainwater harvesting storage tank sizing calculator. | Cleanflo Water Technologies | https://harvestingrainwater.ca/
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By analyzing 55 years of location-specific daily rainfall data, Cleanflo can determine the maximum potential volume of rainwater that is actually usable based on:

  • Daily Rainfall Data
  • Water Demand
  • Storage Tank Volume

In real world scenarios a system can never reach ‘maximum potential’ because there is loss from the roof, pre-filter, and tank overflow events. Also, the size of rainwater storage tank and how much water is used daily from the tank play an important role in determining how much rainwater you can actually collect and use. Actual useable rainwater forms the basis to determine 'return on investment', because storage tanks are the single most costly item in a rainwater harvesting system, determining the smallest tank size that will collect the most water based on the three criteria listed above, will significantly impact the total cost of your rainwater harvesting system.

Rainwater Data

All daily rainfall data was obtained from Environment Canada from almost 400 stations across Canada, for 55 years, from 1955 to 2010.

Our software automatically selects the geographically closest station to your project site, feel free to view our station list (Station List​)

Rainwater Demand

Rainwater harvesting systems are designed with water demand in mind. The use of incorrect water-figures can result in a system that is either over, or under designed. In the case of an under-designed system, the system may be unable to consistently meet the daily water needs of its users requiring the tank to be topped-up from hauled water or a secondary water supply. In the case of an over-designed system, potentially wasted capital investment.

In either situation it is critical that the water demand value employed in the design process be as representative as possible.

There are two methods to determine the the Water Demand.

  1. Historic Records - Water records or meter readings that provide an accurate water demand.
  2. Estimations - Cleanflo estimates your water demand based on:
    1. Uses
    2. Number of occupants
    3. Days used per week

Rainwater Tank Size

The Rainwater storage tank is the heart of the rainwater harvesting system and should be carefully selected. Cleanflo identifies the optimal tank size to maximize water savings, when any larger tank would provide only a small additional amount of 'Useable Rainwater'.

Tanks are sized by the following criteria:

  1. 15% "Dead space" -
    • Rainwater harvesting tanks often have unused volume referred to as “Dead space” to account for, undisturbed rainwater at the bottom of the tank, and the head space at the top of the tank.
  2. Rainfall Collection Loss -
    • Not all rainwater contacting the catchment surface (roof) is collected in the tank, some is lost due to collection loss. Initial loss factor - The initial amount of rainfall loss before collection takes place, Continuous loss factor - the percentage of rainfall that is lost during the rainfall event (due to roof, wind, leaky gutters, etc.)

Rainwater Harvesting - Hydrology Model

How does the tank sizing work?

Our software will automatically analyze the useable rainwater from various rainwater tanks, it starts at 1000 liters (1 cubic meter) and increases by 1000 liters (eg. 1000L, 2000L, 3000L, etc.) The software compares the useable rainwater for each of these tank sizes, and will determine the useable rainwater per-liter of rainwater storage. By comparing the useable rainwater per-liter of rainwater storage it is possible to determine an “optimum” tank size. When selecting a larger tank it will not provide a significant increase in usable rainwater. The software will continue to increase tank sizes until the largest tank is found that provide a 1% increase in useable rainwater for a 1,000 liter increase in rainwater storage capacity.

What does the report include?

The Rainwater Hydrology Report is 9 Pages and provides a complete picture of the systems hydrology, including breakdowns of all the major sections.

  1. Cover Page
  2. Project Details
  3. Rainwater Demand - Indoor
  4. Rainwater Demand - Outdoor
  5. Tank Sizing
  6. Tank Performance Details
  7. Tank Performance Analysis
  8. Rainwater Demand Daily
  9. Tank Overflow Analysis

Assumptions Used for hydrology model

The simulation of an operating Rainwater Harvesting system through the use of the computer software was performed while making some assumptions. The following list the assumption associated with the software projections.

  1. The rainfall data obtained from Environment Canada are assumed to be both accurate representation of historical rainfall patterns and an indicator of future trends.
  2. The initial and continuous loss factor are assumed to be accurate.
  3. Snowfall will accumulate on the roof surface we assume a continuous loss of 50% for snow melt that is collected in the rainwater tank.
  4. The model assumes a uniform rate of daily household water consumption for the winter months and possibly a separate rate for summer months.


System Overview

The system overview provides a complete breakdown of the rainwater harvesting system design. This includes a description and design narrative which explains the system in non-technical terms. The list of approved end uses is to clearly outline for which standard the system is designed and the applicable uses for the collected rainwater. The system specification and bill of materials outlines exactly which products should be used and any requirements of installation or configuration.

Design Drawings

The system design drawings provide a detailed technical explanation of the system. The schematic provides complete system logic, plumbing detail, and instructions on how the system is built and connected. The site plan, piping, and wiring diagram provide a permit ready package, to ensure trades can install the system properly.

Water Safety Plan

The Water Safety Plan (WSP) is intended to recognize and address water quality concerns and improve water quality for rainwater harvesting system for both potable and nonpotable uses. This plan provides a framework to ensure your water is safe.

Operation and Maintenance

The operation and maintenance section clearly outlines the maintenance and inspections required to maintain a safe and clean rainwater harvesting system. This includes the basic operating instructions to setup and troubleshoot operation.