Size your System

There are three basic considerations for sizing your BlueBarrel System:

  1. How much water can you catch?
  2. How much water do you need?
  3. How much space do you want to dedicate to water storage?

In most cases, the space issue will be the final determining factor, but we’ll walk you through all three steps right here.



This depends on the size of your roof area. Measure the flat footprint area covered by the roof (or by each roof segment). Pitch is not a factor!

Roof Footprint


Use our rain catchment calculator to estimate your average annual catchment potential. Enter your annual rainfall average, a peak monthly average for the amount you can catch in a stormy month, or a typical heavy storm event if you want to size your system to fill with a single storm.

Rain Catchment Calculator

Roof Footprint Area:
(square feet)




If you find that your roof will generate an overwhelming amount of water, it may be more practical to consider how much water you will need. Your system will recharge every time it rains, so you don’t need to estimate your water use for the entire year—just your longest dry stretch.

First determine what you will be using the water for. Let us note again that without treatment, stored rainwater is not potable, so unless you will be using a qualified water purification unit, stick with non-potable demands.

Garden irrigation is an ideal use for rainwater. Irrigation needs depend on regional climate, microclimate (such as sun/shade conditions on your site), soil type, and plant type. If you’re on a metered site, you can subtract the water you use during the rainy season from the amount you use when you’re irrigating to estimate your overall irrigation use.

Or you can estimate, using a rough figure of 0.5 gallons per square foot of irrigation area per week during peak irrigation season. Actual irrigation need may vary, but this formula will give you a reasonable estimate:

[garden bed square footage] x [number of weeks in your dry season] x [0.5 gallons per week]

If this still sounds like way too much water…



You will get 55 gallons of storage capacity with each barrel in your BlueBarrel System™. If the outputs from questions 1 and 2 seem like way too much water, simply figure out how many barrels you have space for and start with that.

The BlueBarrel System™ is designed with a hose inlet so that you can refill your barrels for backup when (or should we say if!) they dry up. The system is also modular so you can  add on later if you decide you want to store more water.

Measure the space you are considering for your system. Refer to BlueBarrel’s dimensions page to determine how many barrels you have space for.

You’re well on your way!

NEXT: order your rainkit


BlueBarrel is your one-stop-shop for used barrels, DIY rainwater catchment systems, and gravity-fed irrigation.

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