How to Measure Downspouts for Rain Barrels

BlueBarrel specializes in DIY rain barrel systems. We're here to help you with customized rain barrel solutions, from rooftop to garden, and everything between!

In the rainwater harvesting world, gutters and downspouts together are known as the “conduit system,” along with the downspout diverter that takes your plentiful roof runoff into rain barrels or tanks.

You may want to use our rainwater calculator to see just how much rainwater your roof will generate. This article focuses on the more specific detail of how to measure your downspouts for a tight-fitting downspout diverter.

Understanding Downspout Diverters for Rain Barrels

A well-designed downspout diverter is the key piece that takes water from your downspout into rain barrels. BlueBarrel's specialty rain barrel system includes a diverter that handles system overflow automatically as an additional benefit.

If you're not familiar with downspout diverters, you may want to read this article first:

Which is Which: Gutters vs. Downspouts

Schematic showing labels for roof gutters (running horizontally) and downspouts (running vertically)

Now that you're ready to measure, let's define our terms. It's common to mix terms when referring to gutters vs. downspouts. When measuring for your diverter, it's the downspout you need to measure.

Gutters run horizontally across the bottom of each roof segment.

Downspouts run vertically and bring roof water to the ground. Keep reading for how to measure them.

How to Measure Downspouts for a Rainwater Diverter

Downspout sizes in crossection

We carry downspout diverter kits for 2" x 3" and 3" x 4" rectangular downspouts, and round downspouts between 3" and 4" in diameter.

This covers most standard residential and industrial sizes. These sizes are depicted here in cross-section; these are the dimensions you need to measure.

Please note these sizes are nominal, and your measurements may not match these dimensions exactly. From our experience, plastic downspouts are more likely to measure true to size. A metal 2" x 3" downspout may measure closer to 1-3/4" x 2-3/4". This is okay. Our flex-fit diverters will still work in their respective nominally-sized downspouts, despite the quarter-inch variance.

That said, if you are installing new downspouts specifically for your rain barrel project (or if you are replacing the lower half of your downspout to install a leaf eater) plastic material is the easiest to work with, and will give your diverter the tightest seal.

Where to Insert the Downspout Diverter


If you have rectangular downspouts, make sure you measure to know whether you need a 2" x 3" or a 3" x 4" diverter head. These are the most common sizes in residential and industrial settings, respectively.

Although other sizes are rare, we do occasionally see 3" x 3" downspout material. If this is what you have. you can order the 2" x 3" diverter. In all of these scenarios, the diverter will insert into the 3" wall of the downspout, as shown above and below.

The 2" x 3" diverter head inserts into the 3" side of the downspout.

The 3" x 4" diverter head also inserts into the 3" side of the downspout.

The round diverter head flexes to seal inside downspouts between 3" and 4" in diameter.


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