Big Blue Housings Explained: 10-Inch vs 20-Inch

"Big Blue" is the industry nickname for the most common whole house filter housing format — a 4.5-inch diameter canister that comes in two standard lengths, 10-inch and 20-inch. Both use the exact same cartridge diameter and typically the same 1" NPT ports, so the choice between them isn't about compatibility — it's about capacity, install space, and how often you want to be changing cartridges.

What's Actually the Same Between Them

Both 10-inch and 20-inch Big Blue housings:

• Use the same 4.5-inch diameter cartridge format, just in 10" or 20" lengths

• Typically feature 1" NPT (National Pipe Thread) inlet/outlet ports

• Accept the same range of cartridge types — sediment, carbon block, GAC, and specialty media cartridges — in their respective length

• Are commonly available with or without a pressure relief button, which lets you release trapped pressure before opening the housing for a cartridge change

• Come with a housing wrench and a mounting bracket in most kits

The Real Difference: Capacity, Not Flow Rate

A common misconception is that the 20-inch housing simply "flows more water" than the 10-inch. In practice, flow rate is driven primarily by the cartridge's diameter and micron rating, not its length — both housings use the same 4.5" diameter. What the 20-inch actually gives you is more filter media surface area and dirt-holding capacity in the same micron rating, which means it can filter more total water, or handle a heavier sediment load, before the cartridge needs replacing. In other words: the difference shows up in how often you're changing cartridges, more than in how fast water comes out of the tap on day one.

Side-by-Side Comparison

Installation Space Is a Bigger Factor Than People Expect

Both housings need enough clearance below the cap to fully unscrew and remove the sump for a cartridge change — roughly 15 inches for a 10-inch housing and 25 inches for a 20-inch housing. This is one of the most common installation mistakes: mounting a 20-inch housing in a spot that looks fine at first, only to discover there isn't enough room underneath to actually change the cartridge later. Measure your available vertical clearance before choosing a size, not just the housing's own dimensions.

Port Size Matters Regardless of Housing Length

Both housing lengths typically use 1" NPT ports, and it's worth connecting them with matching 1" fittings rather than reducing down to 3/4" on either side — a reduced fitting creates its own flow bottleneck that defeats the purpose of choosing a larger housing in the first place. If your main line is 3/4", a 3/4"-to-1" adapter at the connection point is the better approach than reducing the housing's own ports. For more on how port size and flow rate interact more broadly, see our guide on sizing a whole house water filter.

Which Size Should You Choose?

• Choose 10-inch if: you have a smaller household, moderate water quality, limited installation space, or you don't mind slightly more frequent cartridge changes in exchange for a smaller footprint and lower upfront cost.

• Choose 20-inch if: you have a larger household, well water, higher sediment levels, or you'd simply rather change cartridges less often — and you have the vertical clearance to accommodate it.

Housing length doesn't change what problem the cartridge inside is solving — that's determined by the cartridge type and micron rating, covered in our guides on choosing the right cartridge by water problem and choosing the right micron rating.

Frequently Asked Questions

What's the difference between a 10-inch and 20-inch Big Blue filter housing?

Both use the same 4.5-inch diameter cartridge, just in different lengths. The 20-inch holds roughly double the filter media, giving it more dirt-holding capacity and a longer interval between cartridge changes for the same water conditions, at the cost of a larger footprint and more required clearance for maintenance.

Does a 20-inch Big Blue housing have a higher flow rate than a 10-inch?

Not significantly, on its own. Flow rate is driven mainly by the cartridge's diameter and micron rating, which are the same across both housing lengths. The main practical benefit of the 20-inch is greater media capacity, not a faster flow rate.

How much clearance do I need to install a Big Blue housing?

Plan for roughly 15 inches of clearance below the cap for a 10-inch housing, and roughly 25 inches for a 20-inch housing, since the sump needs to be fully unscrewed downward for cartridge changes. Measuring this before installation prevents a common mistake of mounting a housing somewhere without room to service it later.

What size ports do Big Blue housings use?

Most 10-inch and 20-inch Big Blue housings use 1" NPT ports. If your main water line is 3/4", use an adapter at the connection point rather than reducing the housing's own ports down to 3/4", which creates an avoidable flow bottleneck.

How often does a Big Blue cartridge need to be replaced?

It depends on the specific cartridge type, micron rating, and your water quality, rather than the housing length alone. A 20-inch housing's larger cartridge will generally need replacing less often than a 10-inch cartridge under the same water conditions, since it has more capacity to begin with.

Can I use any cartridge in a Big Blue housing?

As long as the cartridge matches your housing's length (10" or 20") and the standard 4.5" diameter, most sediment, carbon, and specialty media cartridges are compatible. Confirm the specific cartridge is rated for the length of housing you have before ordering.

Is a 20-inch housing better for well water?

Generally, yes, if you're dealing with moderate to heavy sediment, since the added media capacity handles a higher sediment load before needing a change. Well water with more complex issues like iron or hardness typically needs additional, separate treatment beyond a Big Blue sediment housing regardless of size.