A twenty inch cartridge is the long option in domestic filtration, and it exists in both the slim and the big diameter families. Households usually know they need the twenty inch length because that is what came out of the housing, and the diameter is the part that gets assumed, which is where the ordering error lives.
Slim twenty inch
Around two and a half inches across by twenty long, this is the less common of the two. It appears on staged under sink systems where cartridge capacity is wanted without the width, on some appliance and commercial beverage installations, and on point of use systems serving more than one outlet. It gives roughly double the capacity of a slim ten inch cartridge in the same diameter, which mainly means a longer interval rather than better flow, since flow is governed by diameter.
Big diameter twenty inch
Around four and a half inches across by twenty long, this is the largest cartridge most households will fit and the standard for a serious whole house installation. It combines the low pressure drop that comes from diameter with the capacity that comes from length, so it gives both the longest interval and the least restriction. For carbon duties the length matters qualitatively, since chlorine removal depends on contact time and this size gives the most of it.
Confirming which you have
Measure the old cartridge, not the housing, since wall thickness varies and tells you nothing reliable. Diameter across the widest point of the body is the measurement that matters, because a length mismatch announces itself while a diameter mismatch does not: a slim cartridge in a wide housing lets water bypass entirely and filters nothing while everything looks correct. Sellers list the two numbers in either order, so 20x4.5 and 4.5x20 are the same item.
Handling and clearance
A full twenty inch big diameter sump is heavy and awkward, so the bracket must be fixed to something structural rather than plasterboard, and never supported on the pipework. Clearance below the housing must be at least the sump depth at the actual mounting height, which for this length is considerable, or the cartridge cannot be withdrawn without cutting pipe. Have a bucket ready for a volume that surprises people. Our replacement cartridge guide covers media selection.
Questions people ask about 20 inch water filter cartridge
Do twenty inch cartridges come in two diameters?
Yes, slim at around two and a half inches and big diameter at around four and a half. They are not interchangeable.
Which measurement matters most?
Diameter. A length mismatch announces itself, while a slim cartridge in a wide housing filters nothing and looks normal.
What does the extra length buy?
Roughly double the capacity, so a longer interval, and for carbon it doubles the contact time and therefore the chlorine removal.
Is 20x4.5 the same as 4.5x20?
Yes. Sellers list the numbers in either order, and the larger figure is always the length.