What size pipe do I need when combining two 3-inch pipes?
Combining two 3-inch pipes naturally makes it seem like you need a 6-inch pipe, leaving people unsure if a smaller 4-inch pipe works instead. This confusion is compounded by conflicting advice from YouTube tutorials over whether to install a sanitary tee or a wye-45 combination.
What people tried
Every workaround mentioned in the threads below. We haven’t tested any of them — and nobody here is claiming they worked.
- 1Using a 4-inch pipe assuming the cross-sectional area is sufficient to handle the combined flow of two 3-inch pipes.
- 2putting an arm through the fittings to feel for differences
- 3consulting YouTube tutorials
- 4following advice from other forum users regarding fitting orientation
In their words
Unedited, grouped by where they were said, most upvoted first within each place, each linked to the thread it came from.
r/Landscaping1 person · September 2026
“I'm wondering about when I tie two of these together. On the face, it seems I'd need a 6" pipe since I'm combining two 3" pipes. Is that how it works or can I use a 4" pipe for two 3" pipes?”source ↗
Artie-Choke · r/Landscaping · 1 upvotes
r/Plumbing1 person · September 2026
Where this came up
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