# Did ductwork branches change form 5" to 6" at some point? **URL:** \ **Category:** Factual Questions\ **Created:** [December 6, 2020, 9:26pm UTC](https://boards.straightdope.com/t/did-ductwork-branches-change-form-5-to-6-at-some-point/927409 "2020-12-06T21:26:13Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![LivingGhost371](https://avatars.discourse-cdn.com/v4/letter/l/b9e5f3/32.png) [@LivingGhost371](https://boards.straightdope.com/u/LivingGhost371)\ **Post date:** [December 6, 2020, 9:26pm UTC](https://boards.straightdope.com/t/did-ductwork-branches-change-form-5-to-6-at-some-point/927409/1 "2020-12-06T21:26:13Z")
Looking at my house most of the duct branches are 5" round. I planned to put in some more branches off the trunk and found most of the fittings at Home Depot are for 6" round. This won’t cause any issues since I’m starting back at the trunk, but just am curious if the standard duct size in homes was changed at some point. ---
**Author:** ![What\_Exit](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/what_exit/32/10652_2.png) [@What\_Exit](https://boards.straightdope.com/u/What_Exit)\ **Post date:** [December 6, 2020, 9:31pm UTC](https://boards.straightdope.com/t/did-ductwork-branches-change-form-5-to-6-at-some-point/927409/2 "2020-12-06T21:31:38Z")
I haven’t done HVAC work in 28 years, but if you go to a place like Graingers you can pretty much get any size fitting. Home Depot must have decided that 6" is the most common and that is what they stock. The duct size is suppose to be tied to the cubic feet of the room your heating/cooling. I don’t remember the rule of thumb any more but there are charts available and probably calculators now. It isn’t 1 size fits all. ---
**Author:** ![Snipe70e](https://avatars.discourse-cdn.com/v4/letter/s/51bf81/32.png) [@Snipe70e](https://boards.straightdope.com/u/Snipe70e)\ **Post date:** [December 8, 2020, 5:08am UTC](https://boards.straightdope.com/t/did-ductwork-branches-change-form-5-to-6-at-some-point/927409/3 "2020-12-08T05:08:28Z")
In the large sizes the odd sizes have beeen dropped. I worked in a high rise with some 9 inch ducts. We had to order 9 to 10 inch adaptors. ---
**Author:** ![MrMrmrmr](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mrmrmrmr/32/5828_2.png) [@MrMrmrmr](https://boards.straightdope.com/u/MrMrmrmr)\ **Post date:** [December 9, 2020, 8:32pm UTC](https://boards.straightdope.com/t/did-ductwork-branches-change-form-5-to-6-at-some-point/927409/4 "2020-12-09T20:32:10Z")
We made household exhaust fans, all had a 5” outlet. We also made an “increaser”, an adapter to go from 5-6. I was told some states require 6” so that was the reason for the increaser. ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [December 9, 2020, 8:38pm UTC](https://boards.straightdope.com/t/did-ductwork-branches-change-form-5-to-6-at-some-point/927409/5 "2020-12-09T20:38:29Z")
> [@MrMrmrmr](#): > > We also made an “increaser”, an adapter to go from 5-6. I think “embiggener” is the technical term. ---
**Author:** ![MrMrmrmr](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mrmrmrmr/32/5828_2.png) [@MrMrmrmr](https://boards.straightdope.com/u/MrMrmrmr)\ **Post date:** [December 9, 2020, 9:28pm UTC](https://boards.straightdope.com/t/did-ductwork-branches-change-form-5-to-6-at-some-point/927409/6 "2020-12-09T21:28:55Z")
I could see if the engineering group would go for renaming, but since they are a rather humorous less lot, I would have my doubts. Our guys , on the other hand, made an assembly machine and named it the spedogalizer. It was funny in the morning meeting to have the scheduler ask, with a straight face, if the spedogalizer was available. ---
**Author:** ![Dark\_Sponge](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dark_sponge/32/121_2.png) [@Dark\_Sponge](https://boards.straightdope.com/u/Dark_Sponge)\ **Post date:** [December 10, 2020, 5:28pm UTC](https://boards.straightdope.com/t/did-ductwork-branches-change-form-5-to-6-at-some-point/927409/7 "2020-12-10T17:28:27Z")
My father was a ‘tin banger’ for 30+ years and I worked as his ‘apprentice’ for 5. My guess is ending in 5" was the preference of that particular HVAC contractor. Unless you’re talking about pre-fab homes or something similar each ductwork trunk is a custom design created to fit the layout of the air vents in the building.