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A Stack inserter set to override stack size = 5 will output 4 items onto the belt not 5. 1,2,3 or 4 work so it knows how top place partial stacks but wont do it for 5,6,7. A stack size of 8 works as it outputs 2 stacks.



As you can see in the image above, The Stack inserter is set to stack size 5 but it outputs a stack of 4 items onto the belt. In this case it has a filter set by a control signal. I expected it to put 5 items on the belt in a stack of 4 and a stack of 1 and it took a while to realize this was the issue.

I have also attached a save with this build on Vulcanus.

I tired searching for this, i did not find a relevant bug post, if this is expected behavior sorry to waste your time, as always everyone on the team has made an amazing game thank you.

Thanks for the report however this is working as intended. Stack inserters only output full stacks of items onto belts unless specifically set to be less than the belt stack size. When the inserter limit is set to > the belt stack size but < a multiple of the stack size - it has no way to know "I only have 3 items, I should just put that onto the belt" since it might then *always* put 3 items on the belt which is < than the belt stack size of 4.

This is working exactly as intended: https://lua-api.factorio.com/2.0.66/pro ... belt_stack (also described in https://wiki.factorio.com/Stack_inserter). Rseding91 is right marking this as Not a bug.

Rseding91 wrote: Wed Sep 17, 2025 2:11 pm Thanks for the report however this is working as intended.

boskid wrote: Wed Sep 17, 2025 2:14 pm This is working exactly as intended:

I just came across this.
I accept the reasoning.

What I do not accept is that the UI even allows you to set it to these - essentially forbidden - values.
Please change UI to make this understandable, without searching the internet.
Thank you.

worph wrote: Thu Sep 10, 2026 9:19 am What I do not accept is that the UI even allows you to set it to these - essentially forbidden - values.
Please change UI to make this understandable, without searching the internet.
Thank you.

No. This would create problems when trying to configure an inserter before certain technologies are researched. If your current max belt stack size is 3, should a player be only allowed configure values 1,2,3,6,9,12,15 since those are the real stack sizes, or should a player be also allowed to configure values like 8 or 16 because there is an additional technology up ahead that will increase belt stack size to 4 and you want to configure an inserter ahead of time to those exact values.

boskid wrote: Thu Sep 10, 2026 9:26 am

worph wrote: Thu Sep 10, 2026 9:19 am What I do not accept is that the UI even allows you to set it to these - essentially forbidden - values.
Please change UI to make this understandable, without searching the internet.
Thank you.

No. This would create problems when trying to configure an inserter before certain technologies are researched. If your current max belt stack size is 3, should a player be only allowed configure values 1,2,3,6,9,12,15 since those are the real stack sizes, or should a player be also allowed to configure values like 8 or 16 because there is an additional technology up ahead that will increase belt stack size to 4 and you want to configure an inserter ahead of time to those exact values.

Could you at least include a little warning triangle (like for high memory usage of blueprints), if the player sets the ui to something potentially problematic?
Like 5,6,7, 9,10,11, 13,14,15. Could even include a check for which stack size is researched and exclude mutiples of that size from the warning. But I would be totally fine without that additional check.
Would be much appreciated. I was literally convinced I found a bug but managed to find this thread instead. Cheers.

worph wrote: Thu Sep 10, 2026 10:54 am Could you at least include a little warning triangle (like for high memory usage of blueprints), if the player sets the ui to something potentially problematic?
Like 5,6,7, 9,10,11, 13,14,15. Could even include a check for which stack size is researched and exclude mutiples of that size from the warning. But I would be totally fine without that additional check.

First, what defines certain values as problematic? If we say that in a given set of mods the belt stack size only ever has values of 1,2,3,4, some values could be considered friendly (1,2,3,4,6,8,9,10,12,14,15,16) and some unfriendly (5,7,11,13) but would you agree with that set of friendly values? You mentioned 10 and 14 as candidates for a warning sign, but those are perfectly valid values when you have belt stack size of 2. Even if a current value of belt stack size modifier was already at 3, does it mean all multiples of 2 that are not multiples of 3 or 4 are now getting a warning sign, or are they acceptable because a player may create a blueprint out of it for use in a new game where the modifier goes back at 1.

Second, should that indicator also appear when inserter hand size override is set by circuit network, or should it only appear for manual values?

Third, what should happen in case the estimate of possible belt stack size values (the 1,2,3,4 set) was incorrect because you are playing a scenario where a market entity is present allowing slightly higher values or when playing with mods that add extra technologies that increase this value. Should the game do its best trying to predict all possible belt stack size values or should it revert to not showing warning when mods or custom scenarios are present?

I am not convinced we want to complicate this gui with extra warnings if the inserter is a stack inserter and held stack size override is an unfriendly number.

boskid wrote: Thu Sep 10, 2026 11:06 am I am not convinced we want to complicate this gui with extra warnings if the inserter is a stack inserter and held stack size override is an unfriendly number.

I see that there are a plethora of edgecases and it's correct and fair to push back.
All I can do is tell you where I am coming from.

I wanted a stack inserter to output exactly 10 yumako mash (for carbon fiber not directly-inserted) and I was confused for the better part of 15 minutes, why it only ever put out 8.
This journey ended in this thread. Setting it to 2 did the job (unsatisfyingly).

But the game was unable to tell me why I would get 8 consistently and not 10. I tried all sorts of things before resorting to googling and was primed to write a bugreport. I understand now, obviously.

I cannot give any further input on how to handle any for the edgecases as that would probably only scratch the surface. All I want is that the UI gives me a warning or explanation, when the amount a stackinserter picks up does not align with the value I manually setup in the UI.

Thanks for hearing me out, I'll leave it be.

Surely there's a way to visualise how setting the max stack size affects inserter behaviour. No warnings necessary. You could add another line below the slider to indicate this. For example, if you set it to 17 the visualisation will show 2 stacks of 8 (and maybe the numbers 2x8 for extra clarity), then if you bump it up to 18 it will show 3 stacks of 6 (and the numbers 3x6). Players can then see how this visualisation updates as they move the slider up and down.

If it feels necessary, an (i) icon can be added with information on why this behaviour is as it is. Something like "Inserters can only carry multiple stacks of equal size."

Note that the inserter only applies this logic when dropping onto a belt.

Why use any dynamic elements at all? Wouldn't some static explanation, hidden behind the blue (i) thingy, be sufficient? "When targeting a belt, if the inserters configured stack size is larger than the belts stack size, the inserters stack size is rounded down to whole belt stacks."