Is the funnel icon actually understood, or is everyone quietly guessing?

The short answer from the usability literature is that almost no icon is reliably understood without a label, and the funnel is not one of the rare exceptions.

The exceptions are a very short list, earned over decades of near-universal use: the magnifier for search, the house for home, the printer, the envelope, the trash can. Even some of those degrade — the floppy disk for save is now an abstract symbol to people who have never seen the object.

The funnel is worse than average for a specific reason: it depends on a metaphor about pouring and separating that only makes sense once you already know what filtering is. Somebody who understands the concept recognises the icon immediately, and somebody who does not gets nothing from it. That is precisely backwards from what an icon is meant to do.

Your instinct about your own recognition is exactly right and is the single most common blind spot in icon decisions.

There is a middle path that works well on narrow screens if the label genuinely does not fit: show the state instead of the function.

Rather than a bare funnel, show the funnel with the count of active filters, or replace it entirely with the selected filters as chips. "Vegetarian ×" as a visible tag communicates both what the control does and that it is currently doing it, in less space than a label plus an icon.

This also solves the second problem nobody asks about, which is that people forget filters are on and then think your menu is missing items. A visible state is more valuable than a discoverable control here.

Add a label. That is the whole recommendation and it survives every context I have tested in.

The usual objection is space, and it is worth pushing back on, because in your case the cost of failure is very high. This is a dietary preference control. Somebody avoiding an allergen who does not find this control does not have a slightly worse experience — they scroll a menu that is mostly irrelevant to them, or worse, they order something they should not have.

For safety-relevant filtering I would go further than a label on an icon. Make it a visible, worded control: "Dietary filters" as text, in the flow, not tucked into an icon row. The people who most need it are the least likely to go hunting.

And whatever you do, give it an accessible name. An unlabelled icon button is announced as "button" and nothing else.

One cheap test if you want your own evidence rather than a general principle: show five people the screen with the icon and no label and ask them to find dietary options. Do not explain anything first.

Five is enough. If three of them hesitate, you have your answer, and it takes an afternoon.