That's an excellent and practical question! When comparing a round urban trash bin to a square one of the same height, the square bin will almost always have a greater capacity, assuming their footprints (the space they occupy on the ground) are similar.
The reason is geometry. A square or rectangular bin utilizes its corner space fully. A round bin, by its shape, inherently has "wasted" space in the corners of the square footprint it sits within. Think of it like this: if you drew a circle inside a square of equal width, the circle's area is only about 78.5% of the square's area. Therefore, for two bins with the same height and similar overall width, the square one has a larger base area, translating directly to more volume.
However, the real-world answer is more nuanced. While the square bin wins on pure volume, the round design offers other advantages. Its curved walls are often more resistant to denting, have no corners for trash to get stuck in, and can be easier to empty. For public spaces, the lack of sharp corners is also a safety benefit.
So, in summary: for maximum capacity in a fixed space, choose a square bin. But if ease of use, durability, and safety are higher priorities, a round bin—even with slightly less volume—might be the better choice for your urban environment. The final decision balances pure capacity with operational and practical needs.