That's an excellent and practical question! For a standard outdoor trash can placed on a sidewalk or in a public pedestrian area, the most common and recommended capacity is between 20 to 35 gallons.
This size range is a careful balance struck by city planners and facilities managers. A 20-35 gallon can is large enough to hold a meaningful amount of litter from passersby without becoming overwhelmingly heavy or difficult for maintenance crews to empty. Cans in this range are typically emptied daily or multiple times a day in high-traffic areas to prevent overflow, which attracts pests and creates an eyesore.
You'll often see 32-gallon cans as a very common standard. They are manageable for workers to handle, cost-effective to purchase in bulk, and their size sends a subtle cue to the public to dispose of general waste, not large bags of household garbage. Smaller bins (under 20 gallons) would require constant emptying, while much larger ones (like 50+ gallons) become too heavy and cumbersome for efficient daily service on a sidewalk route.
So, while specific regulations can vary by municipality, if you're looking for a standard, aim for 20-35 gallons, with 32 gallons being a typical sweet spot for public sidewalk use.