Honestly? There is no single universal number, and any answer that gives you one is probably guessing. A street bench can be rated to hold 800 or even 1,000 pounds of evenly distributed weight, yet it can still tip over with far less weight if that load is sitting in the wrong spot.
Here is the part most people miss: a bench's official weight capacity is not the same as its tip-over weight. Manufacturers usually rate outdoor benches for static load, meaning the weight is spread across the seat and people are sitting normally. A typical commercial park bench might be rated for 750 to 1,000 pounds total, or around 250 to 300 pounds per seat. That sounds like a lot, and for normal sitting it is usually fine.
But tipping is about leverage, not just weight. If a 200-pound person sits on the very edge of a freestanding bench, or leans way back on the top rail, that weight has much more mechanical advantage than a 400-pound person sitting in the center. Think of it like a wrench: the farther the force is from the pivot point, the easier it is to rotate the bench. That is why a bench can feel rock solid when people sit in the middle, but suddenly start to lift when two or three people crowd onto one armrest or sit on the backrest.
Whether the bench is anchored matters even more. A street bench bolted into concrete is very hard to tip over under normal use. The bolts and frame may bend or break before the whole bench flips. A freestanding bench is a completely different story. With enough off-center force, even a heavy bench can pivot on its front or rear legs. In the real world, a standard six-foot freestanding park bench can usually handle 400 to 600 pounds of adults sitting normally. But that same bench might start to tip with only 250 to 350 pounds concentrated at one end or on the backrest. I have seen a single adult climb onto the top rail of an unanchored bench and send it over almost immediately.
Design also changes the equation. A heavy cast iron or steel bench with wide legs and a low back is much more stable than a lightweight aluminum bench with a tall back and narrow footprint. Benches with armrests can help keep people from sliding to the extreme ends, but armrests can also be used as a seat by someone who does not care about the rules.
So if you want a practical number: for normal sitting, most sturdy street benches will handle 500 to 800 pounds without drama. For tipping, the dangerous threshold is usually much lower, maybe 200 to 300 pounds of off-center, climbing, or leaning force. The safest rule is to keep the weight centered, keep people off the backrest and armrests, and anchor any bench that might be exposed to rough use.