The most comfortable seat depth for an urban bench that people will actually sit on for long periods is about 18 to 20 inches, or roughly 46 to 51 cm. If you have to pick one general-purpose dimension for a public bench with a backrest, go with 18 inches.
That might sound narrow, but seat depth is one of those details where small changes matter quickly. A bench that is only 16 inches deep starts to feel like a perch. It supports your hips, but leaves too much of the thigh unsupported, so your legs end up doing subtle work just to keep you stable. On the other hand, a bench that is 22 inches deep can feel like a ledge. Taller people may love it, but average and shorter adults cannot sit all the way back and use the backrest without slouching. The front edge then presses into the backs of the knees, which creates pressure, numbness, and the urge to stand up after 20 or 30 minutes.
The sweet spot is 18 to 20 inches because it supports most adult thighs without cutting into the sensitive area behind the knee. At 18 inches, a shorter person can still sit back against the backrest and keep their feet on the ground. At 20 inches, the bench feels a little roomier for taller people, but it is still manageable for most adults. That is why many well-designed public benches settle around 18 inches as a safe baseline and only go deeper on backless benches or long lounge-style seats.
For long sitting, seat depth does not work alone. It works with backrest angle, seat height, and the shape of the front edge. A seat depth of around 18 inches pairs well with a backrest angle of about 100 to 110 degrees from horizontal. That lets people settle into a relaxed but upright position without sliding forward. A seat height of about 17 to 18 inches also helps because it allows most adults to plant their feet flat on the ground, which reduces pressure on the underside of the thighs.
The front edge matters more than people expect. A rounded or waterfall-style front edge reduces pressure on the legs during longer sitting periods. A hard, square-edged 18-inch seat can still feel uncomfortable because circulation gets restricted near the knee. So the most comfortable urban bench is not just about the number. It is about combining 18 to 19 inches of depth with a slightly sloped seat pan, a rounded front edge, and a supportive backrest angle.
If the bench has no backrest, you can go a little deeper, around 20 to 22 inches, because people will sit more toward the middle and shift positions freely. But once a backrest is added, 18 to 20 inches is the practical range. For truly accessible seating, a few benches or end sections at 17 to 18 inches deep with armrests will help older adults and people with shorter legs sit back comfortably and stand up more easily.
So the short answer is: choose 18 to 20 inches of seat depth for an urban bench meant for long sitting. If you can only choose one dimension, choose 18 inches with a slight backrest angle, a rounded front edge, and a seat height around 17 to 18 inches. That combination makes people want to stay longer instead of just sitting for a quick rest and moving on.