Great question—if shoulder support is the goal, the standard urban bench backrest often falls short. Most commercial and city benches have a backrest height of about 18 to 20 inches (46–51 cm) measured from the seat surface to the top of the backrest. That height typically hits around the mid-to-lower back, not the shoulders.
For real shoulder support, you want a backrest height of at least 24 inches (61 cm), and ideally 26 to 30 inches (66–76 cm). A 24-inch backrest usually reaches the lower shoulder blade area for average adults. If you want the bench to actually cradle the shoulders and allow people to lean back without their shoulder blades rolling over the top, 26 inches or taller is a safer specification.
There are a few reasons many urban benches are lower. Lower backrests keep sight lines open, make benches feel less visually heavy, reduce material costs, and discourage people from sleeping across them. That is useful for public space management, but not great for ergonomic support.
If you are specifying benches, ask for “high-back” or “shoulder-height” commercial benches. Check the measurement from the top of the seat, not the ground, because some product drawings list overall height. Also look for a slight backward tilt—about 5 to 10 degrees—paired with a contoured or slatted backrest. That combination makes a 24 to 26 inch backrest feel much more supportive at the shoulders than a perfectly vertical 20 inch back.
In short: for shoulder support, skip 18–20 inch standard urban backs and specify at least 24 inches, preferably 26–30 inches. If you can, test a sample with adults of different heights, because “shoulder height” changes from person to person.