For most standing gardeners, the top rim of an urban planter box should sit about 30–36 inches (75–90 cm) above the ground. That height puts the soil surface roughly around hip or waist level, so you can plant, prune, and harvest with a slight reach instead of a deep bend. If you are taller than about 5'10" (178 cm), consider 36–40 inches (90–100 cm). If you are shorter, 28–32 inches (70–80 cm) may feel more comfortable.
The best height is personal. Stand relaxed and bend your elbows to about 90 degrees. The planter rim should be a few inches below your bent elbows or around your waist. That usually means 30–36 inches for an average adult, but your own proportions matter more than a standard number.
For seated gardening or wheelchair access, choose a lower box, usually 24–30 inches (60–75 cm), with open space underneath for your knees. Keep the planter wide enough to work in but not so deep that you have to reach too far. If you can access it from only one side, about 18–24 inches deep is comfortable. If you can walk around it, 36–48 inches can work well.
Remember that soil depth changes the usable height. A 36-inch planter still needs 10–12 inches of potting mix for most herbs and greens, so the soil surface will be lower than the rim. If you want almost zero bending, go closer to 36–40 inches and use lightweight potting mix. Just avoid making it so high that you have to lift your shoulders or reach overhead.
In short, aim for 30–36 inches for most urban planter boxes, adjust taller or shorter based on your body, and test the rim height before you build or buy. That range is usually the sweet spot: tall enough to save your back, low enough to stay stable, and practical for balconies, patios, and small courtyards.