For most adults who want to stand while gardening, the most back-friendly height for an urban planter box is about 30 to 36 inches (75–90 cm) from the ground to the soil surface. If you measure to the rim instead, make the box roughly 1 to 2 inches taller than the finished soil line. This range lets you reach plants, weed, water, and harvest without deep bending or stooping.
The best height is personal, though. Stand comfortably and bend one elbow to about 90 degrees. The soil surface should sit roughly 2 to 4 inches below your elbow. If it is much lower, you will round your back; if it is much higher, you may lift your shoulders and strain your neck. Taller gardeners may prefer 34–38 inches, while shorter gardeners may find 28–32 inches more comfortable. If you mainly want to garden from a stool or chair, a 24–30 inch soil height can work well, but check that your knees fit under the box and you can reach the center without leaning.
In an urban setting, also consider access. Keep the planter no deeper than about 2 feet from the side you work from, or use a narrow U-shaped bed, so you do not stretch across the soil. Put the box on a firm, level surface and leave enough path space for your feet. Choose a planter with drainage, and use lightweight soil mix if the box is on a balcony or roof.
For most back-friendly standing gardens, start with a 30–36 inch finished soil height. Test it before filling or building if possible: place a box or table at that height and simulate planting. Your body should feel relaxed, with elbows bent, shoulders down, and feet stable. If you have known back, shoulder, or mobility issues, adjust toward the higher end—or consult an occupational therapist—so the planter works for your body, not just an average measurement.