For most culinary herbs, an urban planter box should give the plants at least 8 to 12 inches (20 to 30 cm) of usable soil depth. If you want one versatile box for a mix of common kitchen herbs, choose an internal soil depth of about 12 inches (30 cm). This is deep enough for basil, mint, parsley, chives, oregano, thyme, and cilantro, while still being practical for balconies and small courtyards. Measure the internal depth from the bottom of the box to the top of the soil, not the outside height of the planter. Drainage holes and a small gap below the box reduce the effective root space only slightly, but a box that is 12 inches tall on the outside may offer much less soil depth once drainage and the rim are accounted for.
For shallow-rooted herbs such as thyme, oregano, chives, and mint, 8 to 10 inches (20 to 25 cm) is usually sufficient, provided you water carefully. Mint spreads aggressively, so give it its own container or root barrier. Basil, cilantro, dill, and parsley perform best with 10 to 12 inches (25 to 30 cm), especially in hot urban sites where soil dries quickly. Deep-rooted or larger perennial herbs, including rosemary, sage, lavender, and bay, benefit from 12 to 18 inches (30 to 45 cm) of soil depth. A planter box that is only 6 inches deep can work for microgreens or very short-lived shallow crops, but it is not a reliable long-term home for most culinary herbs because it dries out fast and restricts root growth.
In urban settings, depth also affects weight, moisture, and temperature. Deeper boxes hold more soil and therefore weigh more, so check balcony or roof load limits before filling a large planter. Use a lightweight, free-draining potting mix rather than garden soil, and make sure the box has drainage holes. A 12-inch-deep box is a good compromise: it gives herbs enough root room, buffers watering gaps, and remains manageable on a balcony. If space is limited, group herbs with similar water and depth needs, and choose a box at least 8 inches deep for annual culinary herbs. With adequate depth, drainage, and sun, an urban planter box can produce a useful supply of fresh herbs throughout the season.