For most urban planter boxes where you want to grow herbs and salad greens together, the ideal depth is about 10–12 inches (25–30 cm). Salad greens such as lettuce, spinach, arugula, and kale can manage in shallower soil, often around 8 inches (20 cm), but herbs benefit from a little more room. A 10–12 inch depth gives roots space to spread, holds moisture more steadily, and makes the planter less likely to dry out on a sunny balcony or rooftop.
If your herbs are mainly basil, cilantro, parsley, chives, mint, or oregano, 10–12 inches is a practical choice. If you plan to include more drought-tolerant or deeper-rooted herbs such as rosemary, thyme, or sage, aim closer to 12 inches and use a very free-draining soil mix. A planter that is too shallow can dry out quickly and restrict growth, especially in summer.
Good drainage is just as important as depth. Choose a box with drainage holes, fill it with a loose, fertile potting mix rather than heavy garden soil, and leave a small gap below the rim for watering. Water when the top inch of soil feels dry, and mulch lightly to help keep moisture even. With roughly 12 inches of depth, you can grow a useful mix of herbs and salad greens together while keeping the planter manageable for an urban space.