For most peppers grown in an urban planter box, plan on at least 12 inches (30 cm) of actual soil depth. That is the practical minimum for compact or dwarf varieties. If you want healthier plants, bigger harvests, and more tolerance for hot, dry city conditions, choose 18 to 24 inches (45 to 60 cm). Standard bell peppers, jalapeños, habaneros, and other long-season peppers perform best in that deeper range.
A box that is only 8 to 10 inches deep can keep a pepper plant alive, but it usually limits root growth, dries out faster, and produces fewer peppers. Measure the depth from the top of the soil to the drainage layer or bottom of the box, not from the rim to the ground. Good drainage matters too: make sure there are drainage holes, use a quality potting mix, and avoid filling the bottom with too much gravel.
If your planter sits on concrete, pavers, or a rooftop, deeper is even more important because roots cannot escape into the ground. In hot urban spaces, 18 inches or more helps stabilize moisture and temperature. If space is limited, grow a dwarf pepper and choose a wider box with at least 12 inches of soil depth.
In short: 12 inches is the minimum, 18 inches is the sweet spot, and 24 inches is excellent for large pepper plants.