When choosing an urban planter box for carrots, measure the internal soil depth first. For most standard carrot varieties, the box should hold at least 30 cm (12 inches) of loose, stone-free soil, so an exterior box height of about 35–40 cm (14–16 inches) is a practical target once drainage space and a rim are included. If you grow short, round, or baby carrots, a box with 20–25 cm (8–10 inches) of soil can work, but the carrots will be smaller and need consistent moisture. Long varieties such as Imperator types need 35–45 cm (14–18 inches) of soil, so choose a box 45–50 cm (18–20 inches) tall or use a deep raised bed.
The planter’s height matters because carrots develop a taproot downward. Shallow soil causes stunted, forked, or twisted roots. The soil should be loose and free of rocks, clumps, and fresh manure; mix compost with garden soil or use a quality raised-bed blend. Make sure the box has drainage holes, and do not count a drainage layer as usable soil depth. If the planter sits on a patio, use feet or casters so water can escape.
A box 30–40 cm deep and at least 30 cm wide is enough for a small urban crop; wider boxes make spacing easier. Sow seeds about 1 cm (0.5 inch) deep and thin seedlings to roughly 5–7 cm (2–3 inches) apart. Keep the soil evenly moist during germination. In short, choose a box with at least 30 cm of soil for standard carrots, 25 cm for short varieties, and 45 cm or more for long carrots. Measure the inside depth, not just the outside height.