For most small apartment balconies, the sweet spot is a planter box around 30 inches long, 10 inches wide, and 12 inches deep. In metric, that is roughly 75 cm long, 25 cm wide, and 30 cm deep. It is large enough for herbs, flowers, or a compact vegetable like peppers, but it still leaves room to walk and sit. If your balcony is very tight, choose a slightly smaller box: 18–24 inches long, 8–10 inches wide, and 8–10 inches deep.
The right depth depends on what you grow. Herbs, lettuce, and small annuals are happy with 8–10 inches of soil. Tomatoes, peppers, beans, and root vegetables need at least 12 inches, and 14–18 inches is even better. Width matters too: most plants need at least 8–10 inches across, while larger vegetables do best with 12 inches or more.
For railing-mounted urban planter boxes, look for a long, narrow shape: about 24–30 inches long, 8–10 inches wide, and 8–10 inches deep. These save floor space, but they must be secured safely and checked against your building’s rules. If weight is a concern, use lightweight resin, fiberglass, or fabric planters instead of heavy ceramic or concrete.
Before buying, measure your balcony’s usable floor space, railing width, and door clearance. Two or three medium boxes usually work better than one giant box because they are easier to arrange, rotate for sunlight, and move for cleaning. A 30 x 10 x 12 inch planter is a reliable all-purpose size; size down for herbs and flowers, or size up in depth only if you have strong support and enough room. Always choose drainage holes and a tray if needed.