There is no single safe weight for an urban planter box on a balcony. The real limit is the lower of two things: the planter’s own manufacturer-rated capacity and the balcony’s available load capacity after accounting for people, furniture, and other items. A planter may physically hold a large volume of soil, but the balcony structure may not safely support that concentrated weight.
For a practical estimate, calculate the total weight of the planter, wet growing medium, plants, water reservoir, and any accessories. Wet potting mix often weighs about 0.6–0.9 kg per liter, while wet garden soil can weigh about 1.2–1.6 kg per liter. For example, a 54-liter planter filled with wet potting mix at 0.7 kg per liter carries about 38 kg of soil. Add a 10 kg planter and 5 kg of plants, and the setup is roughly 53 kg. With denser, saturated garden soil, the same box could exceed 80–90 kg.
Many modern residential balconies are designed for live loads in the range of roughly 150–300 kg per square meter, but this varies widely by country, building age, and design. Older, cantilevered, or poorly documented balconies may have much lower limits. Do not rely on a generic figure. Also remember that a heavy planter creates a concentrated point load, which can matter more than the same weight spread evenly.
To stay safer, use lightweight potting mix rather than dense garden soil, avoid overwatering and waterlogged containers, choose planters with good drainage, and keep heavy boxes close to load-bearing walls or columns rather than at the outer edge. Check the planter’s rated load, confirm the balcony’s design load with the building manager or a structural engineer, and distribute weight over a wider area where possible. If you are unsure, have a professional assess the balcony before adding large urban planter boxes.