An average residential balcony is not built to a single “maximum weight” for planters. Most modern residential balconies are designed for a live load of about 40–60 pounds per square foot (roughly 195–293 kg/m²), while some older or lightly built balconies may be closer to 30 psf (about 146 kg/m²) or less. That live load must cover everything you put on the balcony: people, furniture, grills, snow, and movable planter boxes. If planters are permanent, they may also count as added dead load, so the real safe budget can be lower.
To estimate your planter budget, multiply the balcony area by the design load. Example: a 1.2 m × 3 m balcony has 3.6 m². At 40 psf, its live-load capacity is about 700 kg (1,540 lb). At 60 psf, it is about 1,050 kg (2,320 lb). But leave room for people and furniture—say at least 100–200 kg. That leaves roughly 500–900 kg for planters on that balcony, depending on the building’s actual design, condition, and local code.
Urban planter boxes get heavy fast because wet soil and water weigh a lot. A 1 m × 0.5 m × 0.4 m box holds 200 L of soil. Wet potting mix can weigh about 0.8–1.2 kg per liter, so the soil alone is 160–240 kg; dense garden soil may be 1.6–2.0 kg/L, or 320–400 kg. Add the container and plants, and one large planter can easily weigh 180–450 kg. Four such planters could be 720–1,800 kg, which may exceed what a small or older balcony can safely support.
Practical safe limits: for a typical modern balcony, keep total planter weight well below the design live load, and spread it out. A conservative target is often 20–30 psf (100–150 kg/m²) of planter weight if people will also use the space. Use lightweight potting mix, resin or fabric containers, and avoid filling huge boxes with dense soil. Place heavy planters near the building wall or over supporting beams—not at the outer railing edge—and use load-spreading boards under legs. Never hang heavy planters from a railing unless it is rated for that load.
Because “average” balconies vary widely, there is no universal maximum. If you plan several large urban planter boxes, especially on an older, wooden, cantilevered, or visually cracked balcony, have a structural engineer review the design. A quick professional check is much cheaper than a structural failure.