The short answer is: an urban planter box can weigh anywhere from about 50 kg (110 lb) for a small box filled with lightweight potting mix to more than 2,000 kg (4,400 lb) for a large raised bed filled with wet garden soil. The exact number depends on four things: the internal volume of the box, the empty weight of the box, the type and moisture of the soil, and the weight of plants, water, and any drainage material.
Use this simple formula: total weight = empty box weight + (soil volume × soil density) + water/plant/drainage weight. Measure internal dimensions in meters or feet. For metric, 1 m³ = 1,000 L. For imperial, 1 ft³ = about 7.5 gallons.
Soil density is the biggest variable. Bagged potting mix is light: roughly 300–500 kg/m³ (20–30 lb/ft³). Dry topsoil or garden dirt is much heavier: about 1,200–1,600 kg/m³ (75–100 lb/ft³). Wet topsoil can reach 1,600–1,900 kg/m³ (100–120 lb/ft³). That means 1 m³ of wet dirt can weigh nearly 2 tonnes by itself.
For common sizes: a 60 × 30 × 30 cm (0.054 m³) planter with potting mix might weigh 20–40 kg total, but with wet topsoil it could be 90–120 kg. A 90 × 40 × 40 cm (0.144 m³) box might be 60–100 kg with potting mix and 250–320 kg with wet soil. A large 120 × 60 × 60 cm (0.432 m³) box can be 160–300 kg with potting mix, or 750–900 kg with wet topsoil. A 1 m³ raised planter can easily exceed 1,500 kg (3,300 lb) once saturated.
If you are placing planters on a balcony, roof, or deck, assume the worst-case saturated weight and check the structure's load limit. Use lightweight potting mix instead of native soil, choose smaller or taller-narrower planters, avoid heavy gravel layers, and use pots with good drainage. The box may look simple, but filled with wet dirt it can become a serious structural load.