A typical urban planter box is not as light as it looks. Once filled with soil, it can weigh anywhere from about 50 lb (23 kg) for a small balcony box to over 1,000 lb (450 kg) for a large street planter. A useful middle estimate for a medium planter—roughly 3 ft long, 18 in wide, and 18 in deep—is often 250–600 lb (115–270 kg) when filled with moist soil and plants. With lightweight potting mix, it may sit near the lower end; with dense, wet garden soil, it can push toward the upper end or higher.
The reason is simple: soil and water are heavy. Moist garden soil can weigh around 100–120 lb per cubic foot (1,600–1,900 kg per cubic meter). Lightweight potting mix may weigh only 25–50 lb per cubic foot (400–800 kg/m³), especially when dry. Water adds about 62 lb per cubic foot (1,000 kg/m³). Add the planter itself, drainage stones, and plants, and the total climbs quickly.
To estimate your own box, measure the inside dimensions in feet, multiply length × width × depth to get cubic feet, then multiply by 70–100 lb/ft³ for a typical moist soil mix. For example, a 3 ft × 1.5 ft × 1.5 ft box holds about 6.75 cubic feet; filled 80% with soil, that is about 5.4 cubic feet. At 80 lb/ft³, the soil alone weighs about 430 lb (195 kg). Add a 30–50 lb planter and plants, and the total is around 460–500 lb (210–230 kg).
For balconies, rooftops, and patios, weight matters. Use lightweight potting mix instead of heavy topsoil, add drainage holes, avoid filling completely to the rim, and check the structure's load limit. If in doubt, choose a smaller planter or move it close to a load-bearing wall.