The short answer is that an urban planter box usually weighs much more than most people expect once it is filled with soil, water, and plants. A small, well-drained box might weigh 100–300 lb, while a large rooftop planter can easily weigh 800–1,500 lb or more. The exact weight depends on the planter’s size, material, soil mix, moisture level, drainage, and the plants growing in it.
The basic calculation is simple: total weight = empty planter weight + soil weight + water weight + plant weight. Soil is almost always the heaviest part. Dry potting mix is fairly light, but it becomes much heavier after watering. As a planning range, use about 50–75 lb per cubic foot for wet potting mix and about 80–110 lb per cubic foot for wet topsoil or compost-heavy mixes. If you are checking a roof, balcony, or deck, use the wet, saturated weight rather than the dry weight; it is the safer number.
For example, a 4 ft × 2 ft × 1.5 ft planter holds 12 cubic feet of soil. At 50–75 lb per cubic foot, the wet soil alone weighs about 600–900 lb. Add a 40–80 lb container and 10–40 lb of plants and water in the root zone, and the total is roughly 650–1,020 lb. A similar box filled with heavier mineral topsoil could exceed 1,200 lb. A smaller 2 ft × 2 ft × 1 ft box holds 4 cubic feet, so it might weigh around 250–450 lb when wet and planted.
Do not forget that soil compacts, drainage layers, saucers, trellises, and self-watering reservoirs can add weight. Plants also add less than most people think, but wet foliage, large shrubs, and root balls can matter. For any rooftop or structural installation, spread the load across several supports, keep heavy planters near load-bearing walls or beams, and ask a structural engineer if you are unsure. The practical rule is to estimate with saturated soil and then add a safety margin; an urban planter box is a small garden, but it can be a serious load.