An urban planter box can weigh anywhere from about 30 pounds for a small balcony box to more than 1,000 pounds for a large rooftop raised bed. The exact weight depends on four things: the empty container, the growing medium, moisture, and plants. The basic formula is: total weight = container weight + soil weight + water weight + plant weight.
Soil weight is usually the biggest part. A cubic foot of dry potting mix weighs roughly 20–30 lb. Once moistened, it is closer to 30–45 lb per cubic foot. Saturated potting mix can reach 40–55 lb per cubic foot. Garden soil or topsoil is much heavier: dry soil often weighs 70–90 lb per cubic foot, moist soil 90–120 lb per cubic foot, and saturated soil 100–130 lb or more. In metric terms, moist potting mix is about 700–1,000 kg/m³, while wet garden soil is about 1,200–1,900 kg/m³.
To estimate your planter, measure the inside dimensions in feet: length × width × depth = cubic feet. Multiply that volume by the soil weight per cubic foot, then add the container and plants. For example, a 12" × 12" × 12" box holds 1 cubic foot. With moist potting mix, the soil weighs about 30–45 lb; a plastic or resin container adds 3–10 lb, so the filled box is roughly 35–55 lb. A 24" × 12" × 12" box holds 2 cubic feet and may weigh 70–110 lb with moist potting mix. A 48" × 24" × 12" box holds 8 cubic feet: expect 240–360 lb of moist potting mix, plus 10–40 lb for the container, so about 250–400 lb. If you use wet garden soil in the same box, the soil alone can weigh 720–960 lb.
Water matters more than many people expect. After rain or watering, the same planter can be 10–30% heavier. Plants also add weight: herbs and small flowers are minor, but shrubs, small trees, and root balls can add 10–100 lb or more depending on size. Terracotta, concrete, and stone planters add much more than plastic, resin, or fiberglass.
For safe urban gardening, check the load limit of balconies, rooftops, and decks. Use lightweight potting mix instead of garden soil, choose a lighter container, add drainage, and avoid over-saturating. Place heavy planters near load-bearing walls or beams, and spread weight over a larger footprint when possible. As a rough guide, a small balcony box may weigh 30–80 lb, a medium planter 100–300 lb, and a large raised bed 500–1,000+ lb once filled and watered.