Planters look light when they are empty. Filled with wet soil, a long box on a balcony or roof deck can weigh as much as a person, and several together put a real load on the structure. This calculator estimates that weight before you buy soil or ask whether the balcony can take it.
How heavy is wet soil?
The density of soil changes with its type and how wet it is. Typical wet values are:
| Soil | Weight when wet |
|---|---|
| Lightweight roof-garden mix | about 35 lb per cu ft |
| Potting mix | about 50 lb per cu ft |
| Garden soil | about 100 lb per cu ft |
Dry bagged potting mix weighs less, but after a good watering it holds a lot of water. Plan for the wet weight, since that is what the floor carries after a storm.
The calculation
volume = inside length × width × depth ÷ 1,728 (cubic inches per cubic foot)
weight = volume × fill level × soil weight + empty planter
A 36 × 12 × 12 in planter holds 3 cu ft. Filled to 90% with potting mix: 3 × 0.9 × 50 = 135 lb, plus 15 lb for the box, about 150 lb. Three of them weigh about 450 lb.
The load under the planter is its weight divided by its footprint: 150 lb on 3 sq ft is 50 lb per square foot. Large plants, saturated soil, snow and the weight of the people standing nearby all add to it.
Is my balcony strong enough?
Residential balconies and decks in the US are often designed for a live load of 40 to 60 lb per sq ft, depending on the code edition and local rules, but older or damaged structures may hold less. That figure is spread over the whole surface, not concentrated under one heavy box. Place heavy planters near the walls or above beams and supports rather than at the outer edge. For a roof garden, large planters or any doubt, ask a structural engineer or your building manager.
How to lighten a planter
- Use a lightweight mix with perlite, pumice or coir instead of garden soil.
- Fill the bottom of deep planters with a light filler, such as upturned plastic pots, if the plants do not need the full depth.
- Choose fiberglass or plastic containers rather than concrete or stone.
- Make sure every planter drains, so water does not pool and add weight.