| The short answer. A 36-inch round planter holds about 21 cubic feet if you fill it to the rim — roughly 11 bags of potting mix. Most plants only root in the top 12 to 18 inches, so you can fill the lower two-thirds with a lightweight, non-compacting filler and cut your soil volume by half. That saves money, saves your back, and saves a great deal of weight on a deck or balcony. |
Large planters catch people out. A 48-inch planter looks like it might take four or five bags of soil. It takes twenty-five. At current prices that is a meaningful line item, and it is also several hundred pounds of material you have to carry up a set of stairs.
The good news is that almost none of that soil is doing anything. This guide covers what a planter actually holds, how much of it needs to be real growing medium, what belongs in the bottom, and — the part most guides skip entirely — what the finished thing weighs.
How much soil does a large planter hold?
Volume is straightforward geometry. For a round planter:
cubic feet = π × (internal radius in inches)² × internal depth in inches ÷ 1728
For a rectangular planter, it is length × width × depth in inches, divided by 1728. Use internal dimensions, not the nominal outside size — on a heavy-gauge steel planter the difference is small, but it is not nothing.
Here is what that works out to across our standard round sizes. "Full fill" is soil to the rim. "Reduced fill" is 18 inches of growing medium over a filler base, which suits the great majority of planting schemes.
Round planters
| Nominal size (round) | Total volume | Full fill | Reduced fill (18″) | Soil saved |
|---|---|---|---|---|
| 24″ dia × 24″ deep | 6.3 cu ft | 3 bags | 2.5 bags | 25% |
| 30″ dia × 30″ deep | 12.3 cu ft | 6 bags | 4 bags | 40% |
| 36″ dia × 36″ deep | 21.2 cu ft | 11 bags | 5.5 bags | 50% |
| 42″ dia × 42″ deep | 33.7 cu ft | 17 bags | 7.5 bags | 57% |
| 48″ dia × 48″ deep | 50.3 cu ft | 25 bags | 9.5 bags | 62% |
Bag counts assume 2 cubic foot bags of potting mix, rounded to the nearest half bag.
Rectangular planters
| Nominal size (rectangular) | Total volume | Full fill | Reduced fill (18″) | Soil saved |
|---|---|---|---|---|
| 48″ × 18″ × 24″ | 12.0 cu ft | 6 bags | 4.5 bags | 25% |
| 60″ × 20″ × 30″ | 20.8 cu ft | 10.5 bags | 6.5 bags | 40% |
| 72″ × 24″ × 36″ | 36.0 cu ft | 18 bags | 9 bags | 50% |
At a typical $14 per 2 cu ft bag, filling a 48-inch round planter to the rim costs about $350. Filling it properly costs about $135 — a saving of well over $200 on a single planter.
You probably don't need to fill it to the bottom
Root depth is the thing to design around, not planter depth. As a working guide:
- Annuals, herbs and most perennials: 8–12 inches
- Ornamental grasses and small shrubs: 12–18 inches
- Larger shrubs and small multi-stem trees: 18–24 inches, and check the root ball
A 42-inch-deep planter holding lavender is carrying roughly two feet of soil that no root will ever reach. That soil holds water, adds weight, and costs money. Replace it.
What to put in the bottom of a large planter
A good filler is lightweight, does not compact under load, does not decompose into a soggy layer, and does not hold standing water. In rough order of how well they work:
Recommended
- Rigid foam blocks or offcuts. Close to weightless, completely stable, will not rot. Break into fist-sized chunks so the medium above knits into the gaps rather than bridging over a flat sheet.
- Upside-down nursery pots. Free, rigid, self-draining. Nest them open-side-down so they cannot fill with water and silt.
- Capped plastic bottles or jugs. Effective and free, but cap them tightly. An open bottle becomes a small reservoir of stagnant water.
- Coarse wood chips or small logs. Good if you want the planter to develop a richer, more biologically active base over time. They will break down, so expect the soil level to drop by a few inches in the first two seasons and plan to top up.
Avoid
- Packing peanuts, unless you are certain they are polystyrene. The starch-based kind dissolves. You will end up with a compacted paste at the bottom of your planter and no idea why the drainage failed.
- Loose bagged leaves or grass clippings. They compact to nothing and go anaerobic.
- Anything that traps water. Open containers, tarps, plastic sheeting laid flat across the base.
- Gravel, rock or broken pottery. The reason this one is a mistake needs its own section.
Whatever you use, lay a permeable barrier — landscape fabric or weed cloth — between the filler and the growing medium. It stops soil washing down into the void and the level dropping over the season, while still letting water through.
Why gravel in the bottom doesn't improve drainage
This is the most repeated piece of advice in container gardening and it is backwards.
Water does not move readily from a fine-textured material into a coarse one. It has to saturate the fine layer first before it will cross the boundary. So when you put gravel under potting mix, the mix directly above the gravel stays waterlogged — it forms what is called a perched water table, a saturated zone that sits at the interface.
The effect is the opposite of the intent. Adding a four-inch gravel layer does not lower the wet zone in your planter. It raises it by four inches, straight into the root zone. You have made the drainage worse and added forty pounds of rock while doing it.
What actually controls drainage:
- Enough drainage holes, kept clear. This is most of it.
- A gap under the base. Feet, rails or a stand-off let water leave and air circulate. A planter sitting flat on a paved surface can seal itself against the ground.
- The medium itself. A container mix with perlite, pine bark or coarse sand drains. Bagged garden soil or topsoil in a container does not — it compacts into something closer to a brick.
If you want to raise the effective bottom of the planter, do it with a rigid, open filler and a permeable barrier, not a solid layer of stone.
How much will it weigh?
This is the question that matters most and gets asked least — and it is where a lot of balcony and roof-deck planting goes wrong.
Growing medium is dramatically heavier wet than dry. Depending on the mix, saturated weight typically runs two to three times dry weight. The planter that felt manageable on installation day is a different object after a week of rain, and that is the condition you have to design for.
| Material | Dry (lb/cu ft) | Saturated (lb/cu ft) |
|---|---|---|
| Peat-based potting mix | 20 | 50 |
| Engineered / green-roof media | 45 | 75 |
| Bagged topsoil or garden soil | 80 | 100 |
| Rigid foam filler | 2 | 3 |
Applied to a filled planter:
| Planter | Empty | Full fill, saturated | Reduced fill, saturated |
|---|---|---|---|
| 36″ round | 90 lb | 1,150 lb | 670 lb |
| 42″ round | 120 lb | 1,805 lb | 925 lb |
| 48″ round | 160 lb | 2,675 lb | 1,200 lb |
Notice that the planter itself is a small fraction of the total. The soil is the load. Reduced fill more than halves the saturated weight on every size in this table.
On a residential balcony or wood deck, reduced fill is not optional, it is the whole strategy. It is also worth knowing what your structure is rated for before you commit to a size, and positioning heavy planters over joists or near the building wall rather than out at the edge of a cantilever.
On a roof deck, podium slab or any commercial structure, this stops being a rule of thumb and becomes a structural question for the project engineer. We publish dry and saturated load figures, base dimensions and drainage details for every planter we make. If you are specifying for a terrace or amenity deck, the load and spec data is here — and it is far easier to use before the structural set closes than after.
Drainage, liners and freeze-thaw
A few things that will decide whether the planter is still working in five years:
- Keep the drain holes clear. A permeable barrier over the holes stops them silting up. Check them once a season.
- Lift the planter off the surface. Feet or rails give water somewhere to go and stop moisture being trapped between the base and the paving.
- Liners are for protecting finishes, not for drainage. If you use one, it must be perforated at the base. A sealed liner turns a planter into a bucket.
- In freezing climates, water expanding in saturated media is what destroys planters and root systems over winter. Free drainage and a lifted base matter far more here than any amount of wrapping.