Concrete Planter Calculator
Estimate concrete for a DIY poured or cast concrete planter box.
Concrete needed
0.09 yd³
for 1 planter
- Outer volume
- 6.00 ft³
- Net concrete volume
- 2.34 ft³
- Cubic yards
- 0.10 yd³
- Bags needed
- 7 bags
Short answer
A 24×24×18 inch planter with 1.5-inch walls and a 2-inch floor uses about 2.3 cubic feet of concrete, or roughly 7 bags of 50 lb mix. Thicker walls, larger planters, or a design with rebar or wire mesh will need more concrete and reinforcement.
Typical planter sizes by plant type
| Planter use | Outer size | Wall thickness |
|---|---|---|
| Small herb or succulent planter | 18 × 18 × 12 in | 1.5 in |
| Medium shrub or flower planter | 24 × 24 × 18 in | 2 in |
| Large tree or hedge planter | 36 × 36 × 24 in | 3 in, with rebar |
How to plan and pour a concrete planter
- Decide on outer dimensions and wall thickness — thinner walls save material but are more fragile to strip from the mold.
- Build (or buy) a nested double form: an outer mold for the exterior shape and an inner mold sized down by the wall thickness on every side.
- Add a few drainage holes through the floor before pouring, or drill them after the concrete cures.
- Enter your dimensions above to get the net concrete volume and bag count.
- Pour the floor layer first, then fill the wall cavity between the forms, vibrating to remove air pockets.
- Cure under plastic for several days, strip the forms carefully, then seal the surface if it will hold moist soil long-term.
Worked example
- Outer volume
- 24 × 24 × 18 in = 10,368 in³ = 6.00 ft³
- Inner cavity
- (24 – 2×1.5) × (24 – 2×1.5) × (18 – 2) = 21 × 21 × 16 = 7,056 in³ = 4.08 ft³
- Net concrete + 10% waste
- (6.00 – 4.08) × 1.10 = 2.11 ft³
Common mistakes
- Making walls too thin without reinforcement, which crack easily when moving or handling the finished planter.
- Forgetting drainage holes, which traps water and can crack the planter in freezing weather.
- Not sealing the inner mold tightly, letting concrete leak between the forms and creating gaps in the wall.
- Skipping a release agent on the forms, making them very difficult to remove without chipping the planter.
Assumptions
- Standard or fine-aggregate premix concrete at roughly 150 lb per cubic foot.
- Bag yields follow manufacturer averages: 80 lb = 0.60 ft³, 60 lb = 0.45 ft³, 50 lb = 0.375 ft³, 40 lb = 0.30 ft³.
- Wall thickness is applied evenly on all four sides; floor thickness reduces the inner cavity height.
- Does not include forms, release agent, rebar or mesh, sealant, or drainage hardware.
Frequently asked questions
How thick should concrete planter walls be?
1.5 to 2 inches is typical for small to medium planters. Larger or taller planters benefit from 2.5 to 3 inches plus light reinforcement, since thin unreinforced concrete can crack when moved.
Do concrete planters need drainage holes?
Yes, in almost all cases. Without drainage, water pools at the bottom, which can rot plant roots and crack the concrete during freeze-thaw cycles.
Should I seal a concrete planter?
Sealing helps reduce efflorescence (white mineral staining) and slows moisture absorption into the concrete, which is especially useful in freezing climates or for planters that stay wet most of the time.