Planter Box Soil Calculator
Calculate soil volume for planters and raised beds from dimensions.
Returns bag count in 1 and 2 cu ft sizes with total weight for deck load planning.
Deck planter soil volume and plant capacity calculation ensures your container has enough growing medium for healthy roots, adequate drainage, and structural weight you can safely place on a deck.
Soil volume formula (rectangular planter box): Usable Soil Volume = Length × Width × (Height − Drainage Layer)
Subtract 2–4 inches for the drainage layer (gravel or drainage cells) at the bottom, and leave 1–2 inches of headspace at the top for watering.
Effective soil depth = Total depth − drainage layer (2–3 inches) − headspace (1.5 inches)
Plant spacing and capacity: Number of Plants = Floor Area ÷ Per-Plant Space Requirement
Recommended minimum soil depth by plant type:
| Plant Type | Minimum Soil Depth | Space per Plant |
|---|---|---|
| Herbs (basil, parsley, chives) | 6–8 inches | 6–8 inches |
| Lettuce / spinach | 6–8 inches | 6 inches |
| Tomatoes (determinate/bush) | 12–18 inches | 24–36 inches |
| Tomatoes (indeterminate) | 18–24 inches | 36+ inches |
| Peppers | 12–16 inches | 18–24 inches |
| Dwarf citrus | 18–24 inches | One per large container |
| Annuals (petunias, marigolds) | 8–10 inches | 6–10 inches |
| Small shrubs | 18–24 inches | Depends on mature size |
Weight calculation — critical for deck safety: Total Weight = (Soil Volume × Wet Soil Density) + Planter Weight + Plant Weight + Water Weight
- Wet potting mix density: 55–65 lbs per cubic foot (about 880–1,040 kg/m³)
- Premium lightweight potting mix: ~45 lbs/cubic foot
- Deck load capacity: residential decks typically rated 40–60 lbs per square foot
Always check deck structural capacity before placing heavy planters. A 24″ × 48″ × 18″ planter fully planted and watered can weigh 200–350 lbs — concentrated on a small area.
Worked example: Deck planter: 36 inches long × 18 inches wide × 16 inches deep.
- Internal dimensions (subtracting 0.5″ walls): 35″ × 17″ × 16″
- Drainage layer: 3 inches. Headspace: 1.5 inches. Effective soil depth: 16 − 3 − 1.5 = 11.5 inches
- Soil volume: 35 × 17 × 11.5 = 6,842.5 cubic inches = 6,842.5 ÷ 1,728 = 3.96 cubic feet
- Soil weight (wet, 60 lbs/ft³): 3.96 × 60 = 237.6 lbs for soil alone
- Plants + planter weight: + ~40 lbs
- Total: ~278 lbs — check that deck can support this concentration before placing
For herbs: 35 ÷ 6 = 5 columns × 17 ÷ 6 = 2 rows = ~10 herb plants in this planter