You need 0.67-0.75 cubic feet of soil for a 5-gallon pot.
A standard 5-gallon pot holds 0.67-0.75 cubic feet of soil (1 gallon ≈ 0.134 cubic feet). Exact volume varies slightly by pot shape, but most require ~1.5 cubic feet of bagged soil per two 5-gallon pots. Pre-moistened soil settles, so add 10-15% extra for best results.
Key Factors Affecting Soil Volume
- Pot shape: Tapered pots hold less than straight-sided ones.
- Soil type: Fluffy mixes (e.g., peat-based) compress over time; dense mixes (e.g., clay-heavy) settle less.
- Moisture: Dry soil takes up more space; pre-moistened soil reduces volume by ~10%.
- Drainage layer: Adding rocks/pebbles (1-2 inches) reduces soil needed by ~0.1 cubic feet.
Soil Volume Comparison for Common Pot Sizes
| Pot Size | Volume in Gallons | Cubic Feet Needed | Approx. Bagged Soil (40 lb bag = ~0.75 cu ft) |
|---|---|---|---|
| 1 gallon | 1 | 0.13-0.15 | 1/5 bag |
| 3 gallon | 3 | 0.40-0.45 | 1/2 bag |
| 5 gallon | 5 | 0.67-0.75 | 1 bag |
| 7 gallon | 7 | 0.93-1.0 | 1.25 bags |
| 10 gallon | 10 | 1.34-1.50 | 2 bags |
How to Measure Soil for a 5-Gallon Pot
- Check pot dimensions: Measure diameter (top/bottom) and height. Use a pot volume calculator for irregular shapes.
- Convert gallons to cubic feet:
- 1 gallon = 0.133681 cubic feet
- 5 gallons × 0.133681 = 0.668 cubic feet (minimum).
- Account for settling: Add 10-15% extra soil (e.g., 0.67 + 0.10 = 0.77 cubic feet total).
- Test fit: Fill pot 2/3 full, then add plant + top off. Soil should sit 1-2 inches below the rim.
Common Mistakes to Avoid
- Overfilling: Soil should not reach the rim-leave space for watering.
- Underestimating compression: Peat-based soils shrink significantly when wet.
- Ignoring drainage: A 1-inch gravel layer reduces soil volume by ~0.08 cubic feet.
- Using dry soil: Always pre-moisten to avoid post-planting sinkage.
DIY Pot Volume Calculator
For cylindrical pots, use this formula:
Volume (cubic feet) = π × (radius)² × height ÷ 1728
- Measure diameter (in inches), divide by 2 for radius.
- Height = depth of pot (inches).
- Divide by 1728 to convert cubic inches → cubic feet.
Example: A 12" diameter × 10" deep pot:
3.14 × (6)² × 10 ÷ 1728 = 0.65 cubic feet (~5 gallons).