Formula used
Round pots use π × radius² × height. Rectangular containers use length × width × height. Tapered containers use frustum formulas for better accuracy.
| Shape | Formula |
|---|---|
| Round pot | V = πr²h |
| Square pot | V = s²h |
| Rectangular planter | V = lwh |
| Tapered round pot | V = πh(R² + Rr + r²) ÷ 3 |
| Tapered rectangular pot | V = h(A₁ + A₂ + √(A₁A₂)) ÷ 3 |
| Hanging basket | Spherical-cap volume |
How to use this calculator
Choose each container shape and measurement unit. Enter top, bottom, and height dimensions carefully. Add every container before calculating the final soil requirement.
Enter existing soil, drainage depth, and root displacement. Add realistic settling and waste allowances. Select the bag size used by your preferred supplier.
Review the final volume and complete bag count. Use the mix planner for ingredient estimates. Compare products to find the lowest practical purchase cost.
Example data
| Example | Shape | Dimensions | Quantity | Extra settings |
|---|---|---|---|---|
| Patio pots | Tapered round | Top 36 cm, bottom 24 cm, height 32 cm | 4 | 10% compaction, 8% waste |
| Window boxes | Rectangular | 80 × 22 × 20 cm | 3 | 3 cm drainage |
| Grow bags | Round | 35 cm diameter, 40 cm height | 6 | 15% root displacement |
Soil volume conversions
| Volume | Equivalent |
|---|---|
| 1 litre | 1,000 cubic centimetres |
| 1 litre | 0.0353 cubic feet |
| 1 cubic foot | 28.3168 litres |
| 1 cubic yard | 764.5549 litres |
| 1 US gallon | 3.7854 litres |
| 1 Imperial gallon | 4.5461 litres |
Frequently asked questions
How accurate is the container soil estimate?
The result is accurate when dimensions match the usable interior. Tapered containers need both top and bottom measurements.
Should I include the drainage layer?
Yes. Enter its depth so displaced soil is removed.
What compaction allowance should I use?
Five to fifteen percent suits many loose potting mixes. Very fluffy blends may need more.
Why does the calculator round bags upward?
Suppliers sell complete bags. Rounding upward prevents a material shortage.
Can I calculate several container sizes together?
Yes. Add separate rows for every container size.
How do I estimate existing soil?
Estimate the filled percentage of the usable container volume. Use a conservative value when uncertain.
What is root-ball displacement?
It is space occupied by the plant's existing roots. That space does not need new soil.
Can I use inches and receive litres?
Yes. Input and output units operate independently.
Does the hanging basket formula fit every basket?
It models a spherical cap. Unusual profiles remain approximate.
Can the mix percentages exceed 100?
No. A valid custom mix should total exactly 100 percent.
Does price comparison include delivery charges?
No. Add delivery charges manually when comparing final supplier costs.