This calculator treats a tapered planter as a frustum (a cut-off cone or pyramid). Volumes are computed using inside measurements, so the result matches usable capacity.
- Round taper (cone frustum): V = (πh/12) × (Dtop2 + DtopDbottom + Dbottom2)
- Rectangular taper (pyramid frustum): V = (h/3) × (Atop + Abottom + √(AtopAbottom)), where A = length × width
- Drainage depth: The taper is assumed linear, so sizes at any height are interpolated, then the drainage and soil frustums are calculated separately.
- Select the planter shape and your measurement units.
- Measure inside top and bottom sizes, plus inside height.
- Optional: Enter a drainage layer depth to reserve space.
- Optional: Set a soil fill level to leave watering headspace.
- Press Calculate to view soil and capacity results.
| Example | Shape | Top | Bottom | Height | Drainage | Fill | Usable soil (approx.) |
|---|---|---|---|---|---|---|---|
| A | Round | 40 cm diameter | 22 cm diameter | 35 cm | 5 cm | 90% | ~22.0 L |
| B | Rectangular | 48 × 28 cm | 30 × 16 cm | 32 cm | 4 cm | 95% | ~21.6 L |
| C | Round | 16 in diameter | 10 in diameter | 14 in | 2 in | 85% | ~4.3 US gal |
Understanding tapered planter volume
Tapered planters behave like frustums, so volume changes nonlinearly as the sidewalls narrow. Measuring inside dimensions matters because wall thickness can reduce capacity. For round planters, top and bottom diameters define the taper. For rectangular planters, use inside length and width at both ends. Accurate inputs help estimate potting mix purchases and avoid underfilling during transplanting.
Planning drainage without wasting root space
Drainage layers improve aeration and reduce waterlogging, but they also reduce usable soil. This calculator separates total capacity from drainage volume, then reports soil volume at your chosen fill level. A 3–6 cm layer suits many outdoor containers, while deeper layers can be useful for very tall planters or heavy rains. Keep drainage depth below the full height so roots still have room.
Using fill level to manage watering headroom
Fill level controls headspace for watering, mulch, and top dressing. Many gardeners leave 5–20% headspace to prevent spillover during irrigation. Use higher headspace for fast-draining mixes and frequent watering, or lower headspace for drought-tolerant plants that need more root volume. Adjust fill percent to match your routine and fertilizer strategy, not just the planter size.
Reading results across common volume units
Unit handling affects purchasing decisions. Liters are convenient for bagged media, while gallons and cubic feet help when mixing bulk components. Because dimensions may be taken in centimeters or inches, the calculator converts lengths to centimeters internally, then reports liters and the native cubic unit. This approach reduces rounding error and keeps results consistent when comparing different planter shapes side by side.
Scaling soil orders for multiple planters
Batch planning is where volume estimates pay off. Enter the number of planters to scale usable soil volume for an entire patio or raised-display project. Then translate total liters into bag counts by dividing by your bag size, adding 5–10% extra for settling. Recording results as CSV or PDF helps track seasonal repotting and supports consistent soil recipes across similar containers. For indoor use, consider saucer capacity and overflow space; the soil-only value is most relevant for mix ordering, while total volume helps choose liners and inserts.
What planter shapes does this tool support?
It supports round tapered planters modeled as cone frustums and rectangular tapered planters modeled as pyramid frustums. Choose the option that matches your inside measurements.
Should I measure inside or outside dimensions?
Use inside dimensions. Outside measurements include wall thickness and can overestimate usable capacity, especially on thick ceramic or insulated containers.
How is drainage layer volume calculated?
Drainage depth is treated as the lower portion of the frustum. The calculator interpolates the cross‑section at that height and computes the drainage frustum volume separately.
What fill percentage should I use?
Try 80–95% for most plants. More headspace helps watering and mulching; less headspace maximizes root volume for deep feeders and thirsty annuals.
Why do liters and gallons differ from bag labels?
Bag volumes are often nominal and mixes settle after watering. Add 5–10% extra to account for settling, compaction, and top dressing.
Can I estimate soil for multiple planters at once?
Yes. Enter the planter count to scale usable soil volume. Use the total to plan bulk purchases and keep mix ratios consistent across a group.