Calculator Inputs
Choose a planter shape, enter measurements, then estimate soil, drainage, bag counts, cost, and weight.
Example Data Table
| Shape | Dimension Basis | Key Inputs | Adjusted Soil Volume | Bags to Buy | Estimated Cost |
|---|---|---|---|---|---|
| Rectangular | External | 60 × 30 × 35 cm, wall 1.5 cm, base 2 cm, fill 90%, drainage 4 cm, bag 25 L | 42.72 L | 2 | $18.00 |
| Cylinder | Internal | Diameter 40 cm, height 32 cm, fill 92%, drainage 3 cm, bag 20 L, bag cost $8 | 35.88 L | 2 | $16.00 |
| Tapered Round | Internal | Top 42 cm, bottom 30 cm, height 36 cm, fill 88%, drainage 4 cm, bag 25 L | 31.96 L | 2 | $18.00 |
Formula Used
Rectangular Planter
Volume = Length × Width × Height
Use internal dimensions. If you only have external measurements, subtract two wall thicknesses from length and width, then subtract base thickness from height.
Cylindrical Planter
Volume = π × r² × h
Radius equals internal diameter divided by two. This suits round pots, sleeves, and straight-sided container tubs.
Tapered Round Planter
Volume = π × h × (D₁² + D₁D₂ + D₂²) ÷ 12
This frustum formula works for containers with a wider top and narrower base, which is common in decorative garden pots.
Oval Planter
Volume = π × Major Diameter × Minor Diameter × Height ÷ 4
This uses the oval base area multiplied by height, making it useful for trough-style and elliptical planters.
Drainage and Fill Calculation
Net Soil = (Fill % × Internal Volume) − Drainage Volume
Drainage depth is limited to planter height. The net soil result shows the usable growing medium after reserving the lower layer.
Bags, Cost, and Weight
Adjusted Soil = Net Soil × (1 + Waste %)
Bags = Adjusted Soil ÷ Bag Size
Weight = Adjusted Soil × Soil Density
How to Use This Calculator
- Select the planter shape that best matches your container.
- Choose one measurement unit and use it consistently.
- Decide whether your measurements describe the internal cavity or the outside shell.
- Enter all required dimensions for the selected shape.
- Add fill level, drainage depth, soil bag size, bag cost, and density.
- Click Calculate Planter Volume to show the results above the form.
- Review the graph, then export the report as CSV or PDF.
Frequently Asked Questions
1. Should I use internal or external measurements?
Use internal measurements when you can reach the cavity directly. Use external measurements when measuring the outside of the pot, then enter wall and base thickness so the calculator can estimate usable space correctly.
2. Why does the calculator ask for a drainage layer?
A drainage layer reserves space for coarse material, air pockets, or a false bottom. That lowers the actual soil amount you need, which helps prevent overbuying compost or potting mix.
3. What does fill level mean?
Fill level is the share of the internal cavity you want filled. Gardeners often leave a small gap below the rim for watering, mulch, or top dressing, so 85% to 95% is common.
4. Why is there a waste and settling allowance?
Soil settles after watering and planting. Some mix also spills during handling. The allowance adds a safety margin, helping you buy enough material without returning to the garden center mid-project.
5. How accurate is the soil weight estimate?
It is an estimate based on dry bulk density. Actual weight changes with moisture, bark content, compost blend, and mineral additives. Use a higher density value when the mix contains more sand or wet compost.
6. Which shape should I choose for sloped containers?
Choose tapered round for pots with a narrower base and wider top. That formula captures the changing diameter better than a cylinder, so the final soil estimate is closer to real container capacity.
7. Why are bag counts rounded up?
Stores sell whole bags, not fractions. The exact bag count is useful for planning, but the rounded value tells you how many bags to purchase so the job can be completed without shortages.
8. Can I compare several planters with this page?
Yes. Run one planter, export the report, then enter the next planter. The example table also gives a quick benchmark for comparing container sizes, bag counts, and estimated spend.