Calculator
Example data table
| Units | Top diameter | Bottom diameter | Height | Fill % | Allowance % | Pots | Adjusted per pot (L) | Total (L) |
|---|---|---|---|---|---|---|---|---|
| cm | 24 | 16 | 20 | 95 | 10 | 6 | 6.310 | 37.860 |
| in | 10 | 7 | 9 | 100 | 8 | 4 | 8.905 | 35.620 |
Formula used
A tapered pot is modeled as a frustum (truncated cone). Convert diameters to radii, then compute volume.
How to use this calculator
- Measure the inner top diameter, inner bottom diameter, and inner height.
- Select units, then enter your measurements in the fields.
- Set fill level to leave headspace under the rim.
- Add an allowance percentage for settling and spillage.
- Enter the number of pots to estimate total soil needed.
- Click Calculate Volume to view results above the form.
- Use the export buttons to save CSV or PDF reports.
For irregular shapes, use this as a close estimate and round up.
Article: Planning mix volume for tapered pots
1) Why tapered geometry changes volume
Many nursery containers narrow toward the base. Using a straight cylinder often overestimates soil needs. This calculator models the pot as a truncated cone, using top diameter, bottom diameter, and height to compute a realistic internal capacity for potting mix purchases.
2) Typical sizes and practical ranges
Common tapered pots range from 15–35 cm (6–14 in) top diameter and 12–30 cm (5–12 in) height. A 25 cm top, 17 cm base, and 22 cm height pot is often near 6–8 liters, depending on taper and fill level. Use inner measurements, not rim-to-rim exterior values.
3) Fill level and headspace decisions
Gardeners typically leave 2–4 cm (about 1–1.5 in) of headspace under the rim for watering. That reduces usable volume by roughly 5–15% on many containers. Setting a fill level below 100% helps prevent overflow and improves irrigation control during top watering.
4) Allowance for settling and handling loss
Fresh mixes settle after initial watering and root growth. A 5–15% allowance is common for peat/coco-based blends, especially when filling multiple pots. For coarse bark mixes, settlement may be lower, but spillage during transport can increase, so an allowance still protects your order.
5) Turning results into a buying plan
Use the adjusted liters per pot to estimate bags needed. For example, if the calculator reports 6.31 L per pot and you have 6 pots, total is about 37.86 L. A 50 L bag covers the batch with margin, while two 25 L bags may reduce storage and keep mix fresher.
FAQs
1) Is the result exact for every pot shape?
No. The model assumes a smooth frustum. Pots with ribs, thick walls, or curved sides may differ. Use the estimate and round up slightly for safe purchasing.
2) Should I measure inner or outer diameters?
Measure inner diameters and inner height whenever possible. Outer measurements can overstate capacity because wall thickness reduces the available soil volume.
3) What fill level should I use for most plants?
Many growers use 85–95% fill to leave watering headspace. For mulch-topped pots or frequent top watering, choose a lower fill level to avoid runoff and mess.
4) Why add an allowance percentage?
Mix settles after watering and you may lose material to spillage. An allowance of 5–15% helps ensure you do not run short when filling many containers.
5) How do I convert liters to soil bags?
Bag labels are often in liters or cubic feet. Divide your total liters by the bag size in liters. For cubic feet bags, 1 ft³ is about 28.3 liters.
6) Can I use this for pond baskets or fabric pots?
Yes, if the container is roughly tapered and you can measure top, base, and height. For highly flexible fabric pots, measure when filled to typical working shape.
7) What if my pot is wider at the bottom?
Enter the larger diameter as the “top” and the smaller as the “bottom” only if height reference stays consistent. The formula works either way, but keep measurements paired correctly.