Potting Soil Calculator

How to use this calculator

1. Select the container shape and measure its inside dimensions.
2. Enter the planned filled depth, leaving any watering space above the soil.
3. Set the number of identical containers and the labeled volume of the soil bag.

Formula and method

For the assumptions behind these results, read the explanation below. You can also explore Plant Spacing and Compost.

Use the space inside the container

Pot dimensions printed on a label may describe the outside. Thick walls and a raised base reduce the space available for soil. Measure the interior where possible and use the filled soil depth rather than the complete pot height. If you leave a watering lip, the top diameter should correspond to the height where the soil will finish.

Round flowerpots are often wider at the top than the bottom. Treating that shape as a full cylinder using only the top diameter overestimates its capacity. The tapered-pot formula uses both diameters and models a circular frustum. The rectangular option is appropriate for straight-sided planters with a flat base.

Allow for existing plants and settling

The geometric volume is the capacity of the empty space described by the measurements. A root ball, an inner pot, or another solid insert displaces some of that volume. Subtract the known displacement if you only need material around an existing plant. Do not assume the entire container must be refilled.

Loose growing media can settle during filling and watering. Bag labels, moisture, and compaction also affect the volume you obtain in practice. The calculator rounds the order to complete bags but does not add an arbitrary settling allowance. For many identical pots, fill one as a trial and compare its actual usage with the estimate before ordering the entire batch.