Measure the inside of the container

For a box or rectangular planter, use internal length, internal width and the fill depth you intend. For a cylinder, use the internal diameter and fill depth. For a tapered pot, measure the internal top diameter, the internal bottom diameter and the internal height, because a single diameter will misstate the volume. Wall thickness and rim shape can make outside measurements noticeably larger than the space actually holding medium.

Fill depth is a planning choice, not a fixed rule. If you want an optional rim gap, subtract that gap from the internal height before calculating. Keep the gap consistent across containers so the comparison between pots remains meaningful.

  1. Measure internal length, width or diameter with the pot empty.
  2. Measure internal height, then subtract any chosen rim gap to get fill depth.
  3. Record dimensions in one unit system before calculating.

Source context: National Institute of Standards and Technology

Calculate box, cylinder and tapered volumes

A rectangular box volume is internal length times internal width times fill depth. A straight-sided cylinder uses pi times radius squared times fill depth. A tapered pot is treated as a frustum: use the internal top and bottom diameters with the fill depth. These are geometrical calculations. They do not identify a pot as a standard nursery size, and a nominal trade name should not be treated as a measured capacity.

The supplied example is one straight-sided cylinder with internal diameter 16 inches and fill depth 12 inches. Using pi geometry, it calculates to about 1.396 cubic feet or 39.54 liters. That is an example of the formula, not a nursery trade-size capacity and not a recommendation for a particular crop.

  1. Use the box formula for straight-sided rectangular planters.
  2. Use the cylinder formula for straight-sided round pots.
  3. Use the frustum formula when top and bottom internal diameters differ.
  4. Check that every dimension is in the same unit before multiplying.

Source context: National Institute of Standards and Technology

Convert volume to whole bags with explicit units

Bag labels govern the conversion. US liquid quarts and US dry quarts are different volumes, so a bag marked in quarts must state which kind. A cubic foot equals 28.316846592 liters, one US liquid quart equals 0.946352946 liters, and one US dry quart is approximately 1.101221 liters. Convert the calculated volume into the bag's stated unit, then round up to whole bags.

Do not assume that two bags with similar-looking numbers hold the same amount. Compare the labeled unit and volume, and keep the calculated pot volume separate from the purchased bag total. Whole-bag rounding is a purchasing step, not a correction to the geometry.

  1. Read the bag label and identify cubic feet, US liquid quarts or US dry quarts.
  2. Convert the calculated volume into that labeled unit.
  3. Divide by the labeled bag volume and round up to a whole number of bags.

Source context: National Institute of Standards and Technology

Choose a medium and handle reuse carefully

Container growing medium must support roots while holding appropriate water and air. Dense garden soil behaves differently from a container mix, so the calculator's volume does not make garden soil equivalent to a soilless mix. The guide lists commercial soilless mix and an equal-volume soilless-mix and compost blend among its container vegetable options.

For reuse, remove plant residue and do not save medium associated with root disease. The guide describes blending old and fresh material rather than assuming old mix is unchanged. Use the calculator to plan quantity, then check the medium and any reuse decision separately.

  1. Select a medium suited to container use rather than dense garden soil.
  2. Remove plant residue before considering reuse.
  3. Discard medium associated with root disease.
  4. Blend old and fresh material rather than treating old mix as unchanged.

Source context: University of Maryland Extension

A worked planning example

Check your measured example

One straight-sided cylinder with internal diameter 16 in and fill depth 12 in; pi geometry, not a nursery trade-size capacity.

QuantityCalculated result
volume cuft1.3963
volume liters39.5378

The example uses a straight-sided cylinder with internal diameter 16 inches and fill depth 12 inches. Pi geometry gives about 1.396 cubic feet or 39.54 liters. It demonstrates the cylinder formula only; it is not a nursery trade-size capacity or crop advice.

Example inputs explain the method. Review the source context, chosen cultivar and actual site before planting or ordering.

Your before-you-plant checklist

  • Internal dimensions recorded for each container
  • Rim gap subtracted from internal height if used
  • Box, cylinder or tapered formula matched to pot shape
  • Bag unit identified as cubic feet, US liquid quarts or US dry quarts
  • Calculated volume converted and rounded up to whole bags
  • Medium choice and reuse decision checked separately from volume

Questions that come up while planning

Should I measure a pot from the outside or inside?

Use internal dimensions. Outside measurements include wall thickness and rim shape, so they can overstate the space available for medium.

How do I account for a rim gap?

Subtract your chosen gap from the internal height to get fill depth, then calculate with that depth. The gap is optional and is a planning choice rather than a fixed requirement.

Why does the bag unit matter?

US liquid quarts and US dry quarts have different volumes. Convert the calculated volume into the exact unit printed on the bag before rounding up to whole bags.

Sources & method

Horticultural guidance is linked to its original source and regional context. Geometry and unit conversions are calculated separately. Read how we prepare and check a page.