Potting Soil Calculator: How Much Mix a Container Actually Holds
Pot sizes are printed as a rim measurement, never a volume. Enter the shape and the measurements and this works out the mix the container actually holds.
Why the number on the pot is not the volume you need
Containers are sold by the width of the rim. Two pots both labelled 30 cm can differ by a third in what they actually hold, because one tapers sharply to a narrow base and the other runs straight down. That gap is the reason people buy one bag of mix, fill two thirds of a pot, and go back to the shop.
Measure the inside top, the inside base and the inside height, pick the shape, and the figures below update as you type.
| Example | One container | All containers | Same total in cubic feet | Same total in US gallons | Bags to buy |
|---|---|---|---|---|---|
| 30 cm tapered pot — one tomato | 13.9 L | 13.9 L | 0.49 cu ft | 3.7 gal | 1 |
| 25 cm tapered pot — herbs | 7.3 L | 7.3 L | 0.26 cu ft | 1.9 gal | 1 |
| 40 cm half-barrel — potatoes | 37.8 L | 37.8 L | 1.34 cu ft | 10.0 gal | 1 |
| 20 L grow bag — straight sided | 19.1 L | 19.1 L | 0.67 cu ft | 5.0 gal | 1 |
| 80 cm window box × 3 | 25.6 L | 76.8 L | 2.71 cu ft | 20.3 gal | 2 |
Assumptions and limits
- All measurements are INSIDE the container. A pot sold as "30 cm" is measured across the outside of the rim, so the inside is usually 1–2 cm narrower.
- Bagged potting mix is sold loose and compressed. It settles roughly 10–20% after the first few waterings, so treat the bag count as a minimum and buy one spare for every five.
- The gap below the rim is growing volume you give up on purpose: without it, water runs over the edge instead of soaking in.
- Litres and cubic feet here are volume, not weight. Wet mix is far heavier than dry — check the load rating before filling large containers on a balcony or roof.
How the three shapes are calculated
A straight-sided container is a cylinder: the area of the circle multiplied by the usable height. A rectangular trough is simply length times width times height. A tapered pot is a frustum — a cone with its point cut off — and it uses the top diameter, the base diameter and the height together. That third formula is the one that matters in practice, because almost every moulded plant pot is tapered so it can be stacked in the factory.
The practical consequence: if you estimate a tapered pot as if it were straight-sided, you will over-order by roughly a third.
Reading the result
The litres figure is what one container holds to the fill line. The total multiplies that by your container count, and the bag count rounds up because you cannot buy two thirds of a bag.
If the bag count sits just above a whole number — 3.1 bags rounding to 4 — check whether dropping the headroom by half a centimetre across the batch brings it back down. On a run of ten containers that is often a whole bag.
Where people get caught out
- Measuring the outside. Rim lips are thick on cheap plastic and very thick on glazed ceramic. Measure the soil space, not the pot.
- Filling to the brim. Every centimetre of headroom you skip is a centimetre of water that runs off the top instead of soaking in.
- Forgetting settling. Mix arrives compressed. The level after three waterings is not the level on planting day.
- Ignoring weight on balconies. Volume is the easy half; a large container of wet mix can weigh more than the person who carried it up.