Calculate How Much Potting Mix Your Pot Needs Before Repotting
This tool was built by Serge (MSc) using established science and government sources. My academic background covers plant biology, environmental biology, and biogeochemistry. My tools are for educational purposes and give practical guidance, not professional advice.
When you repot a plant, it helps to know how much potting mix the new pot will take before you open a bag.
This calculator works out the volume of a pot from its size, so you can buy the right amount.
It handles round pots and square or rectangular planters, and gives the answer in litres, since that is how most potting mix is sold.
Potting mix needed:
Most pots taper inwards, so the real amount is usually a little less. The plant and its root ball also take up space.
How this works
For a round pot, the calculator uses the area of the circle, which is pi times the radius squared, then multiplies by the height.
For a square or rectangular planter, it multiplies length by width by height.
The result is then converted into litres.
The top width is used, so the figure sits a touch high, which is useful, since it means you will not run short.
Understanding your result
The figure is the full volume of the pot.
In practice you use a little less, because most pots are narrower at the base and the plant's root ball fills part of the space.
Buying to this figure leaves you a small buffer rather than a shortfall.
When you repot, go up only one or two pot sizes at a time, since a pot that is too large holds wet soil around the roots for too long.
Important limitations
This is the geometric volume of the pot, not an exact bag figure.
It does not subtract the root ball, drainage material, or the taper of the pot, all of which reduce how much mix you pour in.
Treat it as a sensible upper guide for how much to have ready.
Scientific basis
The volume method and container guidance behind this tool come from university extension services, the public gardening advice arm of the land-grant university system:
- University of Maryland Extension: Soil to Fill Raised Beds
- Mississippi State University Extension: Constructing Raised Beds
- Purdue University Extension: Container and Raised Bed Gardening
Keep Planning With These Calculators
I recommend these calculators alongside this one because a repotting job also needs the right amount of soil, the right feed, and the right light.
For beds and larger containers, the Soil Volume Calculator works out the fill in litres, cubic feet, and bags.
Once the plant is in its new pot, the Fertiliser Dilution Calculator turns a label dose into the exact feed for a watering can.
To check that the new spot gives enough light, the DLI Calculator turns a light-meter reading into a daily light integral.
Related reading
My guide on how to repot a houseplant without killing it walks through the whole job.
My article on why houseplant leaves turn yellow covers the watering problems a wrong pot size can cause.
This calculator is for educational purposes and general guidance only. It is not professional horticultural advice. Always check product labels and your local supplier before buying.
