How much potting mix do I need?
It comes down to the volume of the pot. A small pot on a windowsill needs a couple of quarts, while a large patio planter can swallow a whole bag. The figures below are for straight-sided round pots filled to about an inch below the rim.
Pots that narrow toward the base hold roughly a fifth less than these figures, so use the tapered pot option for a closer answer.
How does the calculation work?
The calculator works out the volume of soil in one pot, multiplies it by the number of pots and converts the answer into the units used on the bag.
Round pot = π × radius² × depth
Rectangle = length × width × depth
Tapered pot = ⅓ × π × depth × (R² + R × r + r²)
R is the radius at the top and r the radius at the bottom. Example: a 10 in round pot filled 9 in deep holds 3.14 × 5² × 9 = 707 cubic inches, which is about 10.5 dry quarts or 11.6 litres.
Potting mix bags in the US are labelled in dry quarts or cubic feet. One cubic foot is about 25.7 dry quarts, or 28.3 litres.
Which pot shape should I pick?
Measure inside the pot, not across the rim, and measure the depth from the base to where the soil will stop.
Round
Straight-sided pots, buckets and grow bags. You only need the diameter and depth.
Best for nursery pots and grow bags
Rectangle
Window boxes, troughs and square planters. Measure length, width and depth.
Best for window boxes and troughs
Tapered pot
Classic flower pots that are wider at the top. Measure both diameters and the depth.
Best for terracotta and plastic flower pots
Large planters
For anything bigger than about 2 ft across, a raised bed mix is often cheaper than bagged potting mix.
Best for patio beds and big planters
Why add extra potting mix?
Fresh potting mix is light and fluffy, and it sinks by about a tenth once it is watered and the roots settle in. Adding 10% means you can top the pots up straight away instead of going back to the store.
Can I use garden soil in pots?
It is best not to. Garden soil packs down hard in a container, drains poorly and can carry weeds and pests. Potting mix is made from peat or coir, bark and perlite, so it stays light and holds both air and water. For a raised bed, a blend of topsoil and compost works better and costs less.