Home Container gardening & soil volume
How much new potting mix do you need when repotting?
Published by Potting Ledger
Start with the volume inside the new pot up to the intended soil line. Subtract the approximate volume of the root ball and old mix that will move with the plant. The difference is the fresh mix needed around and beneath it.
Measure the plug you are moving
Suppose your new planter holds 12 liters to the soil line. The transplant has a roughly cylindrical root ball 16 cm across and 14 cm tall. Its radius is 8 cm, so π × 8² × 14 ÷ 1000 gives about 2.81 liters. If the ball holds together, plan on 12 − 2.81, or about 9.2 liters of fresh mix.
Use the root ball’s own dimensions if it slips free cleanly. A nursery pot marked with a nominal size can have headroom, tapered walls or a root ball that occupies only part of it. If the bottom crumbles away, estimate the smaller plug that actually goes into the new pot.
Allow for the part you cannot measure neatly
If the retained plug could be anywhere between 2 and 3 liters, a 12-liter pot will need about 9 to 10 liters of new mix. Two such plants would use about 18 to 20 liters. Record that range before opening a second bag. Gently watering and allowing loose mix to settle can change the final top-up.
Set the plant at the depth appropriate to the species and keep the planned watering space. Fill gaps around the sides as you go. Rounded volume calculations are for the shopping list. The plant’s previous growing level and actual fit guide the final handful.