How many litres in a 200 mm, 300 mm or 400 mm pot?
Pot sizes in Australia and New Zealand are labelled by the top diameter in millimetres, but the useful number is the volume. Because nursery pots taper, a "300 mm pot" holds far less than a 30 cm cylinder would. These are typical figures for standard plastic nursery pots, to the brim and at a practical 90 % fill:
| Pot size (top diameter) | Brim volume | Mix needed (≈90 %) | Pots per 25 L bag |
|---|---|---|---|
| 100 mm tube / pot | 0.5 L | 0.45 L | 55 |
| 140 mm | 1.3 L | 1.2 L | 20 |
| 200 mm | 3.9 L | 3.5 L | 7 |
| 250 mm | 7.8 L | 7 L | 3 |
| 300 mm | 13.5 L | 12 L | 2 |
| 400 mm | 30 L | 27 L | 0.9 (buy 2 bags for 1 pot… or a 30 L bag) |
| 500 mm | 60 L | 55 L | 0.45 |
| Half wine barrel (≈600 mm) | 120 L | 110 L | 0.2 (5 bags) |
Terracotta and glazed pots vary much more than plastic nursery pots, and self-watering pots lose 20–30 % of their height to the reservoir. When in doubt, measure and use the calculator.
The maths behind a tapered pot
R = top radius, r = bottom radius, h = inside height (all in cm) → divide by 1,000 for litres
Worked example for a 300 mm pot: R = 15 cm, r = 11 cm, h = 27 cm. R² = 225, R·r = 165, r² = 121, sum 511. π × 27 ÷ 3 = 28.27. 28.27 × 511 = 14,447 cm³ = 14.4 L to the brim. At 90 % fill that is 13 L, so a 25 L bag fills one pot with almost enough left for a second.
For a straight-sided cylinder the formula collapses to π × r² × h, and for a rectangular trough it is simply length × width × height.
Potting mix, not garden soil
Containers need a mix that drains freely and holds air, which is why potting mixes are built on composted pine bark rather than mineral soil. Garden soil or "garden mix" in a pot slumps, waterlogs and sets like concrete when it dries. A few things worth knowing when you are standing in the aisle:
- The Australian Standard ticks. AS 3743 potting mixes are sold with a red-tick logo (regular) or a black-tick logo (premium). Premium mixes contain enough fertiliser for the first few weeks and have tighter limits on wettability and air-filled porosity; regular mixes need feeding from day one. For vegetables, herbs and anything you want to perform, buy premium.
- Specialty mixes exist for a reason: orchid and cactus mixes are much coarser; native mixes are low-phosphorus; seed-raising mix is fine and low in nutrients. Do not raise seeds in premium mix – the fertiliser can burn them.
- Handling. Potting mix carries a safety warning in Australia because it can harbour Legionella longbeachae. Open bags away from your face, wet the mix before handling, wear gloves and a mask if you are doing many pots, and wash your hands afterwards.
- Re-potting. When you move a plant up a pot size, you only need the difference in volume. A 200 mm plant going into a 300 mm pot needs about 12 – 3.5 = 8.5 L of fresh mix.
- Choosing the pot. The pot size guide lists the minimum litres each vegetable, herb and fruit tree needs.
NZ has no compulsory potting mix standard, but most major brands sold in NZ are made to the Australian AS 3743 specification or an equivalent in-house one. The same "premium vs regular" logic applies.
Potting mix questions
How many 25 L bags of potting mix do I need for a 400 mm pot?
A typical 400 mm nursery pot needs about 27 L, so two 25 L bags (with 23 L left over) or one 30 L bag. Measure your pot – decorative 400 mm pots are often shallower.
Can I put gravel in the bottom of a pot for drainage?
It does not improve drainage – water hangs in the mix above a coarse layer (the "perched water table") – and it wastes volume. Use a good mix, make sure the drainage holes are open, and put the pot on feet instead.
How much does a bag of potting mix weigh?
A 25 L bag is usually 8–12 kg depending on moisture; a 50 L bag 16–22 kg. Bark-based mixes are much lighter than soil, which is another reason not to use soil in pots on balconies.
Sources and method
- Standards Australia, AS 3743:2003 Potting mixes – the basis of the red-tick and black-tick marks. standards.org.au
- Pot volumes computed with the frustum formula from typical nursery pot dimensions; figures rounded and cross-checked against manufacturers' stated capacities.
- Potting mix handling advice follows Australian state health-department guidance on Legionella longbeachae.
Written by DueRiva · Last reviewed 22 September 2026 · Report an error