Growing note 02 · Metric planning
How many litres does a vegetable need?
Litres help compare containers, but they are not a complete growing specification. A deep narrow pot and a shallow wide trough can hold the same volume and suit different crops.
Use these as planning ranges, not minimum guarantees
The ranges below are editorial starting estimates for modest home growing, not figures quoted from RHS and not experimentally tested here. Larger containers generally buffer moisture better. RHS recommends generous containers and warns that small pots need more frequent watering and feeding.
| Crop / planting | Working volume | Shape / caution |
|---|---|---|
| Loose-leaf salads, small group | 10–20 L trough | About 15–20 cm deep; thin to packet spacing |
| Radishes, short row | 10–15 L trough | About 15–20 cm deep; not for long-rooted types |
| Short-root carrots, small sowing | 20–30 L | About 25–30 cm deep; check variety root length |
| One compact bush tomato | 25–40 L | Generous depth; support if needed; sunny shelter |
| One courgette | 40–60 L | Large footprint and thirsty foliage; not ideal for a tiny balcony |
RHS specifies a 30–45 cm pot for one tomato plant; that diameter alone does not define litres. Do not convert a diameter into capacity without knowing depth and shape.
Measure a box you already own
For a rectangular container, internal length × width × compost depth in centimetres ÷ 1,000 gives approximate litres. A 60 × 25 × 20 cm filled space holds about 30 L. Tapered sides reduce the true capacity; the example is not a weight or structural-load calculation.
Before buying more compost
- Check that drainage holes are open. Do not leave roots standing in collected water.
- Choose a suitable peat-free growing medium and follow its feeding guidance; plain garden soil is often unsuitable for pots.
- Leave watering space below the rim. A container’s advertised brimful capacity is not the usable compost volume.
- Check stability, access and balcony load permission. Larger pots are not automatically safe on a balcony.
Make one comparison at a time
Record the crop, pot capacity, variety and watering pattern. If a pot repeatedly dries out before you can water, reduce the planting density or try a larger suitable container next season. Capacity cannot compensate for insufficient light.
Look for stated capacity, internal dimensions and drainage rather than a decorative photo alone. Compare a deep pot with a trough only after checking the crop’s root needs.
Sources & limits
Sources consulted 7 November 2026. Local conditions vary; seed-packet instructions and cultivar needs take precedence over general examples.