Just a heads up, this post contains some affiliate links. That means that we earn a small commission if you buy the product at no extra cost to you. That allows us to continue to research and share interesting articles with you. :)
Choose a plant pot by its usable capacity, depth and the mature plant’s needs, rather than diameter alone. Small pots suit propagation and temporary growing; established vegetables and perennial plants need enough root space and a container you can keep watered.
A pot that looks generous beside a tiny seedling can feel very small by July. Before buying, think about the plant you will have later, where the container will stand and how often you can check it.
What do plant pot sizes mean?
A centimetre measurement usually describes the width across the top, while a litre measurement describes capacity. Check the supplier’s definition: external width includes the rim, and two pots with the same width can differ considerably in depth and taper.
There is no universal conversion chart that makes every 20cm pot the same number of litres. A shallow bowl, tall nursery pot and straight-sided container can share a top diameter while holding different amounts of compost.
| Measurement | Why it matters | What to check |
|---|---|---|
| Capacity in litres | Indicates approximate growing-medium volume | The stated capacity and space needed below the rim |
| Internal depth | Determines available vertical root space | Crop requirements and any raised internal base |
| Top and base width | Affect planting space and stability | Taper and support for tall plants |
| Drainage outlets | Allow excess water to leave | Open holes and an unobstructed route out |
Match the container to the job
For seedlings, choose manageable propagation pots and move plants on as they grow. For a plant staying in one container through the season, check the final size recommended for that crop and variety.
- Salad leaves: a broad container can provide useful sowing space, but very shallow compost dries quickly.
- Root vegetables: match depth to the roots you intend to harvest, including the named variety.
- Tomatoes: use a generous final container and plan support and watering. A cherry-fruited variety is not necessarily a small plant.
- Perennial herbs: allow for several seasons of growth, with repotting or division when appropriate.
Our UK tomato guide explains growth types. If you are still deciding what to grow, start with the kitchen garden planning guide.
Is a bigger pot always better?
Larger containers generally hold moisture longer, which can make summer care easier. However, putting a small plant into a vast volume of wet compost does not remove the need to judge watering carefully. Choose a sensible progression when potting plants on.
Weight matters too. Plastic may be light when empty, but a large container full of moist compost is a different proposition. Position it before filling, keep access clear and use equipment suited to the load if it needs moving.
The RHS guide to growing plants in containers explains the practical differences between small, large and shallow containers.
Plastic pots: reuse what works
Sound plastic growing pots can be useful, affordable and reusable. Assess the individual pot rather than assuming every plastic container is either harmful or indefinitely durable. Replace brittle or damaged pots and avoid using containers that previously held hazardous substances.
Clean reusable pots before planting. Do not throw away serviceable containers simply to buy a supposedly greener matching set. If buying, consider durability, replacement needs and what local recycling services actually accept.
Get drainage and watering right
Use compost suitable for container growing. Make sure drainage holes are open and that a decorative outer pot is not trapping water around the growing pot. A layer of gravel is not a substitute for drainage outlets.
Leave watering space below the rim. Check moisture in the compost rather than judging only the dry-looking surface, and adjust watering to the weather and plant. A small container in wind may dry quickly even when it is not particularly hot.
Place sun-loving crops where they receive enough light; moving every plastic pot into shade is not a solution to overheating. For a genuinely shaded site, choose suitable crops using our vegetables for partial shade guide.
How much compost should you buy?
Use the manufacturer’s capacity as a starting estimate, remembering that an existing root ball takes up space. For a rectangular planter, internal length × width × filling depth in metres × 1,000 gives litres. Curved and tapered containers need a different calculation.
For rectangular beds and planters, our soil-volume guide includes worked examples. Buy for the containers you intend to plant now, rather than filling every spare pot because it is there.
Before you buy: a quick check
- Identify the crop and variety.
- Check its mature root-space and support needs.
- Compare capacity, depth and stability.
- Confirm drainage and choose suitable compost.
- Check you can water, reach and safely move the container if needed.
A modest set of well-chosen pots is easier to manage than a patio full of thirsty little containers. For the equipment to look after them, our essential gardening tools guide separates useful purchases from things that can wait.
For seasonal growing ideas that fit real life, join The Wealth Pantry, our free fortnightly newsletter.
Reviewed and updated October 2026. By Niki Wilkinson, Back To The Good Life.
Back To The Good Life is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com. We also participate in other affiliate programs which compensate us for referring traffic.
