What this checker works out
This checker calculates a pond's approximate volume from its dimensions and gives guidance on whether it suits fish, along with a check on any fish number you enter.
Its underlying position is that a wildlife pond is usually better without fish. Fish eat amphibian spawn, tadpoles and aquatic invertebrates — the very things a wildlife pond is intended to support.
Enter your details
Before you stock a pond
- Use average depth, not maximum. A pond with shelves is considerably shallower on average than at its deepest point.
- Decide what the pond is for. A wildlife pond and an ornamental fish pond are different projects with different requirements.
- Fish need filtration, aeration and winter management. Wildlife ponds need none of those.
- Never introduce fish or plants from the wild, and never release pond contents into natural water.
Even a few small fish will eat frog and newt spawn, tadpoles, and much of the invertebrate life a pond would otherwise support. Ponds with fish consistently hold far fewer amphibians than ponds without. It is not a question of numbers so much as presence — this is the single most consequential decision in designing a garden pond.
How this calculator works
Volume, then guidance bands:
Volume in litres = length × width × average depth × 1,000
Under 1,000 litres — best as a wildlife pond without fish
1,000 to 3,000 — very low fish numbers at most
Above 3,000 — a modest stock if filtered and maintained
Fish load flagged as high above roughly one fish per 500 litresThe one-per-500-litres figure is a conservative rule of thumb for a lightly managed pond, not a stocking density for an ornamental fish pond, which would be assessed on filtration capacity and fish size rather than volume alone.
Goldfish grow considerably larger than the size they are sold at, and koi far more so. Stocking to a fish count without allowing for growth is a common route to an overstocked pond.
Worked example: a 2 m by 1.5 m pond
Using the default figures — 2 m by 1.5 m at 0.6 m average depth, no fish:
- Approximate volume: 1,800 litres
- Advice: keep fish numbers very low and prioritise plants and oxygenation
- Stocking check: wildlife-friendly, no fish entered
1,800 litres is a decent garden pond and a poor fish pond. Adding four goldfish would put it near the flag threshold immediately — and those goldfish will reach 20 cm or more, breed, and require a filter within a season or two. The same pond left unstocked will hold frogs, newts, dragonflies and pond skaters within a year, at no running cost.
Common mistakes
- Adding a few fish to a wildlife pond. Even a few change what the pond will support.
- Using maximum depth for volume. Average depth is what fills the pond.
- Stocking to the fish's current size. Goldfish and koi grow substantially.
- Introducing wild-caught animals or plants. It spreads disease and invasive species.
- Releasing unwanted fish or plants into the wild. It is illegal for some species and damaging for all.
Frequently asked questions
How do I attract wildlife to a new pond?
Build it and wait — most colonisation happens naturally and surprisingly fast. Include a sloping edge or beach so animals can get in and out, plant native marginals and oxygenators, leave some rough vegetation nearby for cover, and do not add fish. Water boatmen and dragonflies typically arrive within weeks; amphibians within a season or two.
Should I add tap water?
Rainwater is much better — tap water contains chlorine and nutrients that encourage algal blooms. Filling from a water butt is ideal. If a pond must be topped up from the tap in a drought, do it slowly and in small amounts rather than all at once.
Do I need to worry about safety?
Yes, where young children have access. Even shallow water is a drowning risk for toddlers. Rigid mesh set just below the surface, a secure fence, or delaying the pond until children are older are the usual approaches. This is a genuine hazard rather than a theoretical one.
What plants should I use?
A mix of oxygenators, floating-leaved plants for surface cover, and marginals in the shallows. Native species suit wildlife best. Several vigorous non-native aquatic plants are subject to sale bans and must never be put into or near natural water; check current restrictions before buying anything unfamiliar.
Is what I enter stored?
No. Measurements are processed in your browser and never transmitted or retained.
Related tools
References
- Royal Horticultural Society — wildlife pond design, planting and management
- GOV.UK — invasive non-native species, restricted aquatic plants and wildlife law
Sources are checked at publication and can change — how I choose and check references.