Use the free Pond Stocking Calculator below, right in your browser. No sign-up, no software to install, and nothing is sent to a server.
How many fish a pond can hold is one of the most asked pond questions, and also one of the most answered with a single number that ignores the details. The truth is that volume, filtration, fish size and feeding all change the answer. This calculator gives a sensible planning figure from litres, filtration level and adult fish size, so you can stock with a plan instead of a guess.
Enter your pond volume in litres, choose your filtration level honestly, and enter the adult size of the fish in centimetres. The tool allows a number of litres for each centimetre of fish: fifty litres per centimetre with basic filtration, thirty five with good filtration, and twenty five with excellent filtration. It then divides your total fish centimetres by the size of one fish to give a count.
Adult size is the key input, and it is the one most often fudged. A goldfish bought at five centimetres may reach twenty centimetres or more, and a koi can pass sixty centimetres. Stocking for the size in the shop bag guarantees an overstocked pond within two years. Always use the adult size, and if you keep mixed species, run the calculation for the largest species first.
Filtration level is about capacity and maintenance, not about owning an expensive box. Excellent filtration means a filter rated well above your pond volume, cleaned on schedule, with mature bacteria and good water movement. A filter that is undersized, clogged or newly installed is basic filtration no matter what the box claimed. Be honest here, because the calculator can only work with the level you report.
The result is a ceiling for a mature pond, not a shopping list for day one. New ponds need weeks for bacteria to colonise, and each new fish adds waste that the bacteria must learn to process. Add fish a few at a time, wait at least two weeks between additions, and test ammonia and nitrite after each batch. If readings rise, stop adding fish until the water settles.
Feeding changes everything the calculator assumes. Heavy feeding with rich food produces far more waste than light feeding, and uneaten food decays directly into ammonia. Feed only what fish eat in two or three minutes, remove leftovers, and cut feeding in cool weather when fish digest slowly. A lightly fed pond can comfortably hold the calculated number, while a heavily fed pond may struggle with half of it.
Plants and shade help, but they do not double your stocking capacity. Submerged plants take up nutrients and add oxygen in daylight, and floating plants shade the water and slow algae. They are a real benefit, yet at night plants consume oxygen too, and a pond crammed with fish can still run short of oxygen on a warm still night. Treat plants as a buffer, not as a licence to overstock.
Watch the fish themselves as the final check on any number. Fish gasping at the surface, clamped fins, loss of appetite or frequent disease outbreaks all point to a pond under strain, even when the maths looked fine. Water tests confirm it: rising nitrate, falling oxygen at dawn, or any ammonia reading above zero means the load is too high. In that case the fix is fewer fish, better filtration or larger water changes, not another product.
Seasonal changes also matter. Fish grow through summer and produce more waste, while winter slows everything down. A pond that copes in spring may struggle in late summer when fish are biggest, water is warmest and oxygen is lowest. If you stock near the calculated ceiling, plan extra aeration for summer and be ready to rehome fish rather than squeeze them through a hot August.
Finally, keep a written record of what you stock. Note species, sizes and dates, and re-run this calculator each year as fish grow. A pond stocked sensibly at the start stays a pleasure for years, while an overstocked pond becomes a cycle of treatments, losses and frustration. The calculator takes a minute, but the patience it encourages is worth far more than the number itself.
Versión en español: Población de estanque – Cuántos peces según litros