Koi Pond Calculators

Free pond calculators for koi keepers — work out your pond volume in gallons, estimate how many pounds of fish you are actually carrying, find out how many your filtration can handle, check whether your air pump keeps up with them, and get exact doses for salt and KH. No sign-up, no email required.

Fish in trouble right now? Go straight to the Free Ammonia Calculator. An ammonia reading on its own tells you almost nothing — the same number can be harmless or lethal depending on your pH and temperature, and that is the first thing to establish. If they are gasping at the surface in warm weather, check oxygen instead.

Otherwise, start here: if you do not know your pond volume, run the Pond Volume Calculator first. Almost everything else depends on it, and a bad gallon figure is the most common reason pond treatments go wrong.

How these fit together

Koi pond chemistry runs on one number: gallons. Get that wrong and every dose after it is wrong by the same margin — which is how people end up with salt at 0.5% when they were aiming for 0.3%.

Stocking is the exception, and it is the one most people get backwards. Gallons do not hold fish; filtration does. A 5,000 gallon pond with an undersized filter carries fewer koi than a 2,000 gallon pond with a properly built one. What actually sets the limit is how much ammonia your media can process, and that is driven by how much you feed.

Feeding, in turn, is driven by the weight of fish — which is the second number worth writing down, and the one almost nobody knows.

The usual order:

1. Volume — measure once, write it on the filter housing, never guess again.
2. Fish weight — estimate the pounds of fish you carry. Feed rate, ammonia production and oxygen demand all scale off it.
3. Stocking — find out what your filtration can genuinely carry before you buy another fish.
4. Biomedia — if the answer above was disappointing, this tells you how much media would fix it.
5. Oxygen — media only works if there is air to run it, and the fish need it first. Check this before summer, not during it.
6. KH — a pond below 50 ppm has no buffer and its pH can crash overnight. Heavy feeding strips alkalinity fast, so this and stocking are linked.
7. Salt — only once the pond is stable, and only to a level you have measured rather than assumed.

The ammonia calculator sits outside that sequence, because it is a diagnostic rather than a plan. Reach for it any time a test kit shows ammonia — and always before adding a KH buffer, since raising pH converts more of what you already have into the toxic form.

Common questions

How many fish can I put in my pond?

As many as your filtration can keep up with. The old advice — one fish per 250 or 500 gallons — is close to meaningless, because a three-inch fish and a twenty-four-inch fish are not remotely the same load. A 24" koi weighs around 7.6 lb; a 10" koi weighs about half a pound. Fifteen times the fish, in the same water.

What matters is ammonia. Fish eat protein, and the leftover nitrogen comes back out as ammonia, measured as TAN. Your biofilter grows bacteria that remove it. So the honest calculation is: work out how much ammonia your feeding produces, then check whether your media can process it. The stocking calculator does exactly that, and it also asks how hard you intend to feed — passive holding at 3% of body weight, grow-out at 5%, or power feeding at 9% — because that choice changes the answer by a factor of three.

How much does a koi weigh?

An average-bodied koi weighs about its length in inches, cubed, divided by four — in grams. That puts a 24" koi at roughly 7.6 lb, a 12" koi at just under a pound, and a 10" koi at a little over half a pound.

The cube is the part that catches people out. Fifteen koi at 10" is about 8 lb of fish and a pond that looks empty. The same fifteen fish grown to 24" is 114 lb — fourteen times the load, with nothing added. That is why ponds fail in year four rather than year one. The fish weight calculator works out your whole pond by size class, and converts it into a daily feed amount and a grams-of-ammonia figure.

Is my air pump big enough?

It depends far less on the pump than people expect. Only a small fraction of the air ever dissolves, and that fraction is set by how deep the diffuser sits and how fine the bubbles are — roughly 2% of the oxygen per foot of depth for a fine-bubble diffuser, and about half that for coarse bubble. Move the same stone from a 2 ft shelf to a 4 ft bottom and you have doubled your oxygen for nothing.

Put your pump's rated airflow into the oxygen calculator along with your fish weight and it will tell you what percentage of demand you are covering, and what airflow would be needed to cover it outright. Bear in mind that warm water holds less oxygen no matter what you do — at 86 °F the ceiling is about 7.4 mg/L against 9.1 mg/L at 68 °F.

My ammonia reads 0.5 ppm — is that dangerous?

Impossible to say without your pH and temperature, and that is the entire point. Ammonia exists in two forms: ammonium (NH₄⁺), which fish tolerate well, and free ammonia (NH₃), which burns gills. pH decides the split, and it does so steeply.

At pH 7.5 and 68°F, only 1.2% of your total ammonia is the toxic form. At pH 9.0 and 77°F, it is 36% — thirty times more damage from an identical test result. This is also why ponds often lose fish in the late afternoon: pH peaks as plants and algae strip carbon dioxide out of the water, and a reading that was survivable at dawn is not by evening. Run your numbers through the free ammonia calculator rather than guessing.

How many gallons is my pond?

For a rectangular pond, multiply length × width × average depth in feet, then multiply by 7.48 to get US gallons. Round ponds use π × radius² × depth × 7.48. Average depth matters more than maximum depth — most ponds shelve at the edges, so using the deepest point can overstate volume by 30% or more. The volume calculator handles all of this, including irregular shapes.

How much salt does a koi pond need?

About 8.35 lb of salt per 1,000 gallons raises salinity by 0.1%. Common levels are 0.1% as a general tonic, 0.3% for parasites and stress, and up to 0.5% for short-term heavy treatment. Salt does not evaporate, so you only replace what leaves through water changes, splash-out, or backwashing — which is why the salt calculator asks for your current level rather than assuming you start from zero.

What KH should a koi pond be?

Aim for 100–200 ppm. Below about 50 ppm the pond has almost no buffering capacity and pH can swing dangerously between day and night. Nitrification consumes alkalinity continuously — roughly 7 g of CaCO₃ for every gram of ammonia processed — so a heavily fed pond burns through KH far faster than a lightly stocked one.

Do I need to measure or can I estimate?

Measure. These calculators are only as good as the numbers you put in, and a test kit costs less than one dead koi. Treat every result here as a starting point to verify, not a substitute for testing.

Questions about your specific pond? Get in touch or browse our consultation services.