Fish Weight & Biomass Calculator
You cannot set a feed rate or size a filter without knowing how much fish you have. Enter your pond by size class — you do not need to catch anything.
Your fish
Measure nose to the tip of the tail. Group fish that look about the same size — three rows is usually enough for a whole pond, and eyeballing against a known object (a 12" net ring, a paving slab) is close enough for most purposes.
This is an estimate only. It is here to give you a ballpark idea of your fish load so you are working from a number instead of guessing. It is not a measurement and it is not a substitute for putting a fish on a scale.
What to do with the number
Feed rate follows water temperature, not appetite. Koi digest slowly in cold water and uneaten food becomes ammonia, so the percentage drops as the pond cools.
Off the feed bag. Protein is where the nitrogen comes from, so this sets how much ammonia the pond makes.
Quick reference — average-bodied koi
Weight rises with the cube of length, which is the part almost everyone gets
wrong. Doubling a fish from 12" to 24" does not double its weight — it multiplies it by
eight. That is why a pond that coped fine for three years suddenly cannot, without a single
fish being added.
These are estimates from length, not measurements. A scale is always better. Body shape varies
far more between fish than length does.
How much does a koi weigh?
An average-bodied koi weighs roughly its length in inches, cubed, divided by four — in grams. A 24" koi comes out at about 3,456 g, or 7.6 lb. A 10" koi is about 250 g, just over half a pound. A 12" koi is 432 g, under a pound.
The important part of that formula is the cube. Length is one dimension; a fish grows in three. Every extra inch adds weight faster than the last, so the relationship between how a pond looks and what it actually carries is not intuitive at all.
Why the cube rule catches people out
Take fifteen koi at 10". That is about 8 lb of fish — a light load for almost any filter, and the pond looks empty. Grow those same fifteen fish to 24" and you have 114 lb of fish. Nothing was added. The stocking level went up fourteen-fold.
This is the single most common way a pond fails. It was fine for three years, nothing changed, and then one summer the ammonia will not clear. Nothing did change except the fish, and the fish are the whole load. If you are sizing filtration, size it for the fish you will have at maturity, not the ones swimming today — the stocking calculator takes that number directly.
What you need fish weight for
Feeding
Feed rates are quoted as a percentage of body weight per day — commonly 1% in cool water, around 3% for general holding, and 5% or more for grow-out. Without a weight figure those percentages are unusable, which is why most people end up feeding by eye and either stunting their fish or overloading the filter.
Filtration and ammonia
Fish weight sets feed, feed sets ammonia, and ammonia is what a biofilter has to remove. The chain is: feed × protein × 0.092 = grams of TAN produced per day. From there you can size media with the biomedia calculator, and if a test kit is already showing ammonia, the free ammonia calculator will tell you whether that reading is actually dangerous at your pH and temperature.
Knowing what you actually own
Pond treatments are dosed by gallons, so those run off your pond volume rather than fish weight. Where weight matters is everything on the fish side of the equation — how much they eat, how much waste they make, and whether the pond you built three years ago is still the right size for them.
Common questions
How do I measure a koi without catching it?
You do not need to. Put something of known length near where they feed — a 12" net ring, a paving slab, a length of pipe — and compare. Group the pond into two or three size bands rather than trying to measure individuals. An estimate within an inch or two is fine, because the errors across a group average out.
Does the tail count?
Yes. Measure nose to the tip of the tail. That is the convention these figures are built on, and measuring to the fork instead will understate the weight.
Do goldfish weigh the same as koi?
Close, but slightly less. Using published length-weight data for Carassius auratus, a goldfish at a given total length lands about 3–8% lighter than a koi of the same length, and the gap narrows as they grow. The calculator uses separate curves for each.
One caveat worth knowing: that goldfish data is for standard-bodied fish. Long-finned comets and shubunkins carry a lot of their total length in tail, so at the same nose-to-tail measurement they will weigh less than this estimate. If your pond is mostly long-tailed fish, use the thin body setting.
How accurate is this?
It is a ballpark, and that is the point — a ballpark beats a guess. Length-weight estimates carry real scatter because body condition varies: a well-fed, deep-bodied koi and a thin one of identical length can differ by 40% between them. The thin/average/fatty setting covers ±20% of that. Treat the result as the right order of magnitude for planning feed and filtration, not as a figure to build anything precise on.
What about a fish I can weigh?
Weigh it. Always better. A sock or a soft net and a set of scales beats any formula. This calculator exists for the fish you cannot practically catch, which in most ponds is all of them.
