Koi are large, long-lived pond fish, and feeding them well means thinking in percentages of body weight rather than pinches or handfuls. A small koi and a large koi need very different amounts, and water temperature changes the right percentage sharply. This calculator estimates total koi weight from length, picks a feeding percentage for your water temperature, and returns a daily amount in grams.
Body weight is the honest base for koi feeding. Two ponds with the same number of fish can hold very different total weights, because a thirty-centimetre koi weighs several times more than a fifteen-centimetre one. Length gives a fair weight estimate, because koi grow in fairly predictable proportions when they are healthy. This calculator uses a standard length-to-weight curve, so you do not need to catch and weigh your fish.
Temperature is the second input because koi are cold-blooded. Warm koi digest fast and grow quickly, so they can use about two percent of body weight per day in warm water. As water cools, digestion slows, and the percentage falls step by step. Below about eight degrees, most keepers stop feeding entirely, because koi rest near the bottom and digest almost nothing. Feeding a warm-water percentage into cold water is the classic koi-keeping mistake.
To use the tool, enter your koi count, their average length in centimetres, and the current water temperature in degrees Celsius. Use a pond thermometer, not the air forecast, because pond water lags behind air temperature by hours or days. If your koi vary a lot in size, calculate small and large groups separately and add the totals. That extra step is worth it, because one average can hide a large weight difference.
Here is a worked example. Five koi averaging thirty centimetres weigh about one thousand four hundred and eighty-five grams in total in this calculator. At twenty-four degrees, the feeding rate is two percent, which gives about thirty grams per day. At fifteen degrees, the rate falls to one percent, giving about fifteen grams. At nine degrees, the rate is half a percent, giving about seven grams. The same fish, three very different meals, all driven by temperature.
Split the daily amount into two or three meals while the water is warm. Koi eat eagerly at the surface, and a single large meal often leaves pellets drifting into the skimmer or filter. Smaller meals are eaten cleanly, and they let you watch every fish. A koi that hangs back while others feed may be ill, bullied, or cold, and you will only notice if you watch feeding time. Scatter pellets over a small area so you can see what is eaten.
Food choice should follow temperature too. High-protein growth food suits warm water, when koi build muscle and colour. Wheatgerm-based food suits cool water, because it is easier to digest when metabolism is slow. In very cold water, do not feed at all, even if a koi rises on a sunny afternoon. A brief warm spell does not restart digestion, and food eaten then can sit undigested and harm the fish.
Growth goals change the percentage within reason. Keepers growing young koi for size may feed at the top of the warm-water range, with frequent water changes to handle the waste. Keepers maintaining adult show koi often feed a little less, to protect water quality and body shape. Never exceed about three percent of body weight per day, even for fast-growing young koi, because the waste load rises faster than growth, and water quality falls.
Water quality is the limit on all koi feeding. Koi produce a lot of waste, and a pond filter has a daily capacity. If ammonia or nitrite rises after feeding, the pond is telling you the portion is too large or the filter is too small. Reduce food first, because that is instant, and check filter cleaning second. Clear water, steady tests, and eager feeding together mean the amount is right. Green water or rising nitrate often means the amount is too high.
Seasonal transitions need patience. In spring, increase food slowly over two or three weeks as water warms, rather than jumping to a summer amount on the first warm day. In autumn, reduce food the same way, stepping down as temperature falls. Sudden changes stress digestion and can trigger illness. Keep a simple log of temperature and daily grams through spring and autumn, and the pattern becomes easy to repeat each year.
New koi deserve a gentle start. Quarantine new fish when you can, and feed lightly for the first week while they settle. Stress suppresses appetite, so uneaten food in a quarantine tank fouls the water fast. Once new koi feed eagerly for several days, fold them into the main calculation by recounting the pond. Do not forget to update the count after sales, losses, or gifts, because a stale count quietly overfeeds the remaining fish.