As a koi keeper, one of the most important numbers you should know is your pond’s exact gallonage. Whether you're treating for parasites, adding dechlorinator, or managing water parameters, nearly every product or treatment dosage depends on knowing your pond volume.
Unfortunately, many koi owners don’t know this number — and guesswork can be dangerous. Overdosing or underdosing medications can stress, injure, or even kill your koi. That’s why we always recommend knowing your pond volume with precision.
The problem with Natural Style or Ecosystem Ponds
If your pond is a perfect rectangle — lucky you! Calculating volume is easy:
But many koi hobbyists enjoy a more natural, ecosystem-style designs with boulders, plant shelves, irregular shapes, and varying depths. These make calculating volume extremely difficult — even with rough measurements, you're often just guessing.
Luckily, there's a simple and accurate method for estimating pond gallonage using something you likely already have in your koi toolkit: salt. The only other tool you'll need is a salinity reader which can be purchased off Amazon extremely affordably! Click here to check out the salinity meter we recommend.
Using Salt to Measure Pond Gallonage
Here's the rule you need to know:
1 lb of salt will raise the salinity of 100 gallons of water by 0.1%.
With that, you can reverse-engineer your pond’s gallonage by measuring how much the salt % changes when you add a known amount of salt.
Once you apply the salt, wait a few hours for the salt to dissolve. Also, scatter the salt around the pond so it's not piled up in one area. This will help the salt dissolve faster.
Side Note: If you want an extremely accurate measurement, you can use PPM and convert that to a percentage. For example, if the salinity reader shows 2350 ppm of salt, that roughly equates to .235% salt
The Formula:
Pond Gallons = (Pounds of Salt x 100 )/ Salt Percentage increase
Important: Use the salt % as a number, not a decimal. So for 0.2%, just plug in 2. Or for 2350 ppm, it's 2.35.
Example:
Let’s say you add 2 pounds of salt, and your salinity meter shows a 0.2% increase.
Boom — you now have an accurate estimate of your pond’s gallonage, without needing a tape measure.
Why This Matters?
In emergencies — like a parasite outbreak — you’ll often need to medicate the entire pond quickly. Dosages are always based on gallons. Guessing wrong can lead to:
-
Underdosing, which wastes time and money and allows disease to spread
-
Overdosing, which risks harming or killing your fish
-
Wasted treatments, which can get expensive fast
Knowing your exact pond volume isn’t just a nice-to-have — it's a critical part of responsible koi keeping.
Pro Tips:
-
Always use pure, non-iodized salt (no additives)
- These can be found at your local hardware store. Most brands usually have it in a blue bag of crystallized water softener salt. There's no need to spend money on specialized "pond salt". It's all the same stuff.
- We recommend starting off with 10 pounds of salt and doing your calculations from there. This should be plenty for most ponds without impacting much plant life. For larger ponds, 20 or 30 pounds may even suffice to get a more accurate reading.
- Speaking of plants - remove any salt-sensitive plants or wildlife before testing. For koi and goldfish, they can handle salt levels up to 1% in some cases.
- Do this before an emergency so you already have the number on hand