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How to Calculate the Water Volume of your Garden Pond

In this blog post we are going to discuss how you can calculate the total water volume of your Garden Pond, i.e. how many litres or gallons of water does your koi pond hold when full.

The easiest and most accurate way of calculating the volume of your pond is to fill it up while using a water meter. The issue most of us will have with this is that you need to start with an empty pond and have a hose connected to a standalone water meter. If you are building a new garden pond and haven’t filled it yet, then this is the best method.


You simply note the meter reading before you start filling and once you have finished. Then subtract the initial meter reading from the second to calculate the difference. This is your Pond volume including all your pipework, filtration, UV units, Heaters etc.


Once you’ve filled your pond up its still a useful gadget as it allows you to track how much water is passing through your purification filters, so you know when it’s time to replace them.


For most of us we already have our pond up and running and we don’t want to drain it down and refill just to calculate the volume of water it contains. Luckily, you don’t have to, we’ll look at the various options available below.


Let’s look at calculating the volume in litres first:


It helps if your pond is perfectly square or rectangular, but this is rarely the case so just do your best to use an average length that you feel best describes your ponds length, width, and depth. It’s a simple calculation you just multiply the average length x width x depth to calculate the cubic meters (M3). You then multiply this by 1000 to work out how many litres of water your pond system holds.


In the below example we have an average length of 2m with an average width of 1.5m and an average depth of 1.2m.


The first step is to multiply the average length by the average width:

2 x 1.5 = 3


The you multiply the above number by the average depth:

3 x 1.2 = 3.6m3


Now multiply the above m3 number by 1000 to calculate how many litres your Koi pond contains:

3.6 x 1000 = 3600 litres


The above calculation is not much use if you have a round pond, luckily this is also easily calculated. In the below example our garden pond will circular, 2.2m wide and 1.2m deep:


The formula for calculating the volume in a circular pond is:


π x Radius x Radius x Depth x Volume Multiplier


If we look at each component, the first is π or Pi, which is defined as the circumference divided by the diameter of a circle. The circumference divided by the diameter of a circle is always π, no matter how large or small the circle is. Pi to 3 decimal places is 3.142.


So now we know the number for π we need to figure out the Radius, which is easy as its half the diameter, so in our example it would be:

2.2 / 2 = 1.1


We already know the depth and the volume multiplier is 1000 so we now have all the necessary information to work out the pond volume:

3.142 x 1.1 x 1.1 x 1.2 x 1000 = 4562 Litres


The issue with the above example is that most of the round plastic containers available to Koi Keepers have tapered sides, i.e. they are wider at the top than at the bottom. If you only measure from the top will overestimate the pond volume, which could be extremely dangerous if you then use that incorrect figure to work out treatment dosages. If your round pond has tapered sides, then measure the diameter at the top and bottom, and take the average diameter to calculate the radius.


If you prefer imperial measurements, you can convert into gallons by dividing your total volume in litres by 4.54. So, for the above example 4562 / 4.54 = 1005 gallons. You can of course use the same formula to convert any litre volume total into gallons. You may, however, prefer to work out the water volume of your pond in gallons without having to calculate in litres first.


In the example below we will use an average length of 8ft with an average width of 5ft and an average depth of 4ft.


First you multiply the average length by the average width:

8 x 5 = 40sq ft


You then multiply the square foot figure calculated above by the average depth:

40 x 4 = 160sq ft


Now multiple the above total square feet number by the number of imperial gallons that is in one square foot of water, which is 6.23.

160 x 6.23 = 996.8 Gallons


What to do if your Koi Pond is an unusual shape


If you’re unable to measure the volume of your Koi Pond because it’s an unusual shape, then there is a clever technique you can use by adding PDV salt to your pond. Or you may just want to be more precise than the measurement methods described above.

Calculate the approximate quantity of salt you think you need by using one of the methods above to get as close as possible to an accurate pond volume estimate. You should aim for a salt concentration of 3 ppt, which equates to 3kg of salt for each 1000 litres of water.


Now that you have estimated your koi pond volume, we need to work out how much salt to add. For our example below, our estimation is 11000 litres. To reach a salinity of 3 ppt we need 33kg of PDV salt.


You will need a way to measure the salinity, we recommend the Salinity Concentration Tester from Trans Instruments. If you are visiting us at our premises in South Wales, you are welcome to bring a sample of your pond water with you and we will test the salinity for you, Free of Charge.

When you take your initial reading, you will likely get a low reading, even if you have never used PDV salt before, most likely it will read 0.1. Don’t worry this is absolutely fine. Make a note of this number. So, continuing our 11,000 litre estimate example, you add 33kg of PDV salt, and take a salinity reading 12-24 hours later. Make a record of the reading as this is the starting point for calculating the pond volume.


In our example we had an initial reading before the salt was added of 0.1 ppt and a second reading 24 hours after adding the salt of 3.4. We know how much salt we added, 33kg so we now have all the components necessary to figure out the total pond and filtration water volume.


Calculate the change in salinity by deducting the pre salt reading from the reading taken after adding the salt, in our example that would be 3.4 – 0.1 = 3.3ppt. You then divide the weight of salt in kg, by the salinity change figure:

33000 / 3.3 = 10000 litres


Our estimate of 11000 litres was pretty close, but now that we have used the salt volume calculation method we know our pond is 10000 litres. Because of the possible variance in accuracy from the salt meter, it’s wise to put in as much salt as you safely can.


For example, the initial reading of 0.1 could be rounded up or down, resulting in readings anywhere between 0.05 and 0.14. The second reading of 3.4 could result in extreme readings of either 3.35 or 3.44. This accounts for a margin of error of around +/-3%. If we had intended to raise the salinity by 1ppt and therefore only put 11kg of salt, then the margin of error is between closer to 9%.

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