Calculating Volume of Ponds

By Ray Jordan

Pond owners need to know the accurate volume in gallons of their ponds. Many pond chemical treatments can be fatal to the fish if volume is over estimated and ineffective if under estimated. There is a narrow therapeutic range with a difference of only 10% being significant to the point of being ineffective or damaging to your koi. Exact calculation of dosage, and careful mixing and slowly and evenly dispersing the treatment used all over the pond is the key to effective and safe treatment. Knowing the exact volume of your pond also helps in estimating proper stocking levels and amount of aeration and filtration needed.

Water Meter Method: The most accurate method to determine pond volume is a direct water flow measurement using an “accurate” water meter. The little cheap plastic flow meter timer type units sold at nurseries only estimate water volume they are not accurate enough for our purposes. Water meters can be purchased with hose fittings to mate directly with your water hose or you can utilize your city main water meter for your house provided you do not use any water for other purposes while you are filling your pond. Be aware that on average even with a ¾ inch garden hose flow is only about 6-18 gallons per minute depending on your local water pressure. If we use an average of 12 gallons per minute it would take about an hour to flow 720 gallons. If you you estimate your pond to be 5,000 gallons it should take about 7 hours to fill.

To use this method you need to completely drain your pond and filter system. Then record your water meter reading. Fill the pond and filter system separately if possible so you will know the volume of each as some pond treatments need to bypass the filter system to protect the beneficial bacteria. Remember to use the correct type and amount of de-chlorinator treatment before adding your fish or refilling your filter system. Record the water meter reading when the pond is full and when the filter system is full. Subtract the ending number on the meter from the beginning number. If your meter reads gallons the difference will the volume of your pond/filter system. If the meter reads cubic feet (like most city meters) you will need to multiply that number by 7.48 = gallons.

Tip: If you are using your main city water meter you can run multiple hoses into your pond at the same time to speed up the filling process and it will still be an accurate measurement. Also drain and fill your filter system last and keep it aerated if possible to have the least impact on your beneficial bacteria in an existing pond.

Salt Method: An alternative method to calculate pond volume is accomplished by adding a known weight of salt and testing for salt concentration (salinity) change. This method requires an accurate salt test kit* as follows:

1. Test a pond water sample for salt following the instructions of your test kit. Record this concentration in parts per thousand. For our purposes let’s say it is 0.05ppt

2. Add 1 pound of salt per estimated 125 gallons of pond volume. Let’s say we believe our pond is 5000 gallons so we add 40 pounds of salt.

3. Wait until all salt has dissolved plus two hours.

4. Test a pond water sample for the new salt concentration. Let’s say the new concentration is 1.05 ppt giving us an increased salinity of 1.0 ppt

5. Calculate the pond volume using this formula:

( P / T ) x 120 = V

V= pond volume in gallons

P = pounds of salt added

T = actual concentration CHANGE in parts per thousand (or change in % times 10)

Example Calculation = (40/1.0) X 120 = 4800 gallons

*The salt test kit I have used is made by Aquarium Pharmaceuticals and it is labeled “Pond Care Salt Level Test Kit. You fill a test tube to a specified level then add four drops of 1st dropper bottle to sample. Then count the number of drops of the 2nd dropper bottle needed to turn the test solution from clear to purple.

Estimating Volume Method: This method can only be accurate if the pond walls are vertical, bottom is flat, and sides are either rectangular, square, or round in shape. First you calculate the volume in cubic feet then multiply by 7.48 to convert to gallons.

Rectangular/Square Formula (feet) = length x width x depth = volume in Cubic Feet X 7.48 = gallons

Example: 16ft X8ftX5ft = 640 cubic feet = 4,787.2 gallons

Circular Formula (feet) = (3.14 x radius x radius) x depth = Volume in cubic feet X 7.48 = gallons

Radius = ½ the diameter of the circle (tank)

Example: Let’s calculate a 13 foot diameter circular pond that is 5 feet deep.

3.14 x 6.5 X 6.5 X 5 = 663.3 cubic feet X 7.48 = 4,961.7 gallons