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Do You Really Know How Many Gallons You Have?
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Whenever we prescribe water treatments we ask what seemingly should be an easy question to answer. How many gallons are we treating? Some people know the answer, some do not and still others think they know. These are the dangerous ones. Guesses and wrong calculations are often more than 1000 gallons off! A hundred gallons can mean the difference between a healthy dose or an overdose of medication and, in turn, the difference between life and death of pond inhabitants. Even if the pond builder tells you how many gallons you have it is in the best interest of the life inside your pond for you to do your own check.
The best way to find out how many gallons your pond holds is to monitor it when it’s filling up. Whenever the pond must be completely filled, like when it’s cleaned or filled for the first time, note your water meter’s beginning reading so you can simply subtract it from the reading on the meter after the pond has finished filling.
Unfortunately, most of us don’t think about it until we actually need to know our correct gallons. In that case we try to get the best measurements possible so that we can calculate a reasonably safe guess. For folks like us who aren’t mathematic majors we’ve found a quick and easy way to get very close to the true amount of gallons a pond holds. Here’s how to calculate:
There are 7.48 gallons of water to a cubic foot. For ease of calculation and because it doesn’t make that much difference in the end result, we’re going to say 7.5 gallons per cubic foot.
The Formula for an Exact Square or Rectangle
Length x Width x Depth x 7.5
If the pond is a rectangle or square with straight sides and no plant shelves it’s easy to get an exact measurement of the amount of water in the pond. For example, a pond that is exactly 5 feet wide and 10 feet long and 3 feet deep will be exactly 1125 gallons once we apply the formula.
The Formula for an Exact Circle
3.14 x Radius x Radius x Depth x 7.5
If the pond is an exact circle with straight sides and no plant shelves we can get an exact measurement. To do this, measure across the middle of the circle. This is the Diameter of the pond. 1/2 of the Diameter is called the Radius. A circular pond that is 10 feet wide (10 ft. Diameter) and is 3 feet deep has a Radius of 5 feet (1/2 the Diameter) so the formula would be 3.14 x 5 (R) x 5 (R) x 3 (D) x 7.5 and that would equal exactly 1766.25 gallons.
The Formula for an Exact Oval
3.14 x Radius x Radius x Depth x 7.5
If the pond is an exact oval with straight sides and no plant shelves we can get an exact measurement. Though the formula looks the same as with the circle pond, the 2 Radius measurements will be different. For example, an oval pond measures 10 feet in Diameter one way (which is 5 feet Radius) then measures 6 feet Diameter the other way (3 Radius) and is 3 feet deep. We would calculate it as follows: 3.14 x 5 (R) x 3 (R) x 3 (D) x 7.5 = exactly 1059.75 gallons.
Irregular Shaped Ponds
Avg. Length x Avg. Width x Avg. Depth x 7.0
We must use common sense when measuring a pond that is not a circle or square. Most ponds fit that description. The trick is to find an average for the measurements then use the formula. We choose 7.0 as a measurement for gallons per cubic foot because it will allow for rounded corners. For example: A kidney-shaped pond measures 10 feet in Length. In Width it measures 5 feet but there’s a small section in the middle where it’s only 4 feet wide. So we’ll consider the Width measurement to be 4.75. It has no plant shelves or tiers and is uniformly 3 feet in Depth. This calculates to 997.5, roughly 1000 gallons.
Let’s take that a step further and say that the edge of the pond is completely surrounded by a shelf that is 1 foot in Depth but the rest of the pond is 3 feet. We might average that to be 2.5 feet. This changes our calculations to 10 (L) x 4.75 (W) x 2.5 (D) x 7.0. The gallons are now roughly 830.
More Accurate Measurements for Irregular Shaped Ponds
Sometimes we might want to try to get the most accurate calculation as possible, such as in the case of performing a potassium permanganate treatment which requires careful dosing. If a pond can be divided into squares, triangles, ovals or circles we can literally divide them and calculate more accurately the number of gallons a pond holds. For example, a pond that is a “figure 8” can easily be divided into 2 separate circles. Calculate each circle with the formula and it will result in a better calculation than by averaging the measurements.
From The Summer 2002 Edition of What's
Up, Doc?, May thru Aug 2002
© 2002, The Pond Doc's Water Garden Center. All rights Reserved.
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