Q: How do I know how many gallons are in my pond?
Jolene – New Rochelle, NY
A: Gallons matter. Knowing how much water your pond holds will help you determine the size of aeration and filtration systems you need. It will also help you properly dose your pond with commonly used pond products and determine how many fish your pond can house.
So how do you calculate this all-important figure? Put on your thinking cap, because we’re revisiting some high school geometry.
The easiest ponds to measure are those that resemble a rectangle. If you’ll recall from geometry class, you can calculate a rectangle’s volume by multiplying its length by its width by its height, so L x W x H.
First, to get your length and width, measure your pond at its longest point and its widest point. Figuring out its depth is a bit trickier, particularly if you have plant shelves or if the pond has a slope. If it is the same depth throughout, use that number in your formula. If you have a plant shelf or the depth varies, measure the maximum depth and cut it in half to create an average depth. For instance, let’s say the pond is four feet at its deepest but has some shallow areas for plants. Use half of that depth, or two feet, for your formula.
So let’s put this in real terms: If your pond is 15 feet long by 10 feet wide by 2 feet deep, your pond’s volume is 300 cubic feet. One cubic foot holds 7.48 gallons of water, so multiply 300 by 7.48 to get your total – which equals 2,244 gallons.
If your pond is round or oval, you’ll start by using the formula to find the surface area of a circle – which is Pi x Radius2 , or 3.14 x R x R. The radius of your pond is half the distance across, so if your pond is a 10 foot circle, the radius is 5 feet. Your formula will look like this: 3.14 x 5 x 5 = 78.5.
Next, figure out your pond’s depth, just like we did with the rectangular pond. If it’s 4 feet deep with plant shelves and sloped sides, use 2 feet. Then multiply that number by your surface area: 78.5 x 2 = 157 cubic feet.
Finally, convert that number into gallons. One cubic foot holds 7.48 gallons of water, so 157 x 7.48 = 1,174 gallons.
Irregular Shaped Ponds
The more irregular your pond’s shape, the less accurate your measurement will become – but it’s OK. In most cases, you don’t need to pinpoint its volume to the exact gallon. Use whichever formula best resembles your pond’s shape. For instance, if your pond is kidney shaped, use the rectangle formula and remember that the final figure will be slightly higher.
Playing with the Numbers
With your newfound numbers, head over to The Pond Guy’s® online calculator where you can plug in those figures and see how many boulders you need, how many fish your pond can hold, what pump size you need and more.
Now aren’t you glad you paid attention in geometry class?
Pond Talk: What kinds of improvements have you made to your pond or backyard this summer?