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We just purchased a house and it has a backyard swimming pond. How do we measure it? | Ponds & Lakes Q&A


Q: We just purchased a house and it has a backyard swimming pond. How do we measure it?

Q: We just purchased a house and it has a backyard swimming pond. How do we measure it?

Rob – Hawthorne, FL

A: Surface area and pond depth are important measurements to know. They’ll help you determine dosage rates with chemicals like algaecide. They’ll help you calculate the right-size aeration system for your pond. And they’ll help you figure out how many and what types of fish to stock.

Finding those magic numbers isn’t difficult – but it does require some tools, the right equation and a little bit of work. Before you head out to your pond, grab a rope marked at 1-foot increments, a weight that’s heavy enough to sink to the bottom, and something for notetaking.

Calculating Surface Area

The easiest ponds to measure are those that resemble a rectangle, but you can figure out the surface area of a circular, triangular or odd-shaped pond, too. First, you’ll need to measure (with your marked rope) or pace off (one step is about 3 feet) some specific distances, depending on the shape of your pond. Then, plug those numbers into one of these formulas:

  • Square/rectangle: Calculate length and width; L x W = Surface Area
  • Circle: Calculate radius; Pi x R2 , or 3.14 x R x R = Surface Area
  • Triangle: Calculate base and height; (0.5 x B) x H = Surface Area
  • Odd-shaped: Use a handy online tool like Bing Maps to measure the pond. Online tools may be easiest, especially for irregularly shaped ponds.

To determine how many acres your pond is, you’ll then divide that surface area figure by 43,560 (one acre).

Calculating Depth

Figuring out your pond’s depth is a bit trickier, particularly if you have plant shelves or if the pond has a slope. Grab your marked string, the weight, something to write with, and a boat or canoe. Then, head out to the pond and follow these directions:

  1. Securely attach the weight to one end of your string.
  2. Climb aboard your boat or canoe with your weighted string and note-taking materials in hand.
  3. Travel to at least five points in various areas your pond, more if your pond is particularly large.
  4. Drop the weight into the water and note where you feel it hit the bottom. Repeat until you’ve gone to all the different spots and gathered a good sampling of your pond’s depth.

Why They’re Important

Now that you know your pond’s size, why are those calculations important?

You’ll need the numbers to determine dosage rates on chemical products like those found in Pond Logic® ClearPAC® PLUS. If the pond has been around more than three to five years, chances are good that you’ll need to address problems like muck accumulation and string algae growth in the spring.

Those numbers will also help you select the right-size Aquastream™ Fountain and Airmax® Aeration System. For instance, if your pond is less than 6 feet deep, our Shallow Water Series™ Aeration System is a good choice; if your pond is greater than 6 feet deep, our Pond Series™ Aeration System is the one for you.

Since you’ll be able to calculate how many acres your pond is, you’ll also be able to determine what types of fish you can have and how many of them you can keep. Finally, because you’ll know the various depths of your pond, you’ll be able to stake out safe swimming areas for children – but don’t forget to put out your life ring!

Pond Talk: What swimming pond maintenance tips can you offer to this new homeowner?

Reduce Pond Muck - Airmax® Pond Series™ Aeration

3 Responses

  1. Boy, proper aeration will really help a swimming area!

  2. This would be a perfect article but you left out the entire formula for figuring out the size. Where do you figure the depth? Width x Length= surface area. Got it. But where does depth go? For total gallons. Thanks

    • Hi Dawn – Typically for a large swimming pond we do not calculate the number of gallons of water in a pond since it is such a large number and chemicals usually refer to square feet or surface acres. There are a few chemicals that would use acre feet for treatment. To figure out acre ft.- (length x width)/43,560 to get surface acres, then multiply that number by your pond’s average depth (half of the max depth). This will give you acre ft. The main reason for knowing pond depth is more for a guide for sizing aeration systems, determining swimming area etc. The deeper the pond the more area one diffuser can aerate. If you are still interested to know the number of gallons of water in your pond use the following- length x width x average depth x 7.48- This will give you the number of gallons of water in your pond. Hope this helps!

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