Irregular shaped ponds – How to Reline & Calculate the Volume

If you’re having a hard time get your heads around irregular shaped ponds. On this web page, I will go into some information about shaped fish pond and answer the following questions. How to line an oddly shaped pond? Also How to calculate the volume of an irregular shape pond?

How to line an oddly shaped pond?

If you’re starting from scratch on your first pond build, then check out these pictures of one of my guy’s training of site but building his first sole pond. As you can see the first step is to clear the site and mark out the shape of your pond area and then start digging.

Kyles first sole pond build.

However, if you have moved into a new home with an irregular shaped garden pond or if you are like Kyle that learnt a lot when building his first pond.  First of all its ok, a lot of new pond owners don’t like their first pond as they create it without a spirit level. As long as you learn by your mistakes, we all make them, but if the garden pond leaks or designed for the wrong reason. You like Kyle might want to rebuild the small garden pond and want the answer to how to line an oddly shaped pond with flexible liners.

Rebuilding and reshaping a garden pond.

When restoring or rebuilding a garden pond, this is the best time to reshape your Irregular shaped​ garden pond! So we have created this quick guide to lining an oddly shaped garden pond. To carry on answering the question how to line an oddly shaped pond.

If you are re-lining a pond, the first task is to remove all the contents of the problem pond. Once all the waste and water has been removed. It’s now time to tackle and get the old flexible rubber pond liner out. Then you can change the groundwork and possibly to dig the pond deeper.

Once all the groundwork has been completed. You can start to measure the new hole. Now to work out the size of the pond liner you need for your newly shaped hole. Start by trying to imagine the oddly shaped garden pond inside a formal rectangle or multiple squares.

What is the most efficient way to line the hole and minimise the size of the pond liner? Once you have done this, you can work out the dimensions by measuring all the maximums (maximum length width and depth).

Example: Pond liner size

So you then know how to line an oddly shaped pond from scrach or restoring an old garden pond.

Let’s start by giving you the answer of how big the liner was for Kyles newly reshaped hole. The newly re shaped fish pond is 3 m long (10 feet) by 2.4 m wide (8 feet) by 60 cm deep(2 feet).

To work out the size of the pond liner. You need to add double the depth to both the maximum length and the maximum width. 

The Pond depth is 60cm (2 feet) doubled is 1.2m (4 feet).

Then we can add this to the length. So that would make it 3 metres long plus double the depth of 1 m 20cm = 4 m 20cm and again to the width 2.4 m wide plus the 1m 20cm = 3 m 60cm. 

Now we need to add extra for the overlap. I always leave this to the end because you need to round up the flat sheet liner anyway.  So I would buy a 5m x 4m piece of rubber liner. This size of pond liner allows for any of the awkward shapes.

Reshaped and rebuilt pond
Reshaped and rebuilt pond, note the two liners one for the pond and an extra for the stream.

You always want to build a garden pond with one sheet of butyl rubber or 1mm EPDM pond liner. Kyles new pond has a 1.5m stream feeding the garden fish pond so has two pond liners.

What we needed for different shaped fish pond

So just for the garden fish pond, you want 20 m² of protective underlay.  The protective underlay goes down first and protects the liner from underneath. When we are building ecosystem ponds, we also protect the top side of the pond liner as well. So we would then double up and buy 40 m² of protective pond fleece (an overlay and underlay). 

So now you’ve got the right size liner you need to fold the flat sheet. And cover the underlay with the pond liner starting at the bottom going around the edges, working your way up the shelves, you can see us doing this in some of our how to build a pond videos.

Once you have the liner in place now, you want to start filling the pond with water before fixing down the edges around the top. If you’re building an oddly shaped ecosystem pond, wildlife pond or garden pond, then you start with the overlay and making the necklace of stones around each shelf. Before filling it with water.  The key to this is starting from the bottom on work your way around and up. Folding the line up as you go it’s better to put prominent folds in rather than lots of small ones, but on tight curves, you have to put in a series of small folds on an irregular shape.

Now is the time, to start thinking about marginal planting pockets before filling the pond with water.  As once the pond this full you can’t begin to create planting pockets. Adding planting pockets is a lovely way to disguise the pond edging, you will not be able to tell whether land finishes and the water begins. Now fill the pond, if you have a flow meter, then you would also know your pond volume. Slowly fill the pond to avoid moving any materials.

Once the pond is full, you can now fix the edging stones or roll out the turf, and now your oddly shaped fish pond has been made. So I hope that helps answer the question, How to line an oddly shaped pond.

This is the finished re-shaped pond.

How to calculate the volume of an irregular shape pond?

Now you know how to line an oddly shaped pond. The next question I get is how to calculate the volume of an irregular shape pond.  This way is without having to pump out all the water, but this is the best way if you want the exact amount of water in your pond. But if you don’t have a pond pump or holding units, then this is the way.

The best way for me to answer this question. Is by starting to figuring out a formal shaped fish pond first. Then we can adapt the results to calculate the volume of water in an irregular shaped fish pond.  Is take the measurements as before and create the boxes in your mind. So let’s use the oddly shaped pond sizes again.

This irregular shaped fish pond is 3 m (10 feet) by 2.4 m (8 feet) by 60 cm (two three).

The best way to do this is the first workout if it was a box or a formal pond.

Take the maximum length by the maximum width of the irregular shaped pond (the pond surface area) and then the surface area is times by the average depth. So this would be 3 m x 2.4 m x 0.6m = 4.32 m² if this was a box and you have no shelves the water volume is 1000 L for every square metre. 

This box or pool would make a maximum volume of the water feature @ 4320 L

But when you have shelves and the irregular shape of the pond. You have to make an educated guess or work out the percentages.  

As a good rule of thumb, irregular shapes are one third smaller. Reducing the pond volume down to 4.32 m²  x 0.67 = 2.894 m²

How much water in a kidney-shaped fish pond like the one below with no shelves same measurements would be 2894 Litres.

How much water in a kidney-shaped fish pond
How much water
in a kidney shaped fish pond

So if we had pond shelves at 30 cm deep and half of the pond was the maximum depth of 60 cm. So in own mind’s eye let’s pretend it was a rectangular sponge cake now if we cut the sponge cake in half so we could put the jam and cream in the middle.  Now we have two halves, and the base layer would be smaller. Example the lower tier of the sponge cake would also be smaller than the top half of the cake like an upside down two-tier cake.

  • So I would calculate the lower tier another third smaller again.
  • 2.894 m² divide by two (top half 1.447 m² bottom half 1.447 m²)
  • Botton half is one third smaller 1.447 x 0.67 = 0.969 m²
  • The top half is still 1.447 m²
  • Add them together 1.447 m² + 0.969 m² = 2.416 m² 

So the irregularly shaped pond volume would be 2416 L of pond water.

For more information about more pond shape’s.

2 responses to “Irregular shaped ponds – How to Reline & Calculate the Volume”

  1. Nancy Harbour says:

    What a very useful article. Thank you very much.

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