Perhaps one of the most difficult components to building a pond is knowing how much pond liner you will need. The most common mistake that people make when they build a pond is under-estimating the amount of liner they need. When I worked at a pond shop, I use to hear horror stories of people building their ponds without enough liner. Here is an example of the stories I use to hear all the time:
A customer decides that they want to build a pond themselves, so they spend top dollar getting everything they need. They purchase the rocks, the pump, the waterfall, pond liner, and even have the koi fish picked out. They have dug the hole, built up the waterfall, and made the stream. They carefully lay the liner in the hole, making sure it doesn’t rip, and place the rocks on top of the liner. They have everything in place, and finally turn the pump on. The pond looks immaculate! But as the hours pass, the customer notices that the water level is dropping drastically. He searches around for the source and realizes that the water level is above the top of the pond liner. He has spent thousands of dollars to make a perfect pond, but has cut the liner too short. Now he must redo everything.
In the case of this particular customer, it was a simple miscalculation. These miscalculations happen all the time, which is why I am going to show you the proper way to estimate the amount of pond liner that you will need to build a pond.
The general rule is that you need 2 feet of liner overhang on all sides of your pond (including stream and waterfall). This 2 feet of overhang is there so the rocks can hold the liner in place. The extra 2 feet also gives you room to miscalculate and get away with it. Then you must add the depth of your pond, then the distance across the bottom. Most people, including that poor customer stop their measurements here. Don’t be one of those people! You need to add the depth again, and then add another 2 feet of overhang.
This is the formula that you should use to estimate the amount of pond liner you need to build a pond:
Length: 2 feet of overhang + depth + distance across the bottom + depth + 2 feet of overhang.
Width: 2 feet of overhang + depth + distance across the bottom + depth + 2 feet of overhang.
So, if you want your pond to be 7 ft x 10 ft x 5 ft deep, then this is the calculation:
Length: 2 ft + 5 ft + 7 ft + 5 ft + 2 ft = 21 ft
Width: 2 ft + 5 ft + 10 ft + 5 ft + 2 ft = 24 ft
Total pond liner needed: 21 ft x 24 ft
One more thing to remember is that if you have more than one drop off in your pond, you need to calculate accordingly. Building a pond is a major undertaking, and knowing how to estimate the amount of pond liner you need to build your pond is the most important step.