Once you have settled on where to site your pond the only remaining thing is to decide how big to make it. While the size of the available space – and the overall look of the garden – can impose their own constraints, generally speaking, larger ponds are easier to manage, principally because they become self-sustaining mini environments more easily. That said, the real question has to be, what do you want your pond to achieve? If the goal is simply to be able to enjoy the sound of running water, then would a simple bubble fountain suffice? Is it to be a wildlife pond? Do you want to keep show-quality koi carp or are you looking for a beautifully planted aquatic garden to showcase exotic specimens of water plants? Sizing the pond depends as much as anything on what job it is supposed to do and it is as well to be clear about what you want at the outset, since it is usually quite difficult to make major changes to a pond once built.
Some purposes come with very specific demands. If the pond is to house koi, or any of the other really large carp species, for instance, then it is going to have to be big – and forget any imaginative planting schemes you may have been toying with trying out! As a good starting point, these ponds need a volume around 7 cubic metres and should be no shallower than 1.2 metres if they are to stand a proper chance of providing suitably stable water quality – aside of the physical space needed to keep such large fish in good health. Smaller ponds can be successful, but the amount of effort involved is likely to be considerably greater. For koi and their relatives, the biggest that available space and budget will allow is the only option – not least because happy koi grow at an alarming rate!
For a simpler, traditional planted fish pond, a surface area of around five square metres and around 60cm or so at its deepest – a volume of 3 cubic metres – should be perfectly adequate and few gardens are unable to accommodate a feature of these dimensions. If space or the garden design limit the pond to anything much smaller, while it can still be an attractive planted feature and valuable to wildlife, it is likely to be unsuitable for fish, particularly if it is sited in full sun, as the temperature fluctuations will probably be too great.
Sizing The Liner
There are various ways commonly suggested for calculating the size of flexible liner needed to build a pond and everyone seems to have their favourite method. The only really important thing is to allow adequate “slack” so if the pond ends up being slightly bigger, or slightly deeper, there will not be a shortage. As a general rule, if you double the depth of the pool, add its maximum length and then add on 30cm or so slack you get the length of liner required, and double the depth plus the maximum width plus 30cm gives you the width. Any surplus which remains after pond construction can be tucked in under stones or turf at the edges and excess cut off – better to have too much than even a small amount too little!
If a pre-formed liner is being used, the size is fixed, but many people making their first pond in this way regret opting for too small a moulding. The largest commonly available sizes are seldom too big for the average garden, though they may look disproportionately big staked up in the suppliers shop, so it is probably a good idea to go for as large a unit as you can accommodate and afford.
Sizing your pond is probably the single most important decision after settling on its location, so it needs some careful consideration. The type of pond being planned is the key to the whole size issue, with some kinds having very particular requirements which must be met, while others are more flexible and forgiving – though the general maxim that bigger is better seems to be fairly universally applicable. Building a pond is a significant undertaking, so it is obviously important to get things right – but with a bit of forethought, sizing the pond should not be a difficult job to do.