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Calculating pond volumes     Calculating pond volumes


Getting medications and treatments correct
We need to know the volume of a pond when it comes to adding a medication for the obvious reason that it is essential that any medication is added at the recommended dose rate.  Too little may not be effective and too much may be harmful or possibly even fatal.

It’s also important to add the correct amount of a water treatment, for example, an algae treatment. Again too much may be harmful to the koi or the biofilter although there is one popular product where experience shows that a half or even one quarter dose is often sufficient. However, even when a lower dose rate is chosen, the pond volume must be known in order to determine what the full dose would have been in order to calculate a half of that amount.

Sodium thiosulphate crystals, when used as a dechlorinator, or any of the proprietary dechlorinators based on sodium thiosulphate are non toxic unless used in ridiculous amounts. This gives the security of knowing that there is no real chance of causing harm by overdosing but there is the danger that not knowing the pond volume may lead to under dosing which could potentially leave the koi at risk from the effects of chlorine.

pH and alkalinity changes
KH buffers such as sodium bicarbonate, which will alter the pH, shouldn’t be added without knowing the pond volume.  As I have repeated many times before, koi can adjust to any pH in the range of 7.0 to 8.5 but, because they have to adjust their internal biochemistry to compensate for different pH values, koi don’t like them to change too quickly. If the pH is to be adjusted, the rate of change is best limited to 20 mg/L per day and the pond volume must be known in order that any pH adjustment is carried out safely.

Hardness shock is another possible consequence of adjusting the KH too quickly.  I have not seen any authoritative sources saying that hardness shock will result from reducing KH too quickly but it is well established that hardness shock is a real possibility if it is increased too rapidly.

There are few practical ways to reduce the KH in a koi pond other than by adding reverse osmosis water and the output flow rate of all normal reverse osmosis purifiers is far too low to allow large volumes to be added very quickly.  However, KH can be raised very easily by adding any one of a range of chemicals that will add carbonates to the water.

Hardness shock
Sodium bicarbonate is a pond chemical that is, to all intents and purposes, non toxic unless added in massive amounts and is one of the most convenient ways to add carbonate. However, while there is no real upper limit to how much can be used in total, even such a benign chemical shouldn’t be added in too great a quantity at any one time.

As with changing values of pH, koi are also very tolerant of changing values of KH but have to adjust their internal biochemistry to compensate.  If the KH changes slowly, they can match it without difficulty but it is stressful for them to have to make these changes too rapidly.

The generally accepted maximum rate of change for KH is 20 mg/L per day and this translates to a maximum dose rate of 30 gm of sodium bicarbonate per 1,000 litres. The notable exception is to rapidly restore the pH after a pH crash when any amount of sodium bicarbonate necessary should be added to restore the pH to 7.0 as quickly as possible but no higher until the ammonia level has been checked and the biofilter confirmed to be working (see the method to deal with a pH crash here)

Measuring pond volumes

Pond volume rectangle 400
Figure 1 standard rectangle pond

The most accurate way to determine the volume of a pond that has been built to any standard geometrical shape is by measurement. One cubic foot will hold 6.2288 Imperial gallons and there will only be a 0.003% error if the usual approximation of 6.25 gallons is used.

If the length, width and depth of a standard square or rectangular pond, as shown in figure 1, can accurately be measured then its volume can be found by the formula:

Length x Width x Depth x 6.25 = Volume (Imperial gallons).  Where Length, Width and Depth are all measured in feet.

Since it’s probable, that the dimensions won’t all be in exact numbers of feet it might be easier to measure all the dimensions in inches and divide the total by 1,728.  For example:

L (inches) x W (inches) x D (inches) ÷ 1,728 x 6.25 = Volume (Imperial gallons)

Note for the non mathematically minded; every dimension is measured in inches, not a mixture of feet and inches. For example for a pond 12 ft 6 ins long by 5 ft 3 ins wide by 4 ft deep the volume is given by:

Pond volume circular 180
Figure 2 standard circular pond

150 x 63 x 48 ÷ 1,728 x 6.25 which equals 1,640 Imperial gallons.

Circular ponds
To calculate the volume of a circular pond, show vat or reservoir, it is only necessary to know the radius (half the diameter) and the depth as shown in figure 2.

If these two dimensions are known the formula is then:

Radius (ft) x Radius (ft) x Depth (ft) x 3.14 x 6.25 = Volume (Imperial gallons).

Or, Radius (ins) x Radius (ins) x Depth (ins) x 3.14 ÷ 1,728 x 6.25 = Volume (Imperial gallons).

Pond volume wedge 375
Figure 3 wedge or stream flow pond

Different shaped ponds

The above formulae are ideal methods for square, rectangular or circular ponds that have flat bottoms and whose walls are vertical.  If this is the case then they will be the most accurate way to determine pond volume.  Ponds that have sloped or benched bottoms may still have their volumes calculated by these formulae if the depth dimension measured is the average depth.

The diagrams in figures 3 and 4 show where the average depths should be measured for some common pond profiles.

Pond volume benched circular 150
Figure 4 benched circular pond

Pond volume circular ends 250
Figure 5 Pond with semi circular ends

There are other freeform shaped ponds and figure 5 shows a common design which is a pond that has double semicircular ends.

The trick to calculating the volumes of ponds such as this one, or other non standard shapes is to break the pond into sections that can easily be calculated then simply add up all the individual volumes to find the volume of the whole pond.

In the case of the shape in figure 5, if the two ends were “pushed together” then they would make a simple circular pond and the volume could easily calculated from the examples in figures 2 and 4, according to whether the bottom of this imaginary pond is flat or sloped. The central part of the pond is a simple rectangle (with or without a sloped floor) and the volume can be calculated from its length, width and depth as shown for the examples in figures 1 and 3.  The volume of this pond is the volume of the central rectangle plus the volume of the imaginary circular pond formed by its two ends.

Provided the dimensions can be measured accurately these formulae can be used to give the most accurate value for the pond volume whether it is empty or has already been filled. However, where the shape gets too complicated another method has to be chosen.  The usual method employed to measure difficult shaped ponds is a water meter and there seems to be a general impression that these are very accurate and that they should be used in preference to measuring or any other method but is that the case?

Measuring volumes with a water meter
Standard water meters aren’t capable of registering the exact amount of water that passes through them because they are limited by the accuracy of the way in which they are made and by the method they use to measure the water flow. There are four classifications of water meters; A, B, C and D and these refer to the ability of the meter to accurately measure a low flow. Class A is the least accurate and Class D is the most accurate.  Unsurprisingly you get what you pay for and the types usually sold for prices considerably under £30 are invariably Class A types.

Water meters also have specifications such as Qmin, Qt, Qn and Qmax but koi keepers are really only interested in two of them:

Qmin (minimum) is the lowest flow rate at which the water meter will register flow.

Qt (transitional); is the flow rate at which the water meter reaches its highest accuracy ± 2%.  The accuracy between Qmin and Qt is no worse than ± 5% which is not too bad but nowhere the 0.003% accuracy of the ideal method of calculating a pond volume by means of measuring its dimensions where ever the shape or profile allows this.

One thing to be aware of when buying a water meter is that we often use them for measuring water volumes after it has passed through a purifier unit. With recommended flow rates through purifiers as low as a few litres per minute these flow rates may fall well below the Qmin of the water meter and the result may be that the accuracy drops off very steeply below its stated Qmin.

Please read the label
Another source of inaccuracy may occur if the meter is not mounted the correct way.  Water meters are usually intended to be mounted vertically on a wall and a spinning turbine is often used in the cheaper water meters as the method by which the flow is measured.  Depending on the internal construction and the bearings for the turbine, if a water meter is simply connected to a hose and allowed to lie on its back, there may be an increase in the frictional losses in the bearings. The additional friction may cause the meter to become very inaccurate indeed.

Manufacturers of equipment, including water meters, invariably go to a great deal of trouble to determine the optimum conditions under which their product will give its best performance.  If there are any special conditions that will allow the best performance, or conditions that should be avoided, they will specify these in the data sheets they supply. And, as I always say, where instructions are supplied with a piece of equipment, please feel free to read them!

PDV salt 150Measuring pond volumes with salt
Let me first state that while salt is a good treatment for a range of fish ailments, ideally, it should be confined to being used as a short term dip or medium term in a hospital tank rather than as a long term or permanent addition to a koi pond.  An exception to this rule is where salt is used to provide a degree of protection against a temporary high level of nitrite due to an immature or damaged biofilter.

Having said that, unless it is used at high levels PDV salt, as sold by koi dealers, isn’t toxic to koi so it may be used in a pond in order to achieve a specific objective. One situation where salt can usefully be added to a pond is finding the volume of a pond that has already been filled and which has a shape that is too difficult to measure.

The principle by which this method works is to take a reading of any salinity in the water first, then add a known quantity of salt and measure how much change in salinity that causes.  The volume of the pond can then be calculated by mathematics in one of two ways.

The hard way
The method that is quite often seen on the Internet requires that the salinity is measured before and after adding salt but the formula is based on American calculations where they use the US gallon as their preferred way to measure volumes.  When that calculation is converted to give an answer in Imperial gallons it becomes:

Pounds of salt added multiplied by 120, divided by the difference in salinity then divided by 1.2 = volume in Imperial gallons.

If that looked hard to follow or unnecessarily complicated, I agree. That was exactly what I thought when I first saw it. There is a much simpler method and that is to work directly in litres.

The easy way
Before adding salt, take a reading of the pond salinity in ppt (part per thousand).

Add a known quantity of salt in grams (e.g. 25 kg = 25,000 grams).

Take a second salinity reading after allowing time for the salt to mix (a couple of hours at least, better still overnight).

Subtract the first reading from the second to get the difference.

The pond volume in litres is given by grams of salt added divided by salinity difference in ppt

Litres = grams ÷ difference

Reading before adding salt = 0.2 ppt

Reading after adding salt = 2.2 ppt

Difference = 2 ppt (2.2 – 0.2)

Salt added = 25,000 gm (25 Kg)

Pond volume = 25,000 ÷ 2 = 12,500 litres

Pond volumes in Imperial or US gallons
Hobbyists who prefer to know their pond volume in gallons can divide the volume in litres (12,500 in the example) by 4.55 to get 2,750 Imperial gallons or divide by 3.79 to get 3,300 US gallons.

Amount of salt to add
You can use any amount of salt, even very small amounts, but the greatest accuracy in calculating the volume is when the amount added produces a large increase in salinity.

Large amounts of salt in a koi pond are undesirable so, in order to obtain the most accurate volume reading without subjecting the koi to any more salt than necessary, a compromise is to use only enough salt to produce a 2 ppt change. That isn't a great amount of salt to add for those of us who don't like to add salt to our ponds but it still gives good accuracy for the pond volume.

To increase the salt concentration by 2 ppt, make an estimate of the pond volume then simply add 2 kg of salt per estimated 1,000 litres.  For a pond volumes estimated in gallons, add 9 kg of salt per 1,000 Imp gallons or add 7.5 kg per 1,000 US gallons.

That may seem like a lot but there is usually a trace of salinity in our tap water supplies, dechlorinators change chlorine or chloramine into sodium chloride (salt) so the 2 ppt increase won't make any great difference to the very low levels of salt in a koi pond that many of us didn’t know was already there.

Type of salt
As for the salt to use, various types of salt may have additives to help granular types of salt flow more freely or to stay in a pelleted form, (such as dishwasher salt), when wet until they are required to dissolve completely in flowing water. Some of these additives may be toxic in large quantities, some aren't necessarily toxic but all the additives are undesirable in a koi pond.

That's why the best type of salt to use is PDV salt from a koi dealer or a trusted source that will guarantee that their salt contains no undesirable additives and that the level of purity is "food grade". That doesn't mean that all non-PDV salt is potentially harmful but we spend a lot of money on our koi so I never recommend saving a small amount of money or convenience by using anything other than the recommended type of salt.


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