Calculating the volume using salt

Norm Meck (Koi Club of San Diego)

Disclaimer: Although the most accurate method to obtain the volume of water in your pond is to use a Water flow meter, in a pinch this method will estimate the volume. Be sure your salinity meter is calibrated correctlyand you know which type of meter you have ppt or percent.


“The amount of salt dissolved in water is termed the salinity and is measured either as a per cent, in parts-per-thousand (ppt), or in parts-per-million (ppm) (where 10 ppt = 1% = 10000 ppm). The more common parts-per-thousand measurement is the weight of the salt in pounds per thousand pounds of water (about 125 gallons). Pond-keepers often talk about the pounds of salt per hundred gallons of water. Since 100 gallons of pure water weighs about 800 pounds, one pound of salt per hundred gallons equates to a salinity of 1.25 ppt (0.125% or 1250 ppm). (1 ppt = 0.8 pounds per hundred gallons)”


The difference between percentage and parts per thousand is that 0.1% equals 1 ppt. i.e. a ppt reading is 10 times the percentage reading. (Percent is just another way of saying parts per hundred).

The simple formula for calculating pond volume using salinity change:


Known:    P = pounds of salt added
T = actual concentration CHANGE in parts per thousand (or change in % times 10)
(i.e. reading after adding the salt minus reading before adding salt)

Find: V = pond volume in gallons

V = ( P / T ) x 120

or pond volume in gallons equals 120 times the pounds of salt added divided by the salinity change in parts per thousand

Why do you multiply by 120?

One part per thousand is one weight of something (say one pound of salt) mixed with 999 weights of something else (say 999 pounds of water) giving a total of 1000 parts. (we normally can round off the 999 to 1000 and still have acceptable accuracy) So this mixture is 1 part per thousand salt (and 999 parts per thousand water)

Since one gallon of water weighs 8.33 pounds, and we are looking at the ratio between what is being added compared to the weight of water; one divided by 8.33 = 0.120048… Since we are talking parts per thousand, need to shift the decimal point 3 places to get the 120. i.e. the 120 is 1000 divided by 8.33. (1000/8.33=120)

Similarly when medicating a pond and we want a dosage of so many parts per million, we normally talk about number of grams of the “medicine” to add. In this case we need to compare grams with grams of water. Can’t use pounds of water here since we have to use the same weight measure. One gallon of water weighs 3790 grams. So the simple formula becomes:

Known: V = pond volume in gallons
M= desired concentration change in parts per million
Find:     G = amount to add in grams

G = V x M x 0.00379