About 80 years ago, a guy named Redfield noted the similarity between the average nitrogen-to-phosphorus ratio in plankton (N : P = 16 by atoms) and in deep oceanic waters (N : P = 15). This ratio is since then called Redfield ratio.

It is important to note that the ratio is by atoms, (ie atom count), often a value expressed as umol/kg, ie a numerical value that does not include the weights of the two elements. (1 mol is aprox 6,022 * 10^23).

ppm values (also often expressed as mg/ liter), do not tell us the atom count but instead how much weight of PO4 or NO3 (in milligrams) we have in each liter of water.

From the periodic table we have the molar mass of phosphate and nitrate:

Molar mass of phosphate (PO4): 30.974 + 4*15.999 g/mol = 94.97 g/mol

Molar mass of Nitrate (NO3): 14.007+3*15.999 g/mol = 62.004 g/mol

Calculation example:

0,05 ppm (mg/liter ~ mg/kg) PO4 translates to 0.526 umol P / kg (since PO4 has a molar mass of 94.97 g/mol), multiply this with 16, and you get 8.42 umol N / kg, (Redfield ratio is 16 and since PO4 has one P-atom and NO3 has one N-atom, according to Redfield ratio: every PO4-"molecule" should correspond to 16 pcs of "NO3-molecules"), convert back to ppm NO3 by multiplication with the molar mass of NO3 (62,004 g/mol = 0,062004 mg/umol), and you get 0,52 mg/kg (ppm) NO3.

From this calculation we see that

PS: I'm note sure if this has any application to our reef tanks, but I've seen so many reefers just multiplying the ppm value of phosphate by 16 to get a ppm value of nitrate, so I did this write up… ie:

/ Peter Nyman

It is important to note that the ratio is by atoms, (ie atom count), often a value expressed as umol/kg, ie a numerical value that does not include the weights of the two elements. (1 mol is aprox 6,022 * 10^23).

**Since the molar mass of PO4 and NO3 are NOT THE SAME, this 1:16 ratio can not be used directly as a ratio of ppm values.**ppm values (also often expressed as mg/ liter), do not tell us the atom count but instead how much weight of PO4 or NO3 (in milligrams) we have in each liter of water.

From the periodic table we have the molar mass of phosphate and nitrate:

Molar mass of phosphate (PO4): 30.974 + 4*15.999 g/mol = 94.97 g/mol

Molar mass of Nitrate (NO3): 14.007+3*15.999 g/mol = 62.004 g/mol

Calculation example:

0,05 ppm (mg/liter ~ mg/kg) PO4 translates to 0.526 umol P / kg (since PO4 has a molar mass of 94.97 g/mol), multiply this with 16, and you get 8.42 umol N / kg, (Redfield ratio is 16 and since PO4 has one P-atom and NO3 has one N-atom, according to Redfield ratio: every PO4-"molecule" should correspond to 16 pcs of "NO3-molecules"), convert back to ppm NO3 by multiplication with the molar mass of NO3 (62,004 g/mol = 0,062004 mg/umol), and you get 0,52 mg/kg (ppm) NO3.

From this calculation we see that

**the Redfield ratio N : P = 16:1 by atom, translates to a ratio NO3 : PO4 expressed in ppm as 10:1**PS: I'm note sure if this has any application to our reef tanks, but I've seen so many reefers just multiplying the ppm value of phosphate by 16 to get a ppm value of nitrate, so I did this write up… ie:

**You can multiply with any number of your liking, but at least do not call it the Redfield ratio unless you do it the right way!**/ Peter Nyman

Last edited: Feb 18, 2020