Enter your nitrate and phosphate test results to see your N:P ratio, how it compares with the Redfield ratio, and whether either nutrient is close to running out. The calculator also converts the units that trip people up: phosphorus in ppb (what ultra-low-range phosphorus checkers read) and nitrate-nitrogen.
How the ratio is worked out
Test kits report nitrate and phosphate as weights of the whole ion (ppm NO₃ and ppm PO₄). The Redfield ratio counts atoms instead: about 106 carbon, 16 nitrogen, and 1 phosphorus in average ocean plankton. To compare the two, divide each reading by the ion's molar mass (62.00 for nitrate, 94.97 for phosphate):
Ratio by weight = ppm NO₃ ÷ ppm PO₄
Molar N:P = (ppm NO₃ ÷ 62.00) ÷ (ppm PO₄ ÷ 94.97) = ratio by weight × 1.53
Redfield 16:1 = about 10.4 ppm NO₃ per 1 ppm PO₄
Unit conversions: ppm NO₃ = ppm NO₃-N × 4.43, and ppm PO₄ = ppm P × 3.066 (or ppb P × 3.066 ÷ 1,000).
Common pairings compared with Redfield
| Nitrate | Phosphate | Ratio by weight | Molar N:P | × Redfield |
|---|---|---|---|---|
| 2 ppm | 0.03 ppm | 67 : 1 | 102 : 1 | 6× |
| 5 ppm | 0.05 ppm | 100 : 1 | 153 : 1 | 10× |
| 10 ppm | 0.1 ppm | 100 : 1 | 153 : 1 | 10× |
| 10 ppm | 0.05 ppm | 200 : 1 | 306 : 1 | 19× |
| 20 ppm | 0.1 ppm | 200 : 1 | 306 : 1 | 19× |
| 0.5 ppm | 0.1 ppm | 5 : 1 | 8 : 1 | 0.5× |
Typical reef targets sit well above the Redfield proportion, so a tank with a ratio far above 16:1 is not automatically out of balance. Our nitrate and phosphate guide lists common target ranges (roughly 2–20 ppm nitrate and 0.03–0.1 ppm phosphate for most reefs). The last row, 0.5 ppm nitrate with 0.1 ppm phosphate, is about half the Redfield proportion: there, nitrogen is the scarce nutrient.
What the ratio can and can't tell you
Redfield describes the average makeup of ocean plankton, not a target proven for reef aquariums, and many healthy tanks run far from it. Read the ratio together with the actual levels. The pattern that matters most is one nutrient bottoming out while the other is plentiful. In a controlled coral study, high nitrogen with limited phosphate led to phosphate starvation in the corals' symbiotic algae and made the corals more prone to bleaching under heat and light stress. Our ultra-low nutrient research guide explains that work, and the nitrate and phosphate balance guide covers how to fix each pattern.
Phosphorus (ppb) to phosphate (ppm)
| Phosphorus reading | Phosphate |
|---|---|
| 5 ppb P | 0.015 ppm POâ‚„ |
| 10 ppb P | 0.031 ppm POâ‚„ |
| 20 ppb P | 0.061 ppm POâ‚„ |
| 30 ppb P | 0.092 ppm POâ‚„ |
| 50 ppb P | 0.153 ppm POâ‚„ |
| 100 ppb P | 0.307 ppm POâ‚„ |
More on reading phosphate tests in our phosphate guide. To see how far a water change will move nitrate or phosphate, use the water change calculator, and keep a record on the free printable reef test log.
FAQ
What is a good nitrate to phosphate ratio for a reef tank? There is no proven ideal ratio for aquariums. The Redfield ratio of ocean plankton, 16 nitrogen atoms per phosphorus atom, works out to about 10 ppm nitrate per 1 ppm phosphate on hobby test kits. Most reef tanks run far more nitrogen-rich than that, for example 10 ppm nitrate with 0.1 ppm phosphate is 100 to 1 by weight. Keep both nutrients in a sensible range and avoid one reading zero while the other is plentiful.
How do I convert a Hanna phosphorus reading in ppb to ppm phosphate? Multiply ppb phosphorus by 3.066 and divide by 1,000. For example, 30 ppb phosphorus is about 0.092 ppm phosphate. The factor comes from the weight of a phosphate ion (about 94.97) divided by the weight of a phosphorus atom (about 30.97).
My test kit reads nitrate-nitrogen. How do I convert it? Multiply ppm nitrate-nitrogen (NO3-N) by about 4.43 to get ppm nitrate (NO3). Most reef hobby kits already report nitrate as NO3, so check the kit's instructions before converting.
Why is nitrate high but phosphate zero in my reef tank? Phosphate is often driven down faster than nitrate by phosphate removers such as GFO or lanthanum, or by phosphate-hungry macroalgae like chaeto. Research on corals links that pattern, plenty of nitrogen with too little phosphate, to phosphate starvation in their algae and a higher risk of bleaching. Easing off phosphate removal or feeding more usually brings phosphate back up.
Sources
- Alfred C. Redfield (1958), "The Biological Control of Chemical Factors in the Environment," American Scientist 46(3): 205–221
- Wiedenmann, D'Angelo, and colleagues (2013), "Nutrient enrichment can increase the susceptibility of reef corals to bleaching," Nature Climate Change 3: 160–164
- Randy Holmes-Farley, "The Units of Measure of Reefkeeping," Reefkeeping Magazine, August 2005
- IUPAC Commission on Isotopic Abundances and Atomic Weights, Standard Atomic Weights