Plants and algae both eat nitrogen and phosphorus, and which one runs short first decides what grows. This nitrate to phosphate calculator takes your measured NO3 and PO4 and returns the ratio, then compares it to the balanced Redfield range.
Nitrate to Phosphate Ratio Calculator
Enter your measured nitrate (NO3) and phosphate (PO4) to get the NO3:PO4 ratio and see how it compares to the balanced Redfield range.
The Redfield ratio points to a balanced NO3:PO4 of roughly 10:1 by weight, with 5:1 to 20:1 workable in a planted tank. A ratio far from that does not poison fish; it just tips which nutrient runs out first and which algae gets an opening. Reef tanks run much lower absolute numbers but watch the same balance.
What the ratio tells you
The Redfield ratio points to a balanced NO3:PO4 of roughly 10 to 1 by weight, with anything from 5:1 to 20:1 workable in a planted tank. A ratio far outside that does not harm fish; it tips the balance so one nutrient limits plant growth and the leftover feeds algae. Green spot algae on the glass usually means phosphate is short relative to nitrate, not that you have too much of either.
How to correct an imbalance
If nitrate is low relative to phosphate, dose nitrate or ease off phosphate; cyanobacteria takes hold when nitrogen bottoms out. If phosphate is the low one, a small phosphate dose brings the ratio down and clears stubborn green spot. Our guide to lowering nitrate covers the nitrate side, and getting rid of algae without chemicals shows why starving a planted tank backfires.
Planted and reef tanks
Reef keepers run much lower absolute numbers than planted tanks but watch the same balance. Either way, keep both nutrients present and steady rather than chasing zero. The full context on nutrients sits in pH, KH and GH explained, and every tool is on the aquarium calculators hub.
Frequently asked questions
What is a good nitrate to phosphate ratio?
About 10 to 1 by weight, following the Redfield ratio, with 5:1 to 20:1 workable. Enter your readings above to check yours.
What does the Redfield ratio mean for aquariums?
It is the balance of nitrogen to phosphorus that keeps plant growth steady. Stray far from it and algae exploits the leftover nutrient.
My phosphate reads zero, what now?
Plants are phosphate-limited. Dose a little phosphate before adding more nitrate.