Tool · free
Ammonia toxicity calculator (NH₃ / NH₄⁺)
Enter TAN (total ammonia nitrogen, N-NH₄), pH and water temperature. The calculator computes what fraction of TAN is the toxic un-ionised ammonia (NH₃) and compares the result with indicative thresholds for aquatic organisms.
Enter TAN, pH and temperature.
How we calculate
We determine the fraction of un-ionised ammonia in TAN with the Emerson equation (Emerson et al., 1975), which relates the dissociation constant to temperature:
pKa = 0.09018 + 2729.92 / T (T in kelvin), and then NH₃ fraction = 1 / (10^(pKa − pH) + 1).
The NH₃-N concentration is TAN multiplied by this fraction. The concentration of NH₃ itself (the whole molecule, not just nitrogen) is a factor of 17/14 higher than NH₃-N (the molar-mass ratio of NH₃ to N).
Frequently asked questions
What is TAN and where do I get it?
TAN (Total Ammonia Nitrogen) is the value reported by a laboratory or probe as N-NH₄ - the sum of the ammonium ion (NH₄⁺) and un-ionised ammonia (NH₃), expressed as nitrogen. If your result is given as NH₄ (the whole ion, not as N), convert it first with the concentration-unit converter.
Why does the result depend on pH and temperature?
NH₄⁺ and NH₃ stay in chemical equilibrium (NH₄⁺ ⇌ NH₃ + H⁺). A rise in pH shifts this equilibrium towards NH₃ - each pH unit is about a tenfold change in proportion. Temperature acts in the same direction, though more weakly. So the same TAN at pH 7 and pH 8.5 gives a completely different concentration of toxic NH₃.
Does exceeding a threshold mean the fish will die?
Not necessarily, and not immediately. The thresholds in the calculator are indicative bands from the literature - sensitivity varies between species (salmonids are far more sensitive than cyprinids), and prolonged exposure to "warning" values harms differently than a brief spike to "high-risk" values. Treat the result as a signal for further observation, not a hard verdict.
Does the calculator account for salt water?
No - the equation used here (Emerson) applies to fresh water. In brackish and marine waters ionic strength changes the equilibrium constant and the result would be slightly underestimated.
TAN, pH and temperature together, as a series
Ammonia responds to pH changes within minutes - a single reading from another time of day can mislead. In LimnoLog you record TAN, pH and temperature in one session (also offline) and see whether you are approaching the risk zone.
Open the app → Explore LimnoLog features →Read more