Free ammonia calculator
Is your ammonia reading dangerous? It depends on your pH and temperature — this works out how much of it is the toxic kind.
Freshwater only. The figures use the dissociation constant from Emerson et al. (1975) — the same equation One Fish Two Fish grades every test with.
Why the same reading means different things
A liquid test kit measures total ammonia: two forms added together. Most of it is ionised ammonium, NH₄⁺, which fish tolerate well. A small share is un-ionised ammonia, NH₃ — the “free” ammonia that crosses gill membranes and does the damage.
The split between the two is set by pH and, to a lesser degree, temperature. Each full pH unit up multiplies the toxic share by roughly ten. So 0.25 ppm in a soft, acidic planted tank is a curiosity, and 0.25 ppm in a warm Rift Lake tank at pH 8.2 is an emergency — even though the vial is the same colour.
How much of your ammonia is toxic
The share of a total ammonia reading that is free NH₃:
| pH | 20 °C | 24 °C | 28 °C |
|---|---|---|---|
| 6.5 | 0.13% | 0.17% | 0.22% |
| 7.0 | 0.39% | 0.53% | 0.69% |
| 7.5 | 1.24% | 1.64% | 2.16% |
| 8.0 | 3.81% | 5.02% | 6.54% |
| 8.5 | 11.12% | 14.31% | 18.12% |
And what a 0.25 ppm reading turns into, in ppm of free NH₃:
| pH | 20 °C | 24 °C | 28 °C |
|---|---|---|---|
| 6.5 | 0.0003 | 0.0004 | 0.0006 |
| 7.0 | 0.0010 | 0.0013 | 0.0017 |
| 7.5 | 0.0031 | 0.0041 | 0.0054 |
| 8.0 | 0.0095 | 0.0125 | 0.0163 |
| 8.5 | 0.0278 | 0.0358 | 0.0453 |
What the numbers mean
- Below 0.002 ppm free NH₃ — effectively nil.
- Around 0.02 ppm free NH₃ — where long-term harm starts: gill damage, stress, poor growth. At this level and above, act the same day.
- Any reading above zero in an established tank — worth attention even when the free fraction is tiny. Something is producing ammonia faster than the filter removes it: overfeeding, a death hidden in the plants, a filter that has been cleaned too hard or stopped.
Turned around: at pH 7.0 and 25 °C a reading would need to reach about 3.5 ppm before the free fraction hits 0.02. At pH 8.0 and the same temperature it takes about 0.37 ppm. At pH 8.2 and 28 °C, 0.2 ppm is enough.
What to do about it
If the free ammonia is 0.02 ppm or more
- Change about a third of the water now, with dechlorinated water at the same temperature.
- Stop feeding for a day — food is where the ammonia comes from.
- Check the filter is running and has not been rinsed in tap water recently.
- Test again tomorrow.
If it registers but is below that
- Not toxic yet at this pH and temperature. Test again tomorrow and feed lightly.
- If it is still there in a few days, or climbing, look for the source rather than just diluting it.
The maths
For temperature T in kelvin (°C + 273.15):
pKa = 0.09018 + 2729.92 / T
free fraction = 1 / (1 + 10(pKa − pH))
free NH₃ = total reading × free fraction
This is the standard freshwater form. Salt changes the constant, so it does not apply to marine or brackish tanks.
One Fish Two Fish does this for every test you log. It already has your pH and temperature from the same test, so ammonia is graded on the free fraction automatically — with the water change worked out in litres for your tank. It is coming to iPhone and iPad; get one email when it launches.
A well-informed second opinion, not veterinary advice. A struggling tank can have causes no water test shows — for a sick or dying animal, an aquatic vet beats an app.