Un-ionized Ammonia Calculator From pH and Temperature

Why one ammonia reading isn't enough

UF/IFAS explains that ammonia in water exists in two forms: un-ionized ammonia (NH₃) and ionized ammonium (NH₄⁺). Test kits measure both together as total ammonia nitrogen (TAN), in mg/L, which is the same as ppm. The un-ionized form is about 100 times more toxic to fish, and its share of the total rises with pH and temperature.

So 1 ppm of total ammonia can be fairly mild in soft, cool, acidic water and dangerous in warm, alkaline water.

How the calculator works

It uses the equation from Emerson and colleagues (1975), the same data UF/IFAS used to build its multiplication table:

• pKa = 0.09018 + 2729.92 ÷ (temperature in kelvin)
• Fraction un-ionized = 1 ÷ (10^(pKa − pH) + 1)
• Un-ionized ammonia = TAN × fraction

Results match the UF/IFAS table closely across typical aquarium temperatures; at the hottest temperatures the table lists, it reads a few percent higher than this equation.

Sample fractions

Share of total ammonia in the un-ionized form, from the same equation:

What the levels mean

UF/IFAS gives these guide levels for un-ionized ammonia:

• Above 0.05 mg/L: damage to fish tissues can occur.
• Rising above that: progressively more damage.
• About 2.0 mg/L: sensitive fish will typically die.

It also notes that fish damaged by ammonia can show effects for weeks after the problem is fixed.

What to do about ammonia

In an established tank, ammonia should read zero. A reading means the waste input has outpaced the biofilter: overfeeding, a dead fish or snail, too many new fish at once, a filter that was cleaned too hard or stopped, or medication that harmed beneficial bacteria. Common responses are to stop or reduce feeding, do a water change with conditioned water, find and remove the source, and keep testing daily until it's back to zero.

In a new tank, ammonia is expected while the nitrogen cycle establishes. See our nitrogen cycle guide.

Testing for this calculator

You need three readings taken at the same time: total ammonia, pH and temperature. UF/IFAS recommends testing ammonia weekly and immediately whenever fish appearance or behavior changes, including reduced feeding. Our testing guide covers kits and technique.

Common questions

Is 0.25 ppm ammonia dangerous?

It depends on pH and temperature. At pH 7.0 and 78°F, about 0.6% is un-ionized: roughly 0.0015 mg/L. At pH 8.2 and the same temperature, it's about 0.021 mg/L. Any ammonia in an established tank means something is wrong, so find the cause.

Why does pH matter so much?

Ammonia and ammonium are in a balance that shifts toward toxic un-ionized ammonia as pH rises. Each full pH unit increases the toxic share roughly tenfold at low percentages.

My test says 'ammonia' in ppm. Is that TAN?

Most freshwater kits report total ammonia (NH3 + NH4+). Check the kit instructions; some report as nitrogen (TAN) and some as total ammonia, which differ by a factor of about 1.2.

Should I lower pH to make ammonia less toxic?

Changing pH quickly stresses fish. The safer response is to remove ammonia: reduce feeding, do a water change with treated water, and fix the cause.