Free Ammonia Part of Total Ammonia Nitrogen (TAN) Calculator
If you know your know your temperature in Celsius skip "Temperature Conversion" fields. Enter your ammonia test results in the first field, then enter the pH from test results in the second field, then enter the temperature in °C in the third field. Click on "Calculate" button to see the amount of toxic ammonia in the TAN. A NH3 level of .6 or greater can kill many species of fish within 24 hours of exposure.
Ref Article: Ammonia Test Result is More than Ammonia