Free tool
Fishless Cycle Ammonia Dosing Calculator
Fishless cycling grows the bacteria that convert ammonia to nitrite and nitrate before any fish are exposed to them. You feed those bacteria by adding a measured amount of ammonia, usually enough to reach about 2 ppm. This calculator turns your water volume, the product you have and the way your test kit reports ammonia into a dose in milliliters, drops or grams.
Dose for this addition
Water volume used: –. Increase to add: –, which equals – ppm as NH3, – ppm as NH4+ or – ppm as NH3-N. Density assumed for household ammonia: – g/mL.
Before you dose: three safety checks
- No fish or invertebrates in the tank. Dosed ammonia at cycling levels is harmful to animals. If fish are already in the tank, use the fish-in cycling approach instead, which avoids adding ammonia.
- Surfactant-free product only. Many cleaning ammonias contain soaps, scents or dyes. Read the ingredients: you want only ammonia (or ammonium hydroxide) and water. Shake the bottle; lasting foam suggests surfactants.
- Add a little less, then test. Dose about three quarters of the result, wait 30–60 minutes with the filter running, test, and top up. Labels round concentrations and droppers vary.
The math behind the calculator
Household ammonia
One ppm in water is one milligram per liter. A solution labeled p% ammonia by weight contains p × 10 × density milligrams of ammonia per milliliter. Rearranging gives the dose:
mL = target ppm × liters ÷ (percent × 10 × density)
Density of dilute ammonia is slightly below that of water: about 0.96 g/mL at 10%. The calculator approximates it as 1 − 0.004 × percent, which is close enough for dosing because the label percentage itself is usually rounded. Worked example for 20 US gallons (75.7 L), 10% ammonia and a 2 ppm target: 2 × 75.7 ÷ (10 × 10 × 0.96) = 151.4 ÷ 96 ≈ 1.58 mL, or roughly 32 drops.
Ammonium chloride
Ammonium chloride (NH4Cl) has a molar mass of about 53.49 g/mol, of which the ammonia-equivalent portion is 17.03 g/mol, so 31.84% of its weight counts as NH3. The dose in milligrams is target ppm × liters ÷ 0.3184. For the same 75.7 L tank at 2 ppm: 2 × 75.7 ÷ 0.3184 ≈ 475 mg, or about half a gram. Most kitchen scales cannot weigh this accurately; a 0.01 g jewelry scale, or dissolving a larger weighed amount in a known volume of water, works better. Pre-mixed ammonium chloride solutions sold for cycling come with their own dose per gallon; follow the bottle.
Why the test kit convention matters
Test kits report the same water in different ways. Most hobby kits read total ammonia (NH3 plus NH4+) expressed as NH3. Some report it as ammonium (NH4+) and many lab-style kits report ammonia-nitrogen (NH3-N), counting only the nitrogen atom's weight. The conversions are fixed by molar mass:
| From | To NH3 basis | Example: 2.0 ppm reading |
|---|---|---|
| NH3 (total ammonia as NH3) | × 1.000 | 2.00 ppm NH3 |
| NH4+ (ammonium) | × 0.944 (17.03 ÷ 18.04) | 1.89 ppm NH3 |
| NH3-N (ammonia-nitrogen) | × 1.216 (17.03 ÷ 14.01) | 2.43 ppm NH3 |
If your kit reports NH3-N, 2 ppm on your card is about 20% more ammonia than 2 ppm on an NH3-basis kit. That difference rarely matters for cycling, but it explains why two guides can seem to disagree. Our test kit guide covers how to tell which convention your kit uses.
How dosing fits into a fishless cycle
- Dose to about 2 ppm and test ammonia daily. Early on, nothing seems to happen for days.
- When ammonia starts falling and nitrite appears, top back up to 2 ppm only after ammonia drops below about 1 ppm.
- Nitrite typically lags ammonia. Keep feeding ammonia so the nitrite-oxidizing bacteria keep growing.
- The cycle is considered complete when a 2 ppm dose is processed to 0 ammonia and 0 nitrite within 24 hours, repeatedly.
- Then do a large water change to bring nitrate down, dechlorinate, match the temperature and add fish gradually.
The full walkthrough, including stalls and pH problems, is in how to fishless cycle a tank.
Frequently asked questions
Why 2 ppm and not 4 or 5?
Older guides often recommended 4–5 ppm or more. Dr. Tim's Aquatics, whose fishless cycling guidance is widely followed, recommends starting at 2–3 ppm (measured as ammonia-nitrogen) and warns not to go above 5 ppm, because letting ammonia and nitrite climb too high is one of the listed causes of problems. Around 2 ppm gives the bacteria enough food and roughly matches the waste of a realistic fish load.
Can I use fish food instead of ammonia?
Yes, rotting food does release ammonia, but the dose is unpredictable and it fouls the tank. Measured ammonia gives cleaner, repeatable results.
My ammonia is not dropping after two weeks. Did I dose wrong?
Probably not. Common causes are chlorinated top-up water, low pH (Dr. Tim's Aquatics links most stalls to pH below 7.0, because nitrification itself produces acid; a 20–25% water change usually restarts it), low temperature, or a surfactant in the ammonia. Check KH and pH with the help of GH, KH and pH explained.
How accurate is the drop count?
Twenty drops per milliliter is a common approximation for water-like liquids, but droppers range roughly from 15 to 30. Use a syringe for doses you care about.