Fishless Cycle Ammonia Calculator
Find out exactly how much ammonia to add to cycle a new aquarium without fish. Works with household ammonia of any strength or ammonium chloride powder, in gallons or liters.
How to fishless cycle with ammonia
- Set up the tank fully: filter running, heater on (bacteria grow faster around 25–30 °C / 77–86 °F) and dechlorinated water.
- Dose ammonia to your target using the amount above. Test about 30 minutes later to confirm the reading, then adjust future doses if needed.
- Keep feeding the bacteria. Re-dose whenever ammonia drops near zero. The bacteria starve if the ammonia supply stops.
- Watch nitrite rise and fall. Ammonia falls first, then nitrite appears and later falls as nitrate builds up.
- Run the 24 hour test. When a full dose is gone (ammonia and nitrite at zero) within 24 hours, the tank is cycled. Do a large water change to lower nitrate, then add fish.
Formulas used
The target ppm is the same as mg per liter, so the amount of ammonia needed is simply the target times the real water volume:
Ammonia needed (mg NH3) = target ppm × real water (L)
For an ammonia solution, divide by how much ammonia each mL of the bottle holds. That depends on the percentage and the solution's density:
The density of ammonia solution falls as it gets stronger: about 0.958 g/mL at 10% and 0.90 g/mL at 28%. The calculator interpolates between these values.
For ammonium chloride (NH4Cl, 53.49 g/mol), only the ammonia part (NH3, 17.03 g/mol) counts, so you need 3.14 times as much by weight:
NH3 or NH3-N? Some test kits and labels express ammonia as nitrogen (NH3-N). One ppm of NH3-N equals 1.22 ppm of NH3 (17.03 ÷ 14.01). If you choose "ammonia-nitrogen", the calculator multiplies your target by 1.216 before working out the dose.
Drops assume 20 drops per mL, the size of a standard pharmacy dropper. Real droppers vary, so measure with a syringe if you can.
Worked example
A 20 gallon tank with 15% substrate and decor holds 17 US gallons, or 64.35 L of water.
For 2 ppm: 2 × 64.35 = 128.7 mg of ammonia.
With 10% household ammonia (0.958 g/mL), each mL holds 95.8 mg. 128.7 ÷ 95.8 = 1.34 mL, or about 27 drops.
Using ammonium chloride powder instead: 128.7 × 3.14 ÷ 1000 = 0.40 g.
Which target: 2 ppm or 4–5 ppm?
Sources disagree. Older fishless cycling guides dosed 4 to 5 ppm. Dr. Tim's Aquatics, whose ammonium chloride is made for cycling, directs 2 ppm per dose and says not to exceed 5 ppm, saying nitrifying bacteria are inhibited above that level. Aquarium Science has questioned whether the study behind that limit supports it.
TankMath uses 2 ppm as the default because it matches the current manufacturer label, stays well under the 5 ppm ceiling and is a realistic daily load for a lightly stocked tank. A tank planned for heavy stocking can be cycled toward 3 to 4 ppm once the bacteria are established.
| Tank (real water) | 10% ammonia for 2 ppm | NH4Cl powder for 2 ppm |
|---|---|---|
| 5 gal (18.9 L) | 0.40 mL | 0.12 g |
| 10 gal (37.9 L) | 0.79 mL | 0.24 g |
| 20 gal (75.7 L) | 1.58 mL | 0.48 g |
| 40 gal (151.4 L) | 3.16 mL | 0.95 g |
| 75 gal (283.9 L) | 5.93 mL | 1.78 g |
Frequently asked questions
How much ammonia should I add for a fishless cycle?
Enough to reach about 2 ppm, measured with your test kit. That is the level on Dr. Tim's Aquatics ammonium chloride label, which also says not to exceed 5 ppm. Older guides used 4 to 5 ppm. For a 20 gallon tank with normal decor, 2 ppm takes roughly 1.3 mL of 10% household ammonia.
Can I use any household ammonia?
No. Use only plain ammonia whose ingredients are ammonia and water. Avoid anything scented, labelled as a cleaner with surfactants or detergents, or that foams when you shake the bottle, because those additives can harm bacteria and fish.
How do I know when my tank is cycled?
Dose ammonia to your target, wait 24 hours and test. If ammonia and nitrite both read zero (in practice 0.25 ppm or less on a hobby kit) and you are seeing nitrate, the bacteria are keeping up. Many keepers wait for two such results in a row before adding fish.
Does my test kit read ammonia as NH3 or as nitrogen?
It depends on the kit. Lab-style kits often report ammonia-nitrogen (NH3-N), and a reading in NH3-N multiplied by 1.22 gives ppm as NH3. Many hobby kits just say ppm total ammonia without stating the basis. Dr. Tim's label uses NH3-N. If you are unsure, use the default and adjust the next dose from what your kit shows.
How long does a fishless cycle take?
Typically several weeks, often 3 to 6, depending on temperature, pH and whether you seeded the filter with bottled bacteria or media from an established tank. Keep dosing ammonia regularly until the tank passes the 24 hour test.
Related calculators
- Tank volume calculator: find your real water volume
- Dechlorinator dose calculator: treat the water before you start
- Water change calculator: bring nitrate down before adding fish
Sources: 2 ppm dose and 5 ppm limit from Dr. Tim's Aquatics ammonium chloride directions, as reproduced by Bulk Reef Supply. Discussion of the 5 ppm limit and the "zero within 24 hours" test from Aquarium Science. Ammonia solution density from Ammonia solution (Wikipedia) and LibreTexts, composition of commercial acids and bases. Molar masses from PubChem. NH3-N to NH3 factor of 1.22 from Fisher Scientific test kit documentation.
Last updated: October 2026