Freshwater tool
Fishless Cycle Ammonia Dosing Calculator
This fishless cycle ammonia calculator turns your tank volume and the strength of your ammonia source into a dose in millilitres for the current step. The schedule steps the target up as nitrite appears, and the tally keeps a running total of the ammonia you have added.
Dosing schedule
Each row shows one dose to reach that step's target from 0 ppm. Treat the running total as a ceiling for the whole cycle, not a prediction of the final reading: nitrifying bacteria consume the ammonia between doses.
| Step | Use it when | Target | This dose | Running total |
|---|---|---|---|---|
| 1. First dose | Day 1, before any ammonia is in the water | 2 ppm | — | — |
| 2. Step up when nitrite appears | Nitrite first reads 0.25 ppm or more | 3 ppm | — | — |
| 3. Step up when ammonia clears | Ammonia falls to 0 ppm within 24 hours | 4 ppm | — | — |
| 4. Maintenance dose | Nitrite is falling; repeat until ammonia and nitrite both clear within 24 hours | 2 ppm | — | — |
The running total assumes each dose starts from 0 ppm and is given once. If your reading is above 0, enter it in the form so the current dose is reduced by what is already there.
How to use the ammonia dosing calculator
- Enter the actual water volume of the tank in US gallons or litres. If you are not sure, the aquarium tank volume calculator works it out from inside measurements.
- Enter the strength of your ammonia source in milligrams of ammonia per millilitre. A 5% solution is about 50 mg/mL; check your own label rather than assuming.
- Enter your current ammonia reading from a hobby test kit and choose the step you are on. Step 1 is the first dose; later steps are triggered by nitrite appearing and by ammonia clearing quickly.
- Calculate the dose, add it, then log it. Logging adds the dose to the running tally and moves you to the next step. Reset schedule returns the tally and the step to day one.
How the dose is calculated
One ppm is one milligram of ammonia per litre of water. If your product contains 50 mg of ammonia per mL, then one mL added to 100 litres raises the reading by 0.5 ppm. The calculator works the same relationship backwards:
dose in mL = (target ppm − current ppm) × water volume in litres ÷ ammonia strength in mg per mL
For example, 50 US gallons is 189.3 litres. To go from 0 ppm to 2 ppm at 50 mg/mL: 2 × 189.3 = 378.5 mg of ammonia, divided by 50 mg/mL = 7.6 mL. The same tank on a 10% solution (about 100 mg/mL) needs about 3.8 mL.
Worked example
A 40 US gallon (151.4 litre) tank with a 50 mg/mL source needs 6.1 mL for the first 2 ppm dose. Once nitrite appears, the step 2 target of 3 ppm works out to 9.1 mL from zero, or 3.0 mL if the tank still reads 2 ppm. Logging each dose keeps a running total in the panel above, so you can see exactly how much ammonia has gone in over the cycle.
Why the schedule steps the target up
The first dose feeds the bacteria that convert ammonia to nitrite. When nitrite appears, those bacteria are working, so the schedule raises the target to 3 ppm to keep feeding them. Once ammonia clears within a day, the colony is strong enough for a 4 ppm dose, which builds the nitrite oxidisers. The final step drops back to 2 ppm maintenance doses until both ammonia and nitrite clear within 24 hours.
This is one reasonable schedule, not the only one. Many experienced keepers hold 2 ppm for the entire cycle and get the same result. What matters is that ammonia is present, that you re-dose in response to readings, and that you do not let the level climb far above 4 to 5 ppm, where cycling can stall.
What "ppm" means on your test kit
Hobby ammonia kits report total ammonia nitrogen (TAN) in ppm, which is the same as mg/L. The calculator uses that number directly. Different kits read in different increments — some use 0.25 ppm steps, some 0.5 ppm — so the dose is limited by what you can actually measure. Use one kit from start to finish and follow its instructions on temperature and timing.
What this calculator does not do
- It does not verify your product. Only use pure ammonia or ammonium chloride with no surfactants, fragrance, dyes or other additives.
- It does not test the water or tell you when the cycle is finished. A 2 ppm dose must clear to 0 ppm ammonia and 0 ppm nitrite within 24 hours, with nitrate present.
- It does not account for ammonia bound by water conditioners or for test kit interference. Retest before adding more if a reading looks wrong.
- It is a dosing aid, not a treatment for sick fish or an established tank. Do not use it to add ammonia to a tank that holds livestock.
When the cycle is done, do a large water change to bring nitrate down before adding fish, then stock slowly. If you are still planning the tank, the tank volume calculator gives the water volume this dose depends on, oursalt mix calculatorcovers mixing saltwater once the tank is running, and ourfreshwater fish for beginners guide covers choosing the first species.
Frequently asked questions
How much ammonia do I add to start a fishless cycle?
Most guides start at 2 ppm total ammonia nitrogen (TAN). This calculator turns that target into millilitres using your tank volume and the strength of your product: target ppm × water volume in litres ÷ mg of ammonia per mL. For example, 50 US gallons (189.3 litres) at 50 mg/mL needs about 7.6 mL to reach 2 ppm from zero.
How do I know the strength of my ammonia?
Read the product label. A 5% ammonia solution is about 50 mg of ammonia per mL, and a 10% solution is about 100 mg/mL. Aquarium ammonium chloride products are usually sold with a stated dose rather than a percentage, so convert that dose or use the strength figure the maker publishes. Only use pure ammonia or ammonium chloride with no surfactants, fragrance, dyes or other additives.
Should I dose to 2 ppm or 4 ppm?
Start at 2 ppm. This schedule steps to 3 ppm once nitrite appears and to 4 ppm once ammonia is clearing within 24 hours, then returns to 2 ppm maintenance doses until nitrite also clears. Many keepers hold 2 ppm for the whole cycle, and that also works. Going beyond 4 to 5 ppm is not better: very high ammonia can stall the bacteria you are trying to grow and is hard on the test kit's useful range.
How often do I add ammonia during the cycle?
Test daily and re-dose when ammonia falls below 0.25 ppm, or to 0 ppm, using the target for your current step. The schedule is driven by test readings, not by calendar days. If ammonia is still well above the target, do not add more; wait and retest in 24 hours.
How long does a fishless cycle take?
Usually two to six weeks, and sometimes longer in cool water or with no seed material. Temperature, pH, carbonate hardness, filter flow and whether you started with filter media from an established tank all change the timeline. A bottled bacteria product can shorten it, but it does not replace testing.
When is the cycle finished?
When a 2 ppm dose of ammonia clears to 0 ppm and nitrite also reads 0 ppm within 24 hours, with nitrate present. Do a large water change to bring nitrate down before adding livestock, then stock slowly. This calculator does not test the water or decide that for you.
Can I use household ammonia?
Only if the label lists ammonia and water and nothing else. Many household cleaners contain surfactants, fragrance, dyes or degreasers that leave residue and can harm fish and filter bacteria. If the label does not confirm a pure ammonia source, use an aquarium-specific ammonium chloride product instead.
Does a water conditioner change the ammonia reading?
Some conditioners bind or detoxify ammonia, and that can change what certain test kits report. Use one ammonia test method consistently, follow the kit's instructions, and do not combine a conditioner dose with an ammonia dose and then trust a single reading. When a result looks wrong, retest before adding more ammonia.