Quick answer: These free aquarium calculators cover the five numbers every fishkeeper needs: your real water volume, how many fish your tank can hold, how much water to change, how much fertilizer to dose and how much CO2 is in your water. They work on phones, need no sign-up, and are built around real hobby methods like EI dosing and the pH drop test rather than rules of thumb. Start with the volume calculator, because every other result depends on knowing how much water is actually in your tank, which is usually 10 to 25 percent less than the size on the box.
All aquarium calculators at a glance
| Calculator | What it tells you | Use it when |
|---|---|---|
| Aquarium volume calculator | Real water volume in gallons and liters, tank weight, substrate needed, heater and filter size | Setting up a tank, or before dosing anything |
| Aquarium stocking calculator | How full your tank is, plus warnings for tank size, group size, aggression and temperature | Planning fish, or before buying new ones |
| Aquarium water change calculator | What percent to change to lower nitrate, how many buckets, how much dechlorinator, and your weekly routine | Nitrate is high, or you want a steady routine |
| Aquarium fertilizer calculator | Grams of KNO3, KH2PO4, K2SO4, Epsom salt and CSM+B for EI, PPS Pro and low tech dosing | Dosing dry fertilizers in a planted tank |
| Aquarium CO2 calculator | CO2 in ppm from KH and pH, the pH drop method or drop checker color | Running injected CO2 in a planted tank |
Which calculator should you use first?
New tank. Start with the volume calculator to plan substrate, heater and filter. Then use the stocking calculator to plan your fish before you buy them, and read our how to cycle a fish tank guide before adding any.
Existing community tank. Check your stocking level, then use the water change calculator to set a weekly routine that keeps nitrate under 20 ppm.
Planted tank. Use the fertilizer calculator for a full weekly dosing plan. If you inject CO2, check it with the CO2 calculator, since unstable CO2 is the top cause of black beard algae.
Problem tank. High nitrate, algae or stressed fish? Run the stocking and water change calculators first. Overstocking and too few water changes cause most problems.
Here’s the honest part. A calculator is only as good as the numbers you put in. The most common mistake is using the tank’s label size instead of the real water volume, which throws off every dose, from fertilizer to medication. That one step makes all the other results more accurate.
Aquarium volume calculator
Works for rectangle, bowfront, cylinder, half cylinder, corner, hexagon and L shaped tanks. It subtracts glass thickness, the gap under the rim, substrate and decor to give your real water volume. It also gives you full tank weight, substrate in liters and kilograms, heater wattage, filter flow and water change amounts, and sends your volume straight to the fertilizer calculator.
Open the aquarium volume calculator
Aquarium stocking calculator
Pick from more than 50 freshwater fish, shrimp and snails. It scores your tank by adult size and waste rather than the old inch per gallon rule, and warns about tanks that are too small, groups that are too few, fin nippers with long-finned fish, aggressive fish, predators, goldfish with tropicals and temperatures that don’t overlap.
Open the aquarium stocking calculator
Aquarium water change calculator
Enter your nitrate now and your target, and it shows the percentage, gallons, buckets and dechlorinator dose. It splits big changes into safer steps, and a second tab shows the level your nitrate settles at with your weekly routine.
Open the aquarium water change calculator
Aquarium fertilizer calculator
Builds a full weekly plan for EI, medium, low tech, daily or PPS Pro dosing, and works out how much potassium your nitrate and phosphate salts already add. It also converts target ppm to grams and grams to ppm, plans stock solutions, and shows the worst case nutrient build up with your water changes.
Open the aquarium fertilizer calculator
Aquarium CO2 calculator
Three methods in one: KH and pH, the pH drop method that works even with aquarium soil and driftwood, and a drop checker color guide. A color bar shows whether your CO2 is low, ideal or dangerous for fish.
Open the aquarium CO2 calculator
FAQ
Are these aquarium calculators free?
Yes. All of them are free, work on phones and computers, and don’t need an account.
How do I calculate my aquarium volume?
Multiply length x width x height in inches and divide by 231 for US gallons, or multiply in centimeters and divide by 1,000 for liters. Then subtract roughly 10 to 25 percent for substrate and decor, or let the volume calculator do it.
How many fish can I put in my tank?
It depends on the adult size and waste of each species, not just inches. The stocking calculator checks this, along with group size and compatibility.
How much water should I change each week?
Usually 25 to 30 percent for community tanks. The water change calculator shows the exact amount your tank needs to keep nitrate where you want it.
Do I need a calculator for aquarium fertilizer?
If you dose dry fertilizers, yes. Small dosing mistakes add up quickly, and the fertilizer calculator gives exact grams for your water volume.