Aquarium calculators

Which aquarium calculator do you need?

Each tool answers one question well. Use this list to jump to the right one for the job in front of you.

• Tank volume: the true water volume after substrate and the rim gap, in gallons and liters.
• Stocking: how full your tank is, using adult size and bioload rather than the inch per gallon rule.
• Heater wattage: the right heater size for your volume and the gap to your target temperature.
• Water change: how much water to change to reach a target nitrate level, with a safe cap.
• Filter turnover: whether your filter GPH moves enough water for your stocking level.
• Tank weight: the full loaded weight and the load on your floor in pounds per square foot.
• Substrate: how much gravel, sand or aqua soil you need for a chosen depth.
• Glass thickness: a safe glass thickness and safety factor for a build or a used tank.
• Salt mix: how much marine salt to reach a target salinity for brackish or saltwater tanks.
• Lighting: the lumens and rough LED wattage for low, medium or high light plants.
• Setup cost: the one time cost of starting a tank, with a cost per gallon.

Use the calculators in order

Start with the tank volume calculator. Almost every other tool depends on knowing your true water volume, which is smaller than the printed tank size once you account for substrate and the gap you leave below the rim. Take that net gallons figure and carry it into the stocking, heater, water change, turnover and weight calculators for consistent results from the first fish to the finished setup.

Planning a new tank rather than running one? Work the other way: use the setup cost and glass thickness tools while you shop, size the substrate and lighting for the look you want, then come back to volume and stocking once the tank is chosen. The tools link to each other, so it is easy to move between them.

Built for freshwater keepers

These calculators are focused on freshwater tanks, from a first betta setup to a planted community aquarium. The stocking tool uses real species data, and the guidance throughout is written for freshwater conditions. The salt mix calculator is the exception, added for brackish and marine keepers, since salinity is one job a freshwater tank never needs.

Every tool is free, works on a phone down to 360 pixels wide, and needs no account. Nothing you enter is saved, and each result is an estimate for planning, so treat the numbers as a strong starting point and keep testing your own water.

Why plan a tank with calculators

Fishkeeping goes wrong most often at the planning stage, not in the daily care. Buying a heater that is too small, stocking a tank beyond what the filter can handle, or setting a heavy tank on a floor that cannot take it are mistakes that are hard to undo once livestock are in. A few minutes with the right calculator turns each of those guesses into a number you can act on.

Planning also saves money. Sizing the substrate and salt mix means you buy the right amount once, the setup cost tool shows the real total before you commit, and the volume tool stops you overdosing medication on a tank that holds less water than the label claims. The tools are free, so there is little reason to guess.

Common aquarium planning mistakes

The classic mistakes repeat across every fish store. People trust the printed tank size instead of the real water volume, follow the inch per gallon rule and end up overstocked, and pick a heater on price rather than the temperature gap in their room. Others skip the cycle and add a full stock list to a tank with no bacteria to process the waste.

Each of these has a tool on this page that heads it off. Work out the net volume, check stocking by bioload, size the heater to your room, and plan water changes around your nitrate. Getting the numbers right before you buy is far easier than fixing a crowded, unstable tank later.

A simple new tank checklist

Setting up a first tank is easier as a sequence. Choose the tank and check the glass and floor can take it, then work out the true volume once you know the substrate depth. Size the heater and filter to that volume and your room, plan the substrate and, for a planted tank, the lighting. Add up the setup cost so there are no surprises.

With the hardware sorted, cycle the tank before any fish go in, then use the stocking calculator to build a livestock list your filter can support. Add fish a few at a time, and set a water change schedule from your nitrate readings. Every step here has a calculator to make the number clear.