Aquarium Calculators

Complete Guide to Free Aquarium Calculators

AquaMath.online is a free suite of aquarium calculators built for freshwater, planted and reef hobbyists who need accurate numbers before they buy equipment, mix salt, dose medication or plan a water change. Every tool starts from a practical question: how much water is really in the tank, how much substrate to order, what heater wattage to buy, how many gallons to remove in a 25 % water change, and whether free ammonia is rising into the danger zone for sensitive fish.

Search terms such as “aquarium tank volume calculator”, “how much substrate for aquarium”, “aquarium heater wattage calculator”, “marine salt mix calculator”, “water change calculator”, “fish stocking calculator”, “free ammonia calculator”, “planted tank CO₂ calculator” and “filter GPH calculator” all lead to the same need — reliable math that converts tank dimensions and water parameters into actionable volumes, weights and equipment sizes. AquaMath answers those queries without accounts, ads that block results, or complicated software.

Why net water volume is the number that matters

Most aquarium products are dosed by volume of water, not by the external size of the glass box. Glass thickness, substrate depth, rock, wood and the freeboard you leave at the top all reduce the true water volume. A 90 × 45 × 45 cm tank that looks like “180 litres” on the sticker may hold only 150–160 litres of water once substrate and décor are in place. Using the external volume for salt mix, medication or heater sizing leads to under- or over-dosing. The aquarium tank volume calculator therefore asks for external length, width and height, glass thickness and fill level, then returns the net water volume in litres and US gallons — the figure you should use in every other tool on the site.

Aquarium Tank Volume Calculator and Cylinder Volume

The rectangular tank volume calculator is the foundation of AquaMath. Enter dimensions in centimetres or inches, set glass thickness (typically 5–12 mm for home tanks) and the fill percentage (often 90–95 % to leave room for surface agitation). The result is net water volume. The companion cylinder / drum volume calculator serves vertical tanks, sumps and temporary holding drums. Both tools display litres and gallons so you can match any product label or equipment rating.

Substrate Calculator and Tank Weight Calculator

Choosing substrate depth affects both plant growth and the structural load on the stand. The aquarium substrate calculator converts tank footprint and desired depth into litres and kilograms of gravel, sand or aquasoil. A common planted-tank depth is 4–6 cm; a deep sand bed for a reef may be 8–10 cm. The tank weight calculator then estimates the total filled weight (water + substrate + rock + glass) so you can confirm that your stand and floor can safely support the aquarium. Water alone weighs about 1 kg per litre; a large reef tank with live rock can easily exceed 300–400 kg.

Water Change Volume and Nitrate Dilution Calculators

Regular water changes are the simplest way to export nitrate, phosphate and organic waste. The water change volume calculator turns a percentage (10 %, 25 %, 50 %) into the exact litres or gallons to remove and replace. The nitrate / parameter dilution calculator goes further: given current and target concentrations and the size of each change, it estimates how many successive water changes are needed to reach the target. These two tools together help you plan a safe reduction of elevated nitrate without shocking the livestock with a single massive change.

Marine Salt Mix and Specific Gravity Converters

Reef and marine aquariums require stable salinity. Most salt mixes aim for approximately 35 g of salt per litre of water to reach 35 ppt (specific gravity ≈ 1.026 at 25 °C). The marine salt mix calculator multiplies your net tank volume by the target grams per litre and returns the weight of salt to dissolve. Always follow the manufacturer’s instructions for the final mix and verify with a calibrated refractometer. The specific gravity ↔ ppt converter lets you move between the two common measurement systems used by hydrometers and refractometers.

Medication Dose and Free Ammonia (NH₃) Toxicity

Medication and water-conditioner labels are written for a stated volume. The medication / conditioner dose calculator scales the label dose to your actual net volume so you neither under-dose (risking treatment failure) nor over-dose (risking toxicity). Free ammonia is far more toxic than ammonium; the fraction of total ammonia that exists as NH₃ rises sharply with pH and temperature. The free ammonia toxicity calculator uses the well-known equilibrium relationship to estimate the dangerous NH₃ concentration from your measured total ammonia, pH and temperature, helping you decide whether an emergency water change or ammonia binder is required.

Heater Wattage, Filter Flow and Lighting Calculators

Heaters are commonly sized at 0.5–1 watt per litre (roughly 2–4 watts per gallon) depending on room temperature and desired tank temperature. The aquarium heater calculator returns a recommended wattage range and often suggests dual heaters for redundancy on larger systems. Filter flow is usually targeted at 4–10 times tank volume per hour for freshwater and higher for many reef setups; the filter GPH / turnover calculator converts volume into the target flow rate. The lighting lumens calculator helps planted-tank keepers estimate the total light output needed for low-, medium- or high-light plants.

Glass Thickness, Stocking and Planted-Tank Tools

DIY aquarium builders need safe glass thickness. The glass thickness calculator offers a conservative starting point based on tank dimensions (always verify with a structural reference or professional fabricator). The fish stocking calculator applies the traditional inch-per-gallon guideline as a rough upper bound; it is a planning aid only and must be tempered by adult fish size, swimming behaviour, filtration and species compatibility. Planted-tank keepers use the CO₂ from pH and KH calculator to estimate dissolved carbon dioxide and the fertilizer dose calculator to scale liquid or dry fertilizers to net volume.

Temperature and Volume Unit Converters

Equipment and literature switch freely between °C and °F and between litres, US gallons and UK gallons. The converters on the Convert tab give instant, accurate translations so you never mis-read a heater range or a medication label written in a different unit system.

How to use AquaMath for a new tank build

1. Calculate net water volume from external dimensions and glass thickness.
2. Decide substrate depth and run the substrate and weight calculators to confirm stand capacity.
3. Size the heater and filter from net volume.
4. For marine tanks, calculate the salt required for the first fill and for future water changes.
5. Plan a water-change schedule and use the dilution tool if parameters need gradual correction.
6. Before adding livestock, review stocking guidelines and, for planted tanks, target CO₂ and fertilizer levels.

All results on AquaMath.online are educational estimates. They are not a substitute for manufacturer instructions, professional aquarium advice or veterinary care. Livestock health always comes first.

Aquarium Calculator FAQ

How do I calculate real aquarium water volume?

Use the aquarium tank volume calculator. Enter external length, width and height, glass thickness and the percentage you actually fill. The tool subtracts glass displacement and freeboard to return net litres and gallons — the volume you should use for dosing, salt and equipment.

How much substrate do I need for my aquarium?

The substrate calculator multiplies the tank footprint by the desired depth and converts the volume into approximate weight for gravel, sand or aquasoil. Typical planted-tank depths are 4–6 cm; deeper beds are used for certain plant species or deep sand beds in marine tanks.

What size heater does my aquarium need?

Open the heater wattage calculator. A common rule of thumb is 0.5–1 W per litre (about 2–4 W per US gallon), adjusted for the temperature difference between the room and the target tank temperature. Larger tanks often benefit from two smaller heaters for redundancy.

How much salt do I need for a marine tank?

The marine salt mix calculator multiplies net water volume by the target grams of salt per litre (often ≈ 35 g/L for 35 ppt). Always mix in a separate container, aerate, and verify salinity with a calibrated instrument before adding the water to the display tank.

How do I calculate a water change volume?

The water change volume calculator converts a percentage of net volume into the exact litres or gallons to remove. The companion dilution calculator estimates how many successive changes are required to bring a parameter such as nitrate down to a target level.

Is free ammonia dangerous at my pH and temperature?

Yes — toxicity rises with pH and temperature. Enter total ammonia, pH and temperature into the free ammonia (NH₃) calculator to estimate the concentration of the toxic form. Even low total ammonia can become hazardous in warm, alkaline water.

How many fish can I keep in my tank?

The stocking calculator applies the classic inch-per-gallon guideline as a rough ceiling. It is only a starting point. Adult size, territorial behaviour, filtration capacity, swimming space and species compatibility matter far more than a simple length-to-volume ratio.

How do I estimate CO₂ in a planted tank?

The CO₂ calculator uses the relationship between measured pH and carbonate hardness (KH) to estimate dissolved carbon dioxide. Target ranges for most planted aquariums are roughly 15–30 ppm; always combine the estimate with observation of plant growth and fish behaviour.

Do the calculators work in both metric and imperial units?

Yes. Volume tools accept centimetres or inches and return both litres and US gallons. Temperature and volume converters on the Convert tab handle °C/°F and litres/gallons (US and UK) so you can match any label or chart.

Are the results accurate enough for real tanks?

The formulas follow standard aquarium-hobby and manufacturer practices. They are excellent for planning and ordering. Always cross-check critical values (salinity, medication dose, structural glass thickness) with product instructions, calibrated instruments and, where appropriate, professional advice. AquaMath results are educational estimates only.

All free AquaMath aquarium calculators

Every tool is free, works on mobile and desktop, and requires no registration. Click any link to jump to the calculator.

• Aquarium tank volume calculator — net water volume from external dimensions
• Cylinder / drum volume calculator — vertical tanks and sumps
• Substrate calculator — gravel, sand or aquasoil volume and weight
• Tank weight calculator — filled weight for stand and floor planning
• Water change volume calculator — litres or gallons to remove
• Nitrate / parameter dilution calculator — changes needed to reach a target
• Marine salt mix calculator — grams of salt for target salinity
• Specific gravity ↔ ppt converter
• Medication / conditioner dose calculator — scale label dose to net volume
• Free ammonia (NH₃) toxicity calculator — from total ammonia, pH and temperature
• Aquarium heater wattage calculator
• Filter flow / turnover calculator
• Lighting lumens calculator — for planted tanks
• Glass thickness calculator — DIY aquarium starting point
• Fish stocking calculator — inch-per-gallon guide
• Planted tank CO₂ calculator — from pH and KH
• Fertilizer dose calculator

Bookmark AquaMath.online for every new tank build, water change and equipment upgrade. Accurate numbers save livestock, money and time.