How We Calculate

Our approach

A calculator is only useful if you can see how it reached its answer. Each AquariumCalcs page explains its method in plain language, says which figures are standard formulas and which are hobbyist rules of thumb, and tells you which numbers are examples you should replace. We do not take payment to favor a product, and we do not give dose rates for particular products.

Tank size, weight and load

Tank volume comes from the inside dimensions: length times width times height in inches, divided by 231 cubic inches per U.S. gallon. A gallon is 3.785 liters. The weight tools use about 8.34 pounds per gallon for water and add the weight of the empty tank, substrate and decor. The floor load tool divides the total filled weight by the footprint in square feet and compares it with a typical residential allowance of 40 pounds per square foot. The substrate tool multiplies the volume of material by a bulk density that depends on the material, since sand packs more tightly than gravel.

Stocking, feeding and quarantine

The stocking tool uses a surface-area method, because oxygen enters the water mostly at the surface, and shows the older one-inch-per-gallon rule beside it for comparison. It cannot account for species, filtration or feeding. The feeding tool gives a starting amount scaled by fish size and number and relies on the two-minute rule: feed what is eaten in about two minutes. The quarantine tool applies the same per-fish space logic with a 5 gallon practical minimum and suggests an observation period of a few weeks.

Water chemistry

The water change tool uses simple dilution: the new reading is the current reading multiplied by the fraction left, plus the tap reading multiplied by the fraction added. The CO2 tool uses the standard pH, KH and CO2 relationship, in which dissolved CO2 in ppm equals 3 times the KH times 10 raised to the power of 7 minus the pH. The hardness converter uses 17.848 ppm as CaCO3 per degree of hardness, the standard German-degree conversion. The ammonia tool turns a target in ppm into milligrams for your volume, then into milliliters for the strength on your bottle. The heater tool uses a watts-per-gallon rate that rises with the gap between your coldest room temperature and your target. The filter flow tool multiplies tank volume by a turnover rate of 5 times an hour for a lightly stocked freshwater tank, 8 times for a heavily stocked one and 10 times for a reef, as a minimum.

Planted tanks

The fertilizer tool follows the Estimative Index approach, in which doses are scheduled somewhat above expected plant consumption and then reset with a large weekly water change. The aquascape tool applies the rule of thirds and the golden ratio to the length and height of your tank. Both are guides and not laws.

Reef and marine

The salinity converter uses a linear approximation between specific gravity and parts per thousand that is most accurate near typical reef salinity, about 1.025 to 1.026 specific gravity or roughly 33 to 35 parts per thousand. The reef dosing tool uses commonly published ratios for homemade two-part recipes, which can differ from commercial products. The skimmer tool starts from the manufacturer’s rated capacity and adjusts it for total system volume and for a lighter or heavier bioload.

Glass thickness

The glass thickness tool uses a table of commonly used hobbyist bands based on water height, with a step up for an unusually long panel. It is conservative planning guidance, not a structural calculation, and it says so.

Finders

Each finder asks a few questions, scores your answers against the options and shows the best match, often with a runner-up. Each result explains why it fits, what to check and when it is not the right fit.

Limits

All results are estimates and general guidance. Real tanks vary, defaults are examples and guidance changes. Please read our disclaimer and safety notes, and use the Report an Issue form if a result looks wrong.