Pick a percentage and TankMath shows the amount in gallons and litres, the number of five-gallon buckets, and how much of today’s water would still be in the tank after four weekly changes. Use the real water volume for an accurate answer.
How tank volume is worked out
Every shape comes down to base area × height. For a rectangular tank the base is length × width. A cylinder uses π × radius², a regular hexagon uses 2.598 × side², and a quarter-circle corner tank uses a quarter of π × radius². A bow front is treated as a rectangle plus a curved segment whose area is two thirds of the length times the extra depth at the centre of the bow, which matches most commercial bow fronts closely. In inches the result is divided by 231 to give US gallons; in centimetres it is divided by 1,000 to give litres.
The nominal size of a tank is rarely the water it holds. TankMath subtracts the substrate depth and the gap you leave below the rim, then a percentage for rock and decor, to estimate the real water volume. That figure drives the water change, dosing and schedule, because medicine, conditioner and salt should be measured against the water actually in the tank. The filled weight adds water at about one kilogram per litre and substrate at about one and a half.
Water changes, conditioner and salt
Regular partial changes keep nitrate low and water chemistry steady. With a 25% weekly change, only about a third of today’s water is still in the tank four weeks later. Water conditioner doses scale directly with volume, so the calculator multiplies the label rate by the gallons you are treating. For marine mixes, specific gravity is converted to salinity in parts per thousand and multiplied by the litres you are making; because brands differ, always confirm with a refractometer or hydrometer before the new water goes in.
A schedule that fits your tank
The maintenance plan scales with stocking: heavier stocking means larger, more frequent water changes and more frequent filter rinses. Planted tanks get trimming and fertiliser reminders and gentler substrate cleaning, and saltwater tanks get daily top-offs with fresh RO water and weekly tests. Everything is calculated in your browser and saved only on this device.
Frequently asked questions
How much water should I change?
20–30% weekly suits most freshwater community tanks. Heavily stocked tanks may need more; lightly stocked planted tanks can go two weeks.
Is a 50% water change safe?
Usually, if the new water matches temperature and is dechlorinated. Avoid big changes if the tank has gone months without one; build up gradually.
Should I remove fish during a water change?
No. Leave them in and keep the change gentle.
Is TankMath free?
Yes. Every calculator is free with no account.
Is my tank information saved?
Only in this browser, so the calculator remembers your tank next time. Nothing is uploaded.